ROTARY CIRCUIT CONTROL DEVICES WITH CHANGEABLE GRAPHICS

Abstract: An electrical circuit control of the type having a knob (12) which is turned by an operator to vary a circuit condition is affixed to the face of an electronic image display device (16) and at least a portion of the knob overlays the image display area (14). This enables display of instantly changeable calibration marks and/or other graphics (13, 18) in close proximity to the circuit control including at locations immediately adjacent to the perimeter of the knob (12). Electronic components of the control are contained within the knob and a base member (31) at the front of the panel. Components of this kind may be embedded in or attached to the knob itself enabling easy repair by simply replaced carrier that fits within the base (31). A signal processor can convert initial values of the control signal produced at successive settings of a control to differing assigned values.

Description: ROTARY CIRCUIT CONTROL DEVICES WITH CHANGEABLE GRAPHICS Cross Reference to Related Applications This application is a contmuation-m-part of copend- mg United States application Serial No: 08/725,201 filed on September 20, 1996. Application Serial No: 08/725,201 is a continuation-in-part of copendmg United States ap- plication Serial No: 08/644,888 filed on May 10, 1996 and which is a contmuation-in-part of copending application Serial No: 08/420,438 filed on April 10, 1995. Copending application Serial No: 08/420,438 is a continuation of application Serial No: 08/225,782 filed April 11, 1994 and which is now abandoned. Application Serial No: 08/225,782 was a contmuation-in-part of application Serial No: 08/147,545 filed November 5, 1993 and which is now abandoned . Technical Field This invention relates to devices which enable operator control of electrical circuits, more particularly the invention relates to circuit control devices of the type having a knob which is turned by an operator about an axis of rotation to vary an electrical signal and wherein an electronic image display screen produces changeable images that convey information pertinent to operation of the cir- cuit control device. Background of the Invention Traditional circuit control devices have a component, such as a turnable knob for example, that is physically moved by the operator in order to change an electrical condition in the controlled circuit. Potentiometers, ro- tary variable resistors and rotary encoders are examples of control devices of this type. Controls which require physical movement of a component by the operator are preferred by many persons and are less prone to erroneous input because of the tactile feedback which the operator receives. Some newer forms of circuit control such as touch screens for example do not provide this kind of feedback. Conventional controls which have a knob that is turned by the operator are not well adapted for use with some newer types of electronic circuit. The control typically requires calibration marks or other graphics around the perimeter of the knob and additional graphics which identify the function of the control and/or other information pertaining to operation of the control. In the conven- tional construction a shaft extends through a control panel. The turnable knob is in front of the control panel. Other components of the control device are behind the panel and are interconnected with the knob by the shaft. The graphics are permanently imprinted on the front of the control panel. Fixed graphics of this kind can severely limit the functions of the control device and can cause operator error under certain circumstances. This problem occurs in electronic systems in which a single control device is used for different purposes at different times. For example, a station selector knob of a radio requires one set of graphics when the radio is operating in the AM mode and a different set of graphics when the radio operates in the FM mode. Multiple function controls of this kind are also found in diverse other types of electronic apparatus such as computer alphanumeric keyboards and synthesizer keyboards for example. In many cases the operator must memorize the alternate functions of such controls or consult a chart or resort to such expedients as disposing an overlay on the controls that is imprinted with changed graphics. Problems of the above described kind can be alleviated by associating an image display screen with the circuit control device to enable display of instantly changeable images on or in the vicinity of the control. Under microprocessor control, graphics for the control device can be caused to change automatically when the function of the control device changes. Under some circumstances this can be beneficial at single function control devices as well as at multiple function controls. For example, the language of the labeling at a circuit control device can be caused to change instantly m response to operation of a language selection key. Prior rotary knob circuit control devices having an image display which provides instantly changeable graphics have characteristics that can be a source of problems at least under some circumstances. The control device and the image display screen are at different locations on the control panel and the image display area of the screen is bounded by a peripheral region of the display which con- tains structural framing, seals and other components. The display cannot provide calibration marks around the perimeter of the knob and the sizable spacing between the graphics and the knob complicate the operator's task of associating graphics with the specific controls to which they relate. Some prior rotary controls of the above described kind have a thicker and bulkier construction then is desirable for some usages such as in constricted locations or where a thin control panel is to be disposed against a wall for far example. Some can be difficult and costly to manufacture particularly if precise control signals and a long operating life are desirable criteria. Others can produce only a very limited variety of different graphics. Repair or replacement of prior rotary knob control devices requires access to the back of the control panel. This complicates such operations and deters many users of control devices from personally undertaking the repair or replacement . The present invention is directed to overcoming one or more of the problems discussed above. Summary of the Invention In one aspect the present invention provides a control device for an electrical circuit which control device has a base and a turnable knob coupled thereto, the knob being turnable to any of a plurality of different angular orientations to change an electrical condition in the circuit. The base is affixed to an image display device which provides changeable images and is at a location which is m front of the display device and which at least partially overlays the image display area of the display device. In another aspect of the invention a control device for providing operator selected control signals to a controlled electrical circuit includes an image display device having a screen with a transparent cover plate and an image area thereat at which information pertaining to operation of the control device is displayed. A base member is affixed to the cover plate at a location which is within the image display area. A knob engages on the base member and is turnable about an axis of rotation that ex- tends at right angles to the cover plate. A control signal producing circuit produces a control signal indicative of the angular orientation of the knob about the axis of rotation, the control signal producing circuit being contained within the knob and base member. In another aspect the invention provides a method for processing control signals produced by an electrical circuit control device of the type in which manual movement of a first component along a series of different settings causes a second component to produce a circuit control signal that progressively changes during the course of the movement to identify the successive settings of the first component Steps in the method include detecting and storing the original value of the control signal at each of a series of different settings of the first component. An assigned value for the control signal at each of the series of different settings of the first component is designated which assigned values may differ from the original values. Each assigned value is stored in association with the corresponding original value. There- after, movement of the first component is responded to by converting detected original values of the control signal to the associated assigned values. In still another aspect of the invention, a control device for an electrical circuit has a base member and a movable member attached thereto which movable member is manipulated by an operator to change a setting of said control device. The base member is affixed to an electronic image display screen at a location which is at least partially within an image display area of the screen. A display screen controller is coupled to the image display screen and is conditioned to cause display of an image which conveys information pertinent to operation of said control device. The control device further includes a signal producing circuit at the location of the base member and movable member which requires input of an electrical voltage. A photovoltaic cell for converting light to electrical energy is disposed within the base member and is connected to the signal producing circuit to supply the electrical voltage thereto. The photovoltaic cell is positioned to receive light which is produced by the image display screen. The operator manipulated turnable knob of embodiments of the invention is engaged on a base member which is affixed to the face of an image display device such as a flat panel display or a cathode ray tube for example. The knob is at a location which is at least partially within the image display area. This enables display of calibration marks and/or other graphics in close proximity to the perimeter of the turntable knobs of the circuit control devices. Consequently, the operator can more easily associate the graphics with the particular knob to which they relate and precise setting of the control is facilitated. The displayed graphics may change instantly and automatically when the function of the control device changes or when other conditions call for changed graphics. The invention further provides for detection of operator adjustment of a turnable knob by any of a variety of different and advantageous motion sensing means. In one form of the invention a data processing circuit alters the control signals which it receives in order to compensate for non-linearities at successive settings of the control device or to compensate for departures of the signal from a desired non-linear response to operator ad ustment of the control. Electrical components of the control device which produce the control signal are contained within the knob and/or the base member at the front of the panel which supports the control device making them easily accessible for maintenance or replacement. In one form of the invention components of this kind are contained within a knob which may have a low cost construction and can be easily replaced as a unit by simply replacing the knob itself . The invention, together with further aspects and advantages thereof, may be further understood by reference to the following description of the preferred embodiments and by reference to the accompanying drawings. Brief Description of the Drawings FIG. 1 is a perspective view of a first embodiment of the invention showing an electrical circuit control device which in this example is potentiometer of the type having a knob which the operator turns to vary a control signal voltage . FIG. 2A is a front view of the first embodiment of the invention depicting graphics which are displayed during a first mode of operation of the control device. FIG. 2B is another front view of the first embodiment of the invention showing changed graphics which are displayed during a second mode of operation. FIG. 3 is a broken out side view of a portion of the first embodiment of the invention taken along line 3-3 of FIG. 1. FIG. 4 is a broken out frontal view of a portion of the first embodiment showing the multi-layered construction of a flat panel display which is a component of the control device. FIG. 5 is a broken out side view of the rotary knob region of the first embodiment showing certain internal components thereof. FIG. 6 is a cross section view taken along line 6-6 of FIG. 5. FIG. 6A is a schematic circuit diagram illustrating a first modification of the control device of FIG. 5. FIG. 6B is a flowchart program depicting operations performed by a digital data processor which is a component of the circuit of FIG. 6A. FIG. 6C is a diagram depicting variations of voltages at locations in the circuit of FIG. 6A as a rotary knob of the control device is turned. FIG. 7 s a broken out side view of the rotary knob region of a control device which is a second modification of the control device of FIG. 5. FIG. 8 is another broken out side view of the rotary knob region of a circuit control device which is a third modification of the control device of FIG. 5. FIG. 9 is still another broken out side view of the rotary knob region of a circuit control device which is a fourth modification of the control device of FIG. 5. FIG. 10 is another broken out side view of the rotary knob region of a circuit control device which is a fifth modification of the control device of FIG. 5. FIG. 11 illustrates a further modification of the apparatus of FIG. 10. FIG. 12 illustrates another modification of the apparatus of FIG. 10. FIG. 12A is an elevation view of the apparatus of FIG. 12 taken along line 12-12 thereof. FIG. 13 is an axial section view of the rotary knob region of another embodiment of the circuit control device wnich is a potentiometer that produces a continuously m- creasing or decreasing output voltage as the knob is turned through more than one full revolution. FIG. 14 is a cross section view taken along line 14- 14 of FIG. 13. FIG. 15 is a cross section view taken along line 15- 15 of FIG. 13. FIG. 16 is a cross section view of rotary knob type of electrical circuit control device which uses a radio frequency circuit to sense and track turning of the knob. FIG. 17 is a plan section view of the circuit control of FIG. 16 taken along line 17-17 thereof. FIG. 18 is a schematic circuit diagram depicting electronic components of the circuit control device of FIGS. 16 and 17. FIG. 18A. is a flowchart program depicting operations performed by a digital data processor component of the circuit of FIG. 18 FIG. 19 is a broken out side view of a modification of the circuit control device of FIGS. 16 to 18. FIG. 20 is a broken out side view showing another modification of the circuit control device of FIGS. 16 to 18. FIG. 21 is a broken out side view of another rotary knob type of circuit control device which also uses a ra- dio frequency circuit to track turning of the knob. FIG. 22 is a broken out side view showing a modified form of the circuit control device of FIG. 21. FIG. 22A is another broken out side view of the circuit control device of FIG. 22 as seen from a point of view that is at right angles to the point of view of FIG. 22. FIG. 23 is a broken out side view of another rotary knob type of electrical circuit control device which uses capacitive effects to sense and track turning of the knob. FIG. 24 is a plan section view of the circuit control device of FIG. 23 taken along line 24-24 thereof. FIG. 25 is a schematic circuit diagram depicting electronic components of the circuit control device of FIGS. 23 and 24. FIG. 25A is a flowchart program depicting operations performed by a digital data processor component of the circuit of FIG. 25. FIG. 26 is a plan section view corresponding gener- ally to FIG. 24 while illustrating a modification of the control device thereof. FIG. 27 is a broken out side view of another rotary knob type of electrical circuit control device which uses photoelectric effects to sense and track turning of the knob . FIG. 28 is a cross section view taken along line 28-28 of FIG. 27. FIG. 29 is a cross section view taken along line 29-29 of FIG. 27. FIG. 29A, is a diagram depicting output signal changes which occur at a pair of photoelectric detectors as the rotary knob of the control device of FIGS. 27 to 29 is turned through a series of successive settings. FIG. 30 is a schematic circuit diagram showing elec- tronic components of the circuit control device of FIGS. 27 to 29. FIG. 30A is a flowchart program depicting operations performed by a digital data processor component of the circuit of FIG. 30. FIG. 30B is a flowchart depicting a subroutine performed by the program of FIG. 30A. FIG. 31 depicts a rotary disk which is a component of the circuit control device of FIGS. 27 to 30. FIG. 32 is a broken out side view of a rotary knob type of electrical circuit control showing a first modification of the circuit control of FIGS. 27 to 31. FIG. 33 is a top view of the control device of FIG. 32. FIG 34 is a broken out side view of a rotary knob type of electrical circuit control showing another modification of the circuit control of FIGS. 27 to 31. FIG. 35 is a top view of the device of FIG. 34. FIG. 36 is a side view of a photoelectric source and detector wafer which is used in certain embodiments of of the invention. FIG. 37 is a frontal view of the wafer of FIG. 36. FIG. 38 is a broken out side view of a bank of circuit control devices of the rotary knob type and which use photoelectric effects to sense and track turning of the knobs . FIG. 39 is a cross section view taken along line 39- 39 of FIG. 38. FIG. 40 is a broken out side view of a rotary knob type of electrical circuit control device which uses the Hall effect to sense and track turning of the knob. FIG. 41 is a section view taken along line 41-41 of FIG. 40. FIG. 42 is a schematic circuit diagram showing elec- tronic components of the circuit control device of FIGS. 40 and 41. FIG. 42A is a flowchart program depicting operations performed by a digital data processor component of the circuit of FIG. 42. FIG. 43 is a graphical depiction of variations of output signal strength as a function of the angular position of the rotary knob of the circuit control device of FIGS. 40 to 42. FIG. 43B is a cross section view, corresponding gen- erally to FIG. 41, showing a modified form of the Hall effect circuit control device of FIGS. 40 to 43. FIG. 43C is a schematic circuit diagram showing electronic components of the circuit control device of FIG. 43B. FIG. 43D is a flowchart program depicting operations performed by a digital data processor component of the circuit of FIG. 43C. FIG 44 is a frontal view of a control panel having a plurality of control devices in accordance with another embodiment of the invention. FIG. 45 is a cross section view taken along line 46- 46 of FIG. 44. FIG. 46 is a broken out frontal view of a rotary knob type of electrical circuit control which uses a magnetic pad to sense turning of the knob and in which certain electronic components are shown in schematic form. FIG. 47 is a section view of another rotary knob circuit control wherein electronic components are contained in a replaceable insert. FIG. 48 is a section view taken along line 48-48 of FIG. 47. FIG. 49 is a section view taken along line 49-49 of FIG. 48. FIG. 50 is an enlarged view of the portion of FIG. 47 that is enclosed by dashed rectangle 50 therein. FIG. 51 is a perspective view of a spring contact component of the apparatus of FIGS. 47 to 50. FIG. 52 is a section view of a modification of the apparatus of FIGS. 47 to 50. FIG. 53 is an enlarged view of the area of FIG. 52 that is encircled by dashed line 53 thereof. FIG. 54 is a section view of another embodiment of the invention in which electronic components are contained within a replaceable insert. FIG. 55 is a cross section view taken along line 55-55 of FIG. 54. FIG. 56 is a section view of a touch sensitive circuit control embodying the invention. FIG. 57 is a front view of another circuit control wherein changeable graphics are provided by a cathode ray tube type of display. FIG. 58 is a frontal view of another circuit control wherein changeable graphics are provided by a flat panel display of the segmented electrode type. FIG. 59 is a cross section view of another circuit control which uses the Hall effect to sense the angular orientation of a turnable knob. FIG. 60 is a section view of the control device of FIG. 59 taken along line 60-60 thereof. FIG. 61 is a side view of a magnet which is a component of the control device of FIGS. 59 and 60 indicating the location of magnetic poles within the magnet. FIG. 62 is a side view of another magnet which may be substituted for the magnet of FIG. 61 and which has a dif- fering arrangement of magnetic poles. FIG. 63 is a top view of the magnet of FIG. 62. FIG. 64 is a section view of a modification of the control device of FIGS. 59 and 60. FIG. 65 is a section view of the control device of FIG. 64 taken along line 65-65 thereof. FIG. 66A is a side view of a magnet which is a component of the control device of FIGS. 64 and 65 showing the location of magnetic poles in the magnet. FIG. 66B is a top view of the magnet of FIG. 66A. FIG. 67 is a section view of another modification of the control device of FIGS. 59 and 60. FIG. 68 is a cross section view taken along line 68- 68 of FIG. 67. FIG. 69 is a section view of still another control device which uses the Hall effect to sense the angular orientation of a turnable knob. FIG. 70 is a section view taken along line 70-70 of FIG. 69. FIG. 71 is a side view of an annular magnetic shield which is a component of the control device of FIGS. 69 and 70 wherein the shield is shown in a flat shape as it appears prior to being formed into the annular configura- tion. FIG. 71 A is a program flowchart depicting operations performed by a digital data processor component of the control device of FIGS 69 and 70. FIG. 72 is a section view corresponding generally to FIG. 70 while showing a modification of the control device of FIG. 70. FIG. 73 is a broken out side view of a control device having a turnable knob disposed at the face of an electronic image display screen wherein knob position signals are transmitted to the edge of the screen without requiring that electrical conductors extend across the screen and wherein electronic components at the location of the knob are energized without requiring that electrical conductors extend across the screen. FIG. 74 is a schematic circuit diagram showing electrical components of the control device of FIG. 73. FIG. 74A is a program in flowchart form depicting operations performed by a digital data processor which is a component of the circuit of FIG. 74. FIG. 75 is a frontal view of a control device having a turnable knob emplaced on the face of an electronic display screen wherein the knob may be emplaced at any desired location of the screen and which may be removed therefrom and repositioned if desired. FIG. 76 is a broken out side view of the control de FIG. 77 is a schematic circuit diagram showing electrical components of the control device of FIG. 76. Detailed Description of the Preferred Embodiments The invention is applicable to electrical circuit control devices of the type which have a knob that is manually turned to vary a circuit control signal or to turn a circuit on and off. As an initial example, FIG. 1 depicts a potentiometer 11 type of circuit control device which has a rotary knob 12 that is grasped and turned by the operator to selectively vary an output voltage. Potentiometers typically have indicia 13 at angular intervals around the perimeter of the knob 12 which identify specific angular settings of the control device. Embodiments of the present invention differ from prior control devices that serve similar purposes in that the component which the operator manipulates, knob 12 in this example, is situated at least partially within the image display area 14 of an electronic image display de- vice 16 of one of the kinds that produces instantly changeable images. In this example of the invention the display device 16 is a flat panel display. At least some of the indicia 13 and/or other graphics pertinent to operation of the control device 11 are images generated by the flat panel display 16. A flat panel display controller 17 which may be of known design is coupled to the flat panel display 16 to cause display of the indicia 13 and/or other appropriate graphics. Situating the knob 12 or other operator manipulated component in front of the image display area 14 in this manner enables display of the indicia 13 at locations which are immediately adjacent to the knob 12 or the like. This avoids operator error with respect to associating the graphics with the particular control device to which they relate and enables precision setting of the control device. The arrangement can also enhance the appearance of control panels. Referring to FIGS. 2A and 2B, graphics 18 may flash on and off, change form and purely decorative graphics may be displayed in the vicinity of the knob 12. The display controller 17 can change the graphics 13, 18 instantly when the function of the knob 12 is changed. For purposes of example, FIGS. 2A and 2B depict changing graphics 13, 18 which are appropriate when the potentiometer 11 is the station selector of a radio which selectively operates in either an AM or FM mode. The po- tententiometer 11 can be used to control diverse other types of known circuits of the kind that respond to control signals in the form of a variable voltage or to digitized control signals which are originated in the form of variable voltage. Referring jointly to FIGS. 3 and 4, the flat panel display 16 may be of any of the known types and may be of known design except as hereinafter described. For purposes of example, FIGS. 3 and 4 depict a flat panel dis- play 16 of the TFEL (thin film electro-luminescent) type. Flat panel displays 16 of this kind are multi-layered and have a flat substrate 19 which may be rigid or flexible and which may variously be formed of glass, ceramic or plastic. The substrate 19 is overlaid, in sequence, by a layer of row busbars 21, a first dielectric layer 22, a phosphor layer 23, a second dielectric layer 24, a layer of column busbars 26, a sealing and passivation layer 27 and a transparent cover plate 28 which may be glass or clear plastic. Row busbars 21 are parallel conductive traces bonded to substrate 19 and extend in an x-coordi- nate direction. Column busbars 26 are parallel conductive traces deposited on the second dielectric layer 24 and extend in a y-coordmate direction. The points at which the row and column busbars 21 and 26 cross each other define an array of image pixels 29 at which the phosphor layer 23 emits light when a voltage difference is applied across the row and column busbars that cross each other at a particular pixel location. Thus any desired image can be produced by applying a voltage difference across the particular row busbars 21 and column busbars 26 that define image pixels 29 at which light needs to be emitted to form the image. Knob 12 is engaged on a base member 31 which is bonded to the front surface 32 of transparent cover pate 28. In an alternate construction the base member may be secured to a second transparent cover plate which is overlaid on the original transparent cover plate of the flat panel display. Referring now to FIGS. 5 and 6 m conjunction, the base member 31 of this example is annular and extends into an opening 33 in the base of the knob 12, the opening being of larger diameter than the base member. The base member has a circular groove 34 which extends around the member within opening 33. A resilient clasp 40 formed of spring steel or the like has a generally ellip- tical configuration and has a first end 35A that is secured to the wall of opening 33. A flattened opposite end 35B of the clasp 40 extends into a slot 36 in the portion of knob 12 that encircles opening 33. The end of base member 31 is beveled and forces a temporary expansion of the clasp 40 as the knob 12 is being forced onto the base member after which the clasp seats in groove 34 and retains the knob on the base member. The knob 12 can be easily removed from the base member 31 by forcing end 35B of clasp 40 inward at slot 36 with the tip of a screw- driver or similar tool. This expands the clasp 40 and forces it out of groove 34. The clasp 40 and groove 34 may be replaced with other structure for engaging the knob on the base member examples of which will be hereinafter described. A trace 37 of electrically resistive material s bonded to the front surface 32 of transparent cover plate 28 within the base member 31 and is of circular configuration except that the resistor trace is discontinuous at one location and thus has spaced apart ends 38, the trace being centered on the axis of rotation 39 of knob 12. A pair of spaced apart electrically conductive traces 41 extend along front surface 32 of cover plate 28 from ends 38 of the resistor trace 37 to a connector 42, shown in FIG. 1, at an edge of the flat panel display 16. Connec- tor 42 provides for connection of the conductive traces 41 to a voltage source 43 shown in FIG. 2B . Referring again to FIGS. 5 and 6, the connection of the ends 38 of resistor trace 37 to a voltage source creates a progressive voltage drop along the trace. To provide a selectable output voltage, a resilient and conductive dual wiper contact 45 extends from knob 12 within the knob opening 33. Contact 45 has a first wiper arm 47 positioned to contact resistor 37 and to travel along the trace as the knob is turned. A second wiper arm 48 of contact 45 extends into contact with a conductive pad 49 which is bonded to the front surface 32 of cover plate 28. Another conductive trace 51 extends along the front surface of cover plate 28, m parallel relationship with traces 41 to connect pad 49 with the previously described connector 42 thereby enabling transmission of an operator selected voltage to the circuit which the potentiometer 11 controls . The conductive traces 41 and 51 which extend along the cover plate 28 within the image display area may be formed of transparent conductive material, such as indium tin oxide, to avoid interference with viewing of the displayed graphics. The circular resistive trace 37 of the potentiometer 11 control device of FIGS. 1 to 6 is discontinuous at one location so that a progressive voltage drop is present along the trace when ends of the trace are connected across a voltage source. The control device does not output an operator selectable signal when the wiper arm 47 is at the location of the discontinuity in the trace. In some usages of control devices of this type it is advantageous if a selectable signal can be produced at any location throughout a full 360° of rotation of the turnable knob 12. FIG. 6A depicts a modification of the circular resistive trace and the associated electrical circuit which effects this result. The control device 11a of FIG. 6A may be similar to the previously described embodiment of the invention except that the circular trace 37a is a continuous annular band of resistive material and the electrical circuit is modified. In particular, one terminal of the DC voltage source 43 is connected to the turnable wiper contact 45 through a resistor 40 and conductive pad 49 and the other terminal of the voltage source is connected directly to the circular trace 37a at a first fixed contact location 50a thereon. The voltage value which is present at a second fixed contact location 50b on trace 37a is transmitted to the microprocessor 55 or other digital data processor through a first analog to digital signal converter 60a and the voltage value at a third fixed contact location 50c is transmitted to the microprocessor through a second analog to digital signal converter 60b. The fixed contact locations 50a, 50b and 50c are preferably at 120° angular intervals around the conductive trace 37a although this is not essential as the microprocessor 55 can be programmed to compensate for a non-uniform angular spacing of the contact locations. Microprocessor 55 is able to determine the angular position of wiper arm 45 at any given time as the voltages at contact locations 50b and 50c vary as the wiper arm is turned and at any given angular position of the wiper arm a combination of the two voltages is present that differs from the combination of voltages that occurs at any other angular position of the wiper arm. In this example, con- tact location 50a is at the 180° position of the wiper arm 45. Thus the 0° position of the wiper arm 45 is midway between contact locations 50b and 50c. As wiper arm 45 rotates through 360° the voltages at locations 50b and 50c vary in accordance with Ohms law as depicted in FIG 6C wherein the solid line depicts the voltage at location 50c and the dashed line depicts the voltage at location 50c. If the value of resistor 40 is small in relation to the total resistance of trace 37a then for practical purposes the voltages at locations 50b and 50c are each equal to two thirds of the output voltage of DC source 43 when wiper arm 45 is at 0°. If arm 45 is at 60°, then location 50c is at the maximum voltage which is the output voltage of the DC source 43. If arm 45 is at 300°, then location 50b exhibits the maximum voltage. If arm 45 is at 180°, then both locations 50b and 50c exhibit no voltage. Referring jointly to FIG. 6A and the flowchart depicted in FIG. 6B, an internal look up table is programmed into microprocessor 55 which stores each of the location signal value pairs that occur in the course of a 360° turning of wiper arm 45 in conjunction with the output signal value which is to be transmitted to a controlled electrical circuit when the particular pair of location signal values is being detected. The microprocessor 55 repetitively obtains the current values of the two loca- tion signals from analog to digital signal converters 60a and 60b, consults the look up table to determine the output signal value which is associated with the current pair of location signal values and transmits that output signal value to the electrical circuit 65 which the control de- vice controls either directly if the controlled circuit responds to digital signals or through a digital to analog signal converter 65a if the controlled circuit requires a varying voltage type of signal. The microprocessor 55 also conditions the flat panel display controller 17, in the previously described manner, to cause display of an indication of the current setting of the control device and/or other graphics at the flat panel display 16. The example depicted in FIG. 6A has two locations 50b and 50c at which output signal voltages are produced and which are situated at the 60° and 300°. These locations can be at other angular positions such as, among other examples, at 90° and 270°. In that variation, the negative side of DC power source 43 may be connected to the trace 37a at the 180° location and also to wiper arm 45 through a sliding contact and resistor 40 may connect the 0° location on the trace with the positive side of the power source. FIG. 7 depicts a modification of the potentiometer Ila in which the base member 31 a is a disk of msulative material and in which the resistor trace 37a is bonded to the base member rather than being directly bonded to the cover plate 28a. Potentiometer Ila may otherwise be similar to the previously described embodiment. FIG. 8 depicts another modification of the potentio- meter 11b in which the conductive traces 41b which connect with the ends of the resistor trace 37b and the trace 51b which connects with wiper contact 45b extend along the back surface 52 of transparent cover plate 28b. This protects the traces 41b, 51b from abrasion. Connector pins 53 extend through the cover plate 28b to connect the traces 41b and 51b in the previously described manner with components that are within the base member 31b. The knob 12b of this embodiment has a tapered side surface 54 that is broadest at its base. The snap engage— ment of knob 12b onto base member 31b is made by forcing the base of the knob into a conforming annular recess 56 in the inner surface of the base member. The dual wiper 45b of this embodiment has a first wiper arm 47b that rides on the resistor trace 37b and a second arm 57 which contacts and travels along a circular trace 58 of conductive material which is coaxial with the resistor trace and which is bonded to the cover plate 28b. One of the connector pins 53b extends through cover plate 28b to connect the circular trace 58 with conductive trace 51b. Referring jointly to FIGS. 4 and 8, disposition of the conductive traces 41b, 51b at the back surface 52 of the transparent cover plate 28b in the above described manner makes it possible to form such traces of non-trans- parent material, such as copper or aluminum for example, without creating any noticeable disruption in images which are viewed through the cover plate. Referring to FIG. 4 in particular, this may be accomplished by positioning the traces 41b and 51b to extend in parallel relationship with the rows of image pixels 29 at locations which are between two rows of pixels. In the modification of the potentiometer 11c which is shown in FIG. 9, the turnable knob 12c does not snap engage on the base member 31c as in the previously described embodiments. The knob 12c simply fits onto a circular base member 31c and is held in place by a turn- able rod 61 which extends along the axis of rotation of the knob. A flanged end 63 of rod 61 extends out of the base of the knob 12c. A set screw 64 extends radially within the knob and engages rod 61. A housing 68 encir- cles and overlaps the flanged end 63 of the rod 61 and is bonded to the face of cover plate 28c in order to hold the knob 12c on base member 31c. The rod 61 can turn freely on the base member 31c but is held in position by the housing 68 and flange. An annular stop 62, formed of a material having a low coefficient of friction, such as Teflon (TM) for example, extends between the peripheral region of the base of knob 12c and cover plate 28c. The stop 62 prevents breakage of the glass cover plate by flange 63 which might otherwise occur if an operator should exert a strong lateral force on the knob 12c. The potentiometer lie is otherwise similar to the previously described embodiment of FIG. 8. The previously described embodiments of the invention each have at least one wiper contact which bears against and travels along a resistor trace. FIG. 10 depicts a form of wiper contact which can prolong the life of the control devices by reducing abrasion of the resistor traces. A small spherical ball 69 formed of electrically conductive material is disposed between each wiper contact 47d or 57d that travels along a resistor trace 37d or conductive trace 58d. Small cylindrical rollers can be substituted for the balls 69. In this example, the wiper contacts 47d and 57d are opposite ends of a conductive leaf spring 71 which bear against the balls 69. Spring 71 is secured at its center to the knob 12d. Wiper contacts 47d and 57d extend into housings 72 in the base of knob 12d which hold the balls 69 at the traces 37d and 58d. The need for housings 72 can be eliminated if, as shown in FIG. 11, the resistive or conductive trace 73 is made sufficiently thick to enable a circular groove 74 to be formed in the trace and the ball 69 is seated m the groove. A similar result is obtained if, as shown m FIG. 12, the trace 73 is a thin lining in a groove 76 formed in a circular base 77 which is bonded to the cover plate 28, the base being formed of msulative material. Automatic cleaning of the trace 73 can be provided for by a brush 75, formed of mildly abrasive flexible fi- bers, which extends from the wiper arm 47d to trace 73 and which bears against the exposed surface of the trace as the arm travels along the trace. The invention is highly advantageous when embodied in a turnable knob type of circuit control device of the kind which does not revert to an initial setting after each full revolution of the knob but instead proceeds to progressively higher settings throughout two or more revolutions of the knob. The invention enables display of progressively higher ranges of output signal values around the perimeter of the knob during successive revolutions of the knob. FIGS. 13, 14 and 15 depict a multi-turn potentiometer of this kind. Referring jointly to FIGS. 13, 14 and 15, the potentiometer lie has an annular base member 31e bonded to the front surface of a flat panel display cover plate 28e and a turnable knob 12e which is snap engaged on the base member in the previously described manner. The knob 12e is typically longer than in the previously described embodiments and has a hollow interior 78 that is open at the base of the knob. A threaded rod 79 within the knob 12e extends along the axis of rotation of the knob and has a pilot projection 81 at one end that seats in a bearing 82 that is bonded to the front surface of cover plate 28e. The other end 83 of rod 79 extends into the material of the knob 12e at the distal end of the knob and is bonded thereto thereby causing the rod to turn with the knob. Base member 31e has a cylindrical sleeve portion 84 which extends up into the interior 78 of knob 12e and which encircles rod 79 in spaced apart relationship there- with. A trace or strip of resistive material 86 s bonded to the inside surface of the sleeve portion 84 and extends in parallel relationship with the rod 79. An internally threaded collar 88 encircles rod 79 and engages the threads of the rod. An arm 89 extends radially from col- lar 88 into a longitudinal slot 91 in sleeve portion 84 of the base member 31 e. This prevents rotation of the collar 88 and causes it to travel along rod 79 when the knob 12e is turned, the direction of travel being determined by the direction of the turning of the knob. Another radially directed arm 92 extends from collar 88 and carries a wiper contact 93 which bears against and travels along the resistive trace 86. Conductors 94 embedded within the sleeve portion 84 of base member 31e extend up from the surface of cover plate 28e to apply differing voltages to the opposite ends of the resistive trace 86. Rod 79 and collar 88 including arm 92 are formed of electrical conductor enabling the output voltage of the potentiometer lie to be picked up by another wiper contact 96 which is bonded to the front surface of cover plate 28e. Provided that the rod 79 and resistive trace 86 are of sufficient length, the potentiometer lie outputs a continuously increasing or continuously decreasing voltage signal as the knob 12e is revolved through a plurality of full revolutions. Referring again to FIG. 1, the flat panel display controller 17 can be conditioned to change the calibration marks 13 each time a full revolution of the knob is completed in order to conform the marks with the current range of output voltages. Any of the previously described potentiometers can be used as a variable resistor if voltage is applied to only one end of the resistive trace. Abrasion over a period of time can adversely affect output signal accuracy in potentiometers which include a moving wiper contact that rubs against a resistor. The function of a potentiometer can be realized without making use of such components. For example, with reference to FIGS. 16 and 17 in conjunction, a radio frequency circuit 97 can be used to sense the setting of a turnable knob 12f and to track angular motion of the knob. The embodiment of FIGS . 16 and 17 again has a turn- able knob 12f snap engaged on an annular base member 31 f which is bonded to the transparent cover plate 28f of a flat panel display 16f within the image display area in the manner previously described. A small integrated circuit board or chip 98 is adhered to the surface of cover plate 28f within the base member 31 f. Chip 98 includes two electrical oscillators 99 and 101, shown in FIG. 18, which may be of any of the known types such as Colpitts oscillators for example. The chip 98 also includes two buffer amplifiers 102 and 103 and two capacitors 104 and 106 which are the capacitive components of the resonant circuits of the oscillators 99 and 101. Referring again to FIGS. 16 and 17, the inductive component of the resonant circuit of each oscillator is one of a pair of inductance coils 107 and 108 which are within the base member 31 f. Coils 107 and 108 are preferably spiral shaped conductive traces bonded to the surface of cover plate 28f. Coils 107 and 108 are at different locations relative to the axis of rotation of knob 12f and are preferably situated at locations which are 90° apart relative to the axis of rotation. A body 109 of electrically conductive metal, which is preferably ferromagnetic material, is se- cured to the base of knob 12f at a location which is offset from the axis of rotation of the knob. Referring jointly to FIGS. 17 and 18, the resonant frequencies of both oscillators 99 and 101 are affected by the presence of the conductive body 109 in close proximity to the coils 107 and 108 and the frequency of each of the oscillators changes as the body 109 becomes closer or further away from the coil 107 or 108 of the oscillator. The frequency shifting effect on the two oscillators 99 and 101 is different at any given time as the two coils 107 and 108 are at different locations along the path of revolution of the body 109. Consequently the ratio of the frequencies of the two oscillators 99 and 101 varies as knob 12f is turned and is different at each angular setting of the knob. This allows a microprocessor 111 to identify the rotational position of knob 12f at any given time and to output a control signal which is a function of the angular position of the knob. Referring to FIG. 18 in particular, the resonant circuit of oscillator 99 is coupled to the input of a counter 112 through buffer amplifier 102, a capacitor 113 and a first conductor 114 which extends between the location of the oscillators and the location of the microprocessor 111. The resonant circuit of the other oscillator 101 is coupled to the input of another counter 116 through the other buffer amplifier 103, another capacitor 117 and a second conductor 118 that extends to the location of the microprocessor 111. Each counter 112 and 116 counts the number of radio frequency cycles that are generated by the associated os- cillator 99 or 101 during successive time periods of fixed duration. The accumulated counts are transferred to shift registers 119 and 121 at the end of each such time period and the counters are reset to a count of zero by the microprocessor 111. FIG. 18A depicts the microprocessor program in flowchart form. Referring jointly to FIGS. 18 and 18A, microprocessor 111 repetitively reads the shift registers 119 and 121, computes the ratio of the frequencies of the two oscillators, and consults a look-up table which is configured within the microprocessor to determine the control signal magnitude that should be outputted at the current angular setting of the turnable knob. The current control signal value is outputted in digital form if the controlled electrical circuit responds to digital control signals or is delivered through a digital to ana- log converter 122 if the controlled circuit responds to a variable voltage type of control signal. Capacitors 113 and 117 are present in the above described circuit to enable transmission of the high frequency signals and also DC operating current on the single pair of conductors 114 and 118 that extend between the location of the oscillators 99 and 101 and the location of the microprocessor 111 which may be at the edge region of the flat panel display. The conductors 114 and 118 each connect with a separate one of the terminals of a DC power supply 124 through a separate one of a pair of inductance coils 126. A capacitor 127 is connected across the DC power supply terminals. This forms a filter which allows DC current to be transmitted to conductors 114 and 118 and which isolates the power supply from the high frequency signals. The other ends of conductors 114 and 118 connect with the DC power inputs of both oscillators 99 and 101 through separate ones of another pair of inductance coils 128 and another capacitor 129 is connected between the positive and negative terminals of the oscillators. This keeps the high frequency signals out of the oscillator power terminals. Capacitors 113 and 117 isolate the buffer amplifiers from the DC current. In FIG. 18 components which are above the dashed line 131 are situated inside the turnable knob and base member assembly of the control device and components which are below the dashed line are in the edge region of the flat panel display. Referring again to FIGS. 16 and 17, the radio frequency circuit which is within the base member 31 f can be adversely influenced by external influences which alter the inductance and/or capacitance of the resonant circuits or create circuit noise. Under some circumstances the operator's fingers can have an effect of this kind. It is preferable that the region inside base member 31 f be isolated from influences of this kind by a lining 132 of Mu metal shielding. The lining 132 substantially encloses the region in which the radio frequency circuit 20 components are situated. The driver circuits within the flat panel display 16f itself can under some circumstances adversely interact with the components on the cover plate 28f. This can be prevented by disposing a thin layer 133 of electrical conductor between the cover plate 28f and the other layers of the flat panel display 16f. Insulation 134 isolates the signal and power conductors 114 and 118 from the conductive layer 133. Referring to FIG. 19, the integrated circuit chip 98 and coils 107 and 108 can alternately be situated at the back surface of the cover plate 28f at a location behind the knob 12f. As shown in FIG. 20, the chip 98 and coils 107 and 108 can also be at the back surface of the flat panel display 16f if the display is sufficiently thin to enable interaction with the body 109 of conductive material within the turnable knob and if there are no solid conductive layers within the display. The control devices of FIGS. 19 and 20 may otherwise be similar to the control device of FIGS. 16 to 18. Referring to FIG. 21, circuit components such as the oscillators 99 and 101, buffer amplifiers 102, 103, inductance coils 128 and capacitors 113 and 117 can also be embedded within the turnable knob 12g. The oscillator coils 107 and 108 in this case are bonded to the knob 12g within an opening 136 m the knob that faces toward the transparent cover plate 28g and are positioned, relative to the axis of rotation of the knob, as previously de- scribed. The circuit components which are within the knob 12g are connected to the DC power and radio frequency signal conductors 114 and 118 that extend along the cover plate 28g through a pair of wiper contacts 137 and 138 which extend from the knob within opening 136. Contact 137 bears against a first beveled conductive ring 139 that is bonded to the surface of the cover plate 28g and which is centered on the axis of rotation of the knob 12g. Contact 138 bears against a second similar ring 141 which overlays ring 139 and which is electrically isolated therefrom by an annular band 142 of insulation. Power and signal conductor 114 is connected to ring 139 and conductor 118 connects to ring 141. The body 109 of conductive material that varies the frequencies of the oscillators 99 and 101 is secured to the second ring 141 by adhesive or other means at a location which is offset from the axis of rotation of the knob 12g. Rings 139 and 141 may be replaced with a pair of annular bands 143 which are bonded to the front surface of cover plate 28g as shown in FIGS. 22 and 22A. The bands 1 3 are centered on the axis of rotation of the knob 12g and are of different diameter in order to be electrically isolated from each other. The wiper contacts 137 and 138 which extend from knob 12g are repositioned to bear against the bands 143. The conductive body 109 is bonded to cover glass 28g between the outermost band 143 and base member 31 g. The control device of FIGS. 22 and 22A may otherwise be similar to the control device of FIG. 21. The rotational position of a knob or the like can be sensed by still other means. Referring jointly to FIGS. 23 and 24, a specialized form of capacitor 144 may be used for this purpose. The control device llh of FIGS. 23 and 24 has a turnable knob 12h snap engaged on an annular base member 31 which is bonded to the front surface of the transparent cover plate 28h of a flat panel display 16h which components may be similar to the corresponding com- ponents of the previously described embodiments. An oscillator 146 circuit board is bonded to the face of cover plate 28h within the base member 31 h. The inductive component of the resonant circuit of oscillator 146 is a spi- ral shaped coil 147 of conductive metal bonded to the surface of cover plate 28h within the base member 31 h. Capacitor 144 is the capacitive component of the resonant circuit and is formed by two discontinuous circular traces 148 and 149 of electrically conductive material that are bonded to the front surface of the cover plate 28h. The traces 148 and 149 are each centered on the axis of rotation of knob 12h and are of different diameter in order to be spaced apart from each other. Each of the traces 148 and 149 has a width which is broadest at one end of the trace and which progressively diminishes along the length of the trace. The broadest ends of both traces 148 and 149 are adjacent to each other. A conductive plate 151 is secured to the base of the turnable knob 12h and spans traces 148 and 149 in spaced apart relationship therewith, the plate having a width in the direction of travel of the plate that is substantially smaller than the length of the traces. Traces 148, 149 and plate 151 jointly form a three plate capacitor the capacitance of which progressively decreases or increases as knob 12h is turned to travel the plate along the traces. This progressively changes the resonant frequency of oscillator 146 and each rotational position of the knob 12h is characterized by a different specific frequency that is used to identify the position and to track rotary motion of the knob 12h. Referring to FIG. 25, a circuit junction 152 between capacitor 144 and inductance coil 147 is coupled to a frequency cycle counter 153 through a buffer amplifier 154 which may be situated at the oscillator circuit board. Referring jointly to FIGS. 25 and 25A, a microprocessor 111, which may be within the marginal region of the flat panel display, cyclically reads the accumulated count in counter 153 and resets the counter in order to detect the current resonant frequency of the oscillator 146. The microprocessor 111 outputs a digital control signal which has a value that changes in response to changes of the frequency caused by changes in the angular setting of the turnable knob. The control signal may be delivered through a digital to analog converter 156 if the controlled electrical circuit is of a type that responds to variable voltage control signals. The microprocessor 111, counter 153 and a DC power supply 157 for the oscillator 146 are situated in the marginal region of the flat panel display. The oscillator 146 of this example is of the harmonic type which has a resonant circuit formed by capacitor 144 and coil 147. The oscillator can also be of the relaxation type in which case coil 147 is replaced with a resistor. In a modified form of the capacitive control device one of the conductive traces of the capacitor 144 is replaced with a relatively thin circular band 155 of electrical conductor of uniform width as shown m FIG. 26. The movable capacitor plate 151 carries a wiper contact 158 which contacts and travels along the band 157. The control device of FIG. 26 may otherwise be similar to the control device of FIGS. 23 to 25. Rotary motion of the control device knob can also be tracked and settings can be identified by photoelectric means. Referring jointly to FIGS. 27, 28 and 29, a control device llj of this kind may have a turnable knob 12j snap engaged on an annular base member 31j at the face of a flat panel display 16j which components may be similar to those of previously described embodiments. The knob 12j has an interior chamber 159 which opens at the base of the knob and a mirror 161 is secured to the knob within the chamber. Mirror 161 is spaced outward from base member 31j and oriented to return light which is received from the region of the base member back towards that region. A layer 162 of opaque material is bonded to the front surface of the transparent cover plate 28j within the base member 31j to prevent entry of external light into chamber 159. This objective can be met without the opaque layer 162 if the flat panel display 16j is conditioned to always present a solid black image at the location of the base member 31j regardless of what is being displayed at other locations . An LED (light emitting diode) 163 or other light source is secured to the center of layer 162 to direct light towards mirror 161. Returned light is detected by two phototransistors 164A and 164B or other light sensors which are secured to layer 162 at opposite sides of LED 163, the photodiodes being equidistant from the axis of rotation of knob 12j. The angular spacing of the phototransistors 164A and 164B from each other, relative to the axis of rotation of the knob, is less than 180°. Light in passage from LED 163 to the phototransistors 164 is modulated by an opaque disk 166 which extends across chamber 159 at a location between the mirror 161 and phototransistors 164. Disk 166 has an opening 167 or transparent region at its center to provide a light path from LED 163 to the mirror. The disk also has an annular array of uniformly spaced apart slots 168 or transparent zones through which reflected light is received by phototransistors 164. Slots 168 are shaped and positioned to create a quadrature code of the kind known to the art which enables a digital data processor to track rotary motion of a knob around a series of angular settings and to output a signal which is indicative of the current setting. In this example there are four slots 168 each of which extends along 45° of arc relative to the axis of rotation and which are separated from each other by opaque regions of the disk 166 of similar arcuate length. Phototransistors 164A and 164B are positioned to create the response pattern which is depicted diagramatically in FIG. 29A and which enables a digital data processor to distin- guish between 16 different positions of the knob 12j. At a first position, shown in FIG. 29, both phototransistors 164A and 164B are illuminated. When the knob is turned clockwise 22.5°. only phototransistor 164B is illuminated. Further turning by the same amount blocks light from both phototransistors 164A and 164B. Upon further turning by the same amount, only phototransistor 164A is illuminated. This pattern is continually repeated as clockwise turning of the knob 12j is continued. Reversed turning of the knob 12 creates a reversed pattern of phototransistor illuminations. Referring now to FIGS. 30 and 30A, each phototransistor 164 is connected between a separate port of a microprocessor 111 and system ground in series with separate resistors 165. This causes the voltage at each such mi- croprocessor port to change from a high status to a low status each time that a pulse of light is received by the phototransistor that is connected to the port. As depicted in FIG. 30A, the microprocessor tracks turning of the knob by counting such changes of status in an additive 10 and subtractive manner. Prior to each incrementing of the count the program executes a direction subroutine, depicted in FIG. 30B, to determine if the knob motion is clockwise or counter clockwise. Referring jointly to FIGS. 29A and 30B. the four possible combinations of the status of the two phototransistors 164A and 164B (sensor A and sensor B) can be represented as 00, 01, 10 and 11. If the status combination changes from 00 to 01 or from 01 to 11 or from 10 to 00 or from 11 to 10 then the knob motion is counter clockwise and the program increments the count in a subtractive manner. Any other change in the status combination is indicative of clockwise knob motion and the count is incremented in an additive manner. Referring again to FIGS. 27 to 29, the resolution of the control device 12j can be increased by providing disk 166 with a greater number of the slots 168. LED 163 is energized by a DC power supply 169 which may be located in the marginal region of the flat panel display . Referring now to FIGS. 32 and 33, LED 163 can be en- circled by an opaque cylindrical light barrier 173 which extends from the opaque layer 162 up into a slot 174 m the light modulating disk 166 to optically isolate the two phototransistors 164 from each other. A similar result can be effected by providing an opaque housing 176 around each phototransistor 164 as is shown in FIGS. 34 and 35. Each housing 176 has a thin slot 177 which faces the light modulating disk 166 to admit pulses of light into the housing The slotted housings 176 have the further beneficial effect of causing the phototransistors 164 to re- ceive light from only one of the slots 168 at any given Only one phototransistor 164, photodiode or the like is needed if the light modulating disk 166 of FIGS. 27 and 29 is replaced with a disk 171 of the form shown in FIG. 31. Instead of slots disk 171 has an annular band 170, centered on the axis of rotation, that is of maximum light transmissivity at one location and which becomes progressively less light transmissive at successive other locations around the band. The output of the single pho- totransistor is then a function of the angular orientation of the knob. The control device of FIGS. 27 to 35 uses an LED 163 and phototransistors 164 which are separate components separately secured to the opaque layer 162. Referring now to FIGS. 36 and 37, the control device of those figures and other control devices to be hereinafter described can be more economically manufactured m large numbers by integrating these components into a single photoelectric chip or wafer 178. For this purpose, an LED 163 and phototransistor 164 are bonded to a flat base 179 and are spaced apart thereon. Both the LED 163 and the phototransistor 164 have one of a pair of lenses 181 bonded thereto, the lenses being formed to focus light from the LED at a point located a predetermined distance away and to focus light which is reflected back from that point at the phototransistor 164. FIGS. 38 and 39 depict another electrical circuit control device Ilk in which a bank of three potentiometers 181, 182 and 183 are secured to the transparent cover plate 28k of a flat panel display 16k within the image display area. The potentiometers 181, 182 and 183 each have a turnable knob, 12k, 121, and 12m respectively, snap engaged on annular base members, 31k, 311 and 31m respec- tively in the previously described manner. The base members 31k, 311 and 31m are spaced apart and each is bonded to the cover plate 28k by an adhesive. Each of the potentiometers 181, 182 and 183 uses one of the above described photoelectric wafers 178 to sense changes in the rota- tional position of the knob of the potentiometer to enable synthesis of a variable control signal for a controlled electrical circuit. The knob 12k of the first potentiometer 181 has an interior chamber 184 and a mirrored disk 186 is bonded is to the knob within the chamber. Disk 186 extends in par- allel relationship with cover plate 28k, is spaced apart from the cover plate and is centered on the axis of rotation of knob 12k. A circular band 187 of light reflecting material is present on the surface of disk 186 that faces cover plate 28k, the band being centered on the axis of rotation of the disk. The density of the reflective material and thus the reflectivity of the material progressively changes from a minimum to a maximum at successive locations around the band, the zone of minimum light re- flection being adjacent to the zone of maximum light reflection. A photoelectric wafer 178 of the previously described kind is embedded m the front surface of cover plate 28k at a location at which the focal point of the light beam that is emitted by the wafer is at the band 187 of light reflective material. Thus light is reflected back to the wafer 178 and the reflected light has an intensity that increases as knob 12k is turned m one direction and decreases when the knob is turned in the opposite direction. This enables detection of the direction of turning of the knob and detection of the current angular orientation of the knob in order to produce a control signal that is indicative of the position of the knob. The electrical circuit which is coupled to wafer 178 for this purpose may be similar to that previously described with reference to FIG. 30A. In the second potentiometer 182 of control device Ilk an opaque mask 188 is disposed between the base member 311 and cover plate 28k so that light which is produced within the flat panel display 16k will not affect the output of the photoelectric wafer 178. The mask 188 has an opening 189 which overlays the wafer 178 to enable passage of light between the wafer and the mirrored disk 186. The lenses of the photoelectric wafer 178 of potentiometer 182 have a shorter focal length than the corresponding compo- nents of the first potentiometer 181. Thus the mirrored disk 186 of the second potentiometer 182 is closer to the wafer 178. Except as described above, the second potentiometer 182 is similar to the first potentiometer 181. In the third potentiometer 183 of control device Ilk, the photoelectric wafer 178 is embedded in the back surface of the transparent cover plate 28k in position to view the mirrored disk 186 through an opaque tube 191 which extends outward from cover plate 28k within the base member 31m and knob 12m. If the disk 186 is beyond the focal point of the lens of wafer 178 a supplementary lens 192 situated at the outer end of tube 191 focuses the light at the disk. If cover plate 28k is formed of glass as in this example, the base member 31m can be unitized with the cover plate by also forming the base member of glass and by bonding it to the cover plate with solder glass . The third potentiometer 183 may otherwise be similar to the first potentiometer 181. Referring to FIGS. 40 and 41 in conjunction, a magnet 193 and a pair of analog Hall effect sensors 194-1 and 194-2 may also be used to sense changes in the angular setting of a turnable knob 12p and thereby enable generation of control signals that vary m response to turning of the knob. The control device lip may be mechanically identical to the radio frequency embodiment previously described with respect to FIGS. 16 and 17 except insofar as the Hall effect sensors are substituted for the two oscillator coils 107 and 108 of the previously described embodiment and the conductive body 109 of the previously described embodiment is replaced with magnet 193. The electrical resistance of each Hall effect sensor 194 decreases as the magnet 193 approaches the sensor during turning of the knob 12p and increases as the magnet recedes from the sensor. Referring to FIG. 42, the first Hall effect sensor 194-1 is connected across the positive and negative terminals of a DC voltage source 196 in series relationship with a first resistor 197 and the second sensor is connected across the terminals in series with a second resistor 198. The first sensor 194-1 and resistor 197 form a first voltage divider in which the voltage at a circuit junction 199 between the sensor and resistor changes when the resistance of the sensor changes. The other sensor 194-2 and the second resistor 198 form a sec- ond voltage divider having a similar circuit junction 201 at which the voltage changes when the resistance of the sensor changes. Referring to FIGS. 40 and 41 in conjunction, the two Hall effect sensors 194 of this example of the invention are positioned at locations which are 90° apart relative to the axis of rotation of knob 12p. The angular spacing of the two sensors may have other values in other examples of the invention. The first sensor 194-1, also identified as sensor I in the drawings, is in this example situated at the 0° angular position of the knob 12p. The second sensor 194-2, also identified as sensor 2 in the drawings, is at the 270° position and thus the output signals of the two sensors are 90° out of phase as may be seen in FIG. 43 wherein curved line 1A represents the varying output sig- nal of the first sensor and curved line 2A represents the output signal of the second sensor. Referring jointly to FIGS. 40 to 43, the two voltage values which are present at circuit junctions 199 and 201 are used to identify the rotational orientation of knob 12p. Each circuit junction 199 and 201 is coupled to a separate port of a microprocessor 111 through a separate one of a pair of buffer amplifiers 202 and a separate one of a pair of analog to digital signal converters 203. The microprocessor 111 converts the output signal 1A of the first sensor 194-1 to a control signal which varies as a function of the angular position of knob 12p . The control signal may be caused to vary in a linear manner or to follow some other desired response curve by establishing an internal lookup table in the microprocessor 111 which con- tains assigned values for the control signal at each angular setting of the knob 12p. The microprocessor 111 replaces the actual signal values that are produced at circuit junction 199 with the assigned values, this method of producing a desired response curve being hereinafter de- scribed in more detail. As depicted in FIG. 43, the output signal 1A of the first sensor 194-1 has a different magnitude at each position of the knob 12p as it is turned from the 0° position to the 180° position. The signal 1A value then returns through the same range of signal values as the knob 12p is turned further in the same direction from the 180° position to the 360° (0°) position. Microprocessor 111 uses the output signal 2A from the second sensor 194-2 to determine if a first output signal 1A of a particular magni- tude represents a position in the 0° to 180° range or a position which is greater than 180°. The second sensor output signal 2A has a particular value, designated as K in FIG. 43, when the knob 12p is at the 180° position. Signal 2A is smaller than K when the knob 12p is be- tween the 0° and 180° positions (at which time magnet 193 is at the right half of FIG. 41) and is greater than K when the knob is in between the 180° and 360° positions (at which time magnet 193 is at the left half of FIG. 41). As depicted in flowchart form in FIG. 42A, the microproc- essor consults a lookup table to translate the current value of signal 1A into an assigned value representing the current angular position of the knob within a range of 0° and 180° which assigned value is designated by letter D in FIG. 42A. If the second sensor signal 2A is greater than K at that time, the microprocessor outputs a control sig- nal equal to 360° minus D. Otherwise, the outputted control signal is equal to D itself. Referring to FIG. 43B, a greater number of the Hall effect sensors may be present in order to increase preci- sion in the tracking of motion of the knob 12p. In th s example three analog Hall effect sensors 194-1, 194-2 and 194-3 are situated at angular intervals of 120° around the axis of rotation of the knob 12p, the location of sensor 194-1 again being the 0° position of the knob 12p, sensor 194-2 being at the 120° position and sensor 194-3 being at the 240° position. The physical structure of the control device llq may otherwise be similar to that of the previously described Hall effect control device 119 of FIGS. 40 and 41. Referring to FIG. 43C, the electrical circuit of the control device llq may also be similar to that previously described with reference to FIG. 42 except insofar as the third sensor 194-3 is connected to the DC voltage source 196 and to microprocessor 111 in a manner similar to the connections of the first two sensors therewith. Thus the third sensor 194-3 is connected across the positive and negative terminals of DC voltage source 196 in series relationship with a third resistor 198-1. A circuit junction 199-1 between the third sensor 194-3 and third resis- tor 198-1 transmits the output signal of the third sensor to the microprocessor 111 through another buffer amplifier 202-1 and another analog to digital signal converter 203- 1. Referring jointly to FIGS. 43D and 41, the microproc- essor 111 is programmed to monitor the strength of the output signals of each of the sensors 194-1, 194-2 and 194-3 which sensors in this example have equal sensitivities. If one of the signals is strongest and the other two are equal to each other than the magnet 193 is at the location of the sensor with the strongest signal. At other times the magnet 193 is between the locations of the two sensors which are producing the strongest signals. The microprocessor 111 detects the exact location between the sources of the two strongest signals by computing the ratio of those two signals which ratio progressively changes as the magnet moves away from one such source and towards the other. The ratio typically does not change in a linear manner. The microprocessor 111 corrects for this by referring to an internal look up table in which each ratio value is correlated with the actual angular position of the knob 12p that produces that ratio value. The microprocessor 111 or other data processor can be programmed to revert to the two sensor mode of operation previously described with reference to FIG. 42A in the event of failure of one of the three sensors 194-1, 194-2 or 194-3. Upon power-up, both the control device lip of FIGS. 40 to 42 and the control device llq of FIG. 43B can detect a change in the setting of the knob that has been made during a period when the power was off. Referring jointly to FIGS. 44 and 45, the control devices which have been previously described have a base member, such as base member 437, which is bonded to the front surface of the transparent cover plate 316 of the flat panel display 317. It can be advantageous if the base member 437 has one or more protrusions 438 which extend into or through the cover plate 316 at conforming openings 439 in the plate. This resists lateral forces on the control devices that might tend to detach them from the display. The protrusions 438 can be formed of metal to provide the hereinbefore described electrical connections to conductors at the back surface of cover plate 316. In instances where more than one control device is to be bonded to the same flat panel display 317, the bases 437 can each have protrusions 438 which are arranged in a different pattern and/or which have different shapes to assure that each control device is bonded to the display 317 at the correct location for the particular control device. Referring to FIG. 46, rotary movement of the knob 441 of a circuit control device which is secured to the face of a flat panel display 317 can be tracked by a magnetic pad 439 of the known design which is bonded to the back surface of the display. The control device 442 of this embodiment may be similar to any of the previously described rotary knob control devices except that the electronic components in the knobs and knob bases are not needed. In the present embodiment a small magnet 443 is embedded in the knob 441 at a location which is offset from the axis of rotation of the knob. Movement of the magnet 443 is sensed by the pad 439 and a signal indicative of the current rotational orientation of the knob 441 is produced by a magnetic pad position signal generator 444 which may be of one of the known circuit configura- tions. The microprocessor 111 outputs a control signal to the controlled circuit 446 in the previously described manner wherein the control signal has a magnitude determined by the rotational orientation of knob 441. The microprocessor 111 also causes the flat panel display con- troller 17 to provide graphics 447 at the flat panel display 317 that are indicative of the current setting of the control device 442. In each of the above described circuit control devices both the knob and base member are situated in front of the supporting panel and electrical components of the devices are contained within the knob and base at the front of the panel. The described knob motion sensing means enable the devices to have a compact configuration that is conducive to the situating of the devices within the image display area of a flat panel display. The con- trol device construction also facilitates maintenance and repairs as all components are accessible at the front of the supporting panel by simply detaching the knob from the base member. Embodiments of the invention in which elec- tπcal components are embedded in or are carried by the knob itself, such as the control device of FIGS. 21 and 22 for example, can be quickly and easily repaired by someone who is unskilled in electrical repairs by simply removing the knob and replacing it with a new one. In the previously described circuit control devices rotary movement of the knob between a series of different settings produces a control signal having a value that progressively changes during the course of the knob movement. In some of these control devices the original con- trol signal value at successive settings of the knob does not change in a linear manner nor follow any of the other response curves, such as a logarithmic curve for example, that are customary in control devices. This effect results from the geometry of the novel knob motion sensing means. The control device of FIGS. 16 to 18, the devices of FIGS. 19 and 20, the device of FIGS. 21 and 22 and the device of FIGS. 40 to 43 are examples of control devices which originally produce a control signal that does not vary in any of the standard manners in response to turning of the knob. As has been heretofore described in connection with the specific embodiments, a microprocessor or other digital signal processor compensates for the irregularity of the original control signal by storing an assigned value in association each initial value of the signal that is produced at successive settings of the control. The control device is conditioned for this by turning the knob to each of its settings and entering the desired assigned value for the control signal at each setting into a look- up table in the signal processor. Thereafter the proces- sor responds to movement of the knob by converting the original signal values to the associated assigned values. The converted signal is then used as the control signal for the controlled circuit. This method of signal processing can be used to impart any desired response curve to successive settings of the control without regard to the actual response curve of the knob movement sensing means. In others of the previously described circuit control devices the response curve can be determined by shaping a particular component to produce the desired curve. For example, the output of the circuit control device of FIGS. 5 and 6 can be linearized by forming the resistive trace 37 to have a uniform degree of electrical resistance atsuccessive locations along its length. The assigned signal value method can also be applied control devices of this kind in order to relax manufacturing tolerances and thereby enable more economical production of such devices. Use of assigned signal values compensates for manufactur- mg irregularities in the shape of resistors or other components . The method can also prolong the useful life of circuit control devices. If changes occur in the control signal at one or more settings of the control over a pe- nod of time due to wear or other causes, the control can be repositioned at those settings and a new assigned value can be entered into the look-up table of the signal processor for each such setting. The method is not limited to use with circuit con- trols which are disposed on the face of an image display device nor to controls of the type which have a turnable knob. It is equally applicable to controls having knobs which move in a different manner such as, for example, knobs which are slid along a linear path as in linear po- tentiometers or faders. As previously described, locating electronic components of a control device m front of the panel which supports the device greatly facilitates maintenance and repair of the control as the components can be accessed sim- ply by detaching the turnable knob from the control rather than by opening a housing and performing operations at the back of the panel. FIGS. 48 to 51 depict another embodiment of the invention which is particularly advantageous in this respect. The control device 448 of FIGS. 48 to 51 again has an annular base member 449 affixed to a transparent overlay plate 451 which is disposed against the face of a flat panel display 452 and a turnable knob 453 is snap fitted on the base member in the previously described manner. The control signal producing means 454 are attached to a removable carrier 456 which is fitted within the base member 449 and which is easily accessed by simply removing the detachable knob 453 from the base member. The carrier 456 of this example is a cup shaped mem- ber having an outside diameter conforming with the inside diameter of the base member 449. Referring to FIGS. 47 and 49 in particular, small protrusions 457 at opposites sides of the carrier 456 snap engage in conforming openings 458 in the inner wall of base member 449. Each such protrusion 457 is located on a deflectable tab 459 portion of the carrier side wall that is formed by slots 461 in the side wall at each side of the protrusion. Thus the protrusions 457 may be disengaged from the openings 458 by finger pressure on the free ends 462 of the tabs 459. The free ends 462 are preferably right angled relative to the other portions of the tabs 459 to facilitate manual squeezing of the tabs. The control signal producing means 454 of this example is a rotary potentiometer of conventional internal design. The turnable shaft 463 of the potentiometer ex- tends nto a conforming passage in knob 453 of the control device 448 and a set screw 464 extends radially within the knob and bears against the shaft to constrain the shaft to turn with the knob. Alternately the shaft 463 may be en- gaged by a screw which extends along the axis of the knob 453 from the center of the face of the knob. The conventional potentiometer 454 of this example may be replaced with any of the knob motion sensing circuits that have hereinbefore been described. Referring to FIGS. 47, 50 and 51 in particular, electrical connection of the potentiometer 454 to signal and power conductors 466 that extend through the bases of base member 449 and overlay plate 451 and along the back of the overlay plate is provided for by spring contacts 467. At the location of each such conductor 466 one spring contact 467 is secured to the base of base member 449 and another is secured to the base of carrier 456 in position to contact and bear against each other when the carrier is fitted into the base member. As best seen in FIG. 51, each such contact 467 has a resilient member 468 of generally U-shaped configuration when in an uncompressed state with one arm of the member being disposed against and secured to a conductive pad 469. A single one of the spring contacts 467 may be used at each conductor 466 if the other spring contact is replaced with a conductive pad. FIG. 52 depicts a modification of the control device in which the spring contacts are replaced with pin connectors 471. Pins 472 which extend from the base of carrier 456a enter conductive sleeves 473, which are embedded in the base of the base member 449, as the carrier is being inserted into the base member. The knob 453 of the embodiment of FIG. 52 does not snap engage on the base member 449a. Referring jointly to FIGS. 52 and 53, an outward protruding annular lip 474 encircles the outer wall of base member 449a. Radially directed passages 476 extend through the side wall of knob 453 at opposite sides of the knob and at locations which cause the passages to be directed toward lip 474 when the knob is engaged on base member 449a. A slidable clamp 477 is disposed in each such passage 476 and has a groove 478 which receives an adjacent portion of the lip 474 when the clamp is pushed into abutment with the knob 453. A set screw 479 within a threaded outer portion of each passage 476 is turnable with a screwdriver or the like to selectively force the clamp 477 into abutment with the knob 453 at which position the clamp holds the knob on base member 449a while allowing rotary motion of the knob. Opposite turning of set screw 479 allows the clamps 477 to retract from the knob 453 when the knob is to be detached from the base member 449a. The portion of the side wall of knob 453 which extends towards the image display device 452a from the location of lip 474 has an inside diameter which is at least equal to the diameter of the lip. This form of engagement of the knob 453 and base member 449a may be substituted for the previously described snap-on engagements in instance where the face of the flat panel display 452a is fragile and the force which is applied to effect a snap-on engagement might create a risk of breakage. Except as described above, the embodiment of FIGS. 51 to 53 may be similar to the embodiment of FIGS. 47 to 50. Referring jointly to FIGS. 54 and 55, electrical connections between a replaceable carrier 501 for electronic components of a rotary knob type of circuit control 502 and conductors 503 which extend along the transparent cover plate 504 of the flat panel display 506 can also be made by spring contacts 507 situated at the side of the carrier and which function to snap engage the carrier in place within the base member 508. The carrier 501 has an indentation 509 in its side which in this example is an annular groove that encircles the carrier. Contacts 507 have curved upper ends which extend into the indentation 509 to clasp the carrier 501 in place after it is inserted into the base member 508. The contacts 501 are formed of resilient conductive material and are tensioned to bear against conductive inserts 511 which are embedded in the outer wall of the carrier 501 at indentation 509. Inserts 511 connect with the electronic components 512 which are within the carrier 501 and which may be of any of the pre- viously described forms. The resilient spring contacts 507 deflect momentarily as the carrier 501 is being entered into the base member 508 and then seat in indentation 509 to hold the carrier in place. The resiliency of the contacts 507 also enables withdrawal of the carrier 501 from base member 508 after the rotatable knob 513 has been disengaged from the base member For purposes of example FIGS. 54 and 55 depict a Hall effect type of control of the kind previously described with reference to FIGS. 40 to 43. Thus a magnet 193 is secured to the rotatable knob 513 of the circuit control 502 at a location adjacent to the carrier 501. Other electronic components 512 of the previously described Hall effect knob position sensing circuit, such as the Hall effect sensors, are within the replaceable carrier 501. This circuit has four of the spring contacts 507. As previously described, other types of knob position sensing circuit may have a different number of such contacts. The spring contacts 501 extend between base member 508 and cover plate 504 and contact conductive pads 520 that are connected to the conductors 503 that extend along the plate to connect the control 502 with a controlled circuit in the previously described manner. The knob 513 of this example is snap engaged on base member 508 by a resilient clasp 40 in the manner previ- ously described with reference to FIGS. 5 and 6. A linear key protrusion 515 on the inner surface of the knob 513 is entered into a conforming groove in the outer wall of base member 508 to assure that the spring contacts 507 are po- sitioned to contact inserts 511. Any of the other previously described forms of engagement of the knob on the base member may also be utilized. Referring to FIG. 56, any of the previously described embodiments of the invention may be adapted to function as a switch in addition to providing a circuit control signal of operator selected magnitude. For purposes of example, the control device 481 of FIG. 54 is of the form which uses a magnet 193 and analog Hall effect sensors 194 to track rotary motion of the knob 482 as previously de- scribed with reference to FIGS. 40 to 43 and in which the knob is held on the base member 483 by a resilient clasp 40 as previously described with reference to FIGS. 5 and 6. To provide switching signals for turning a controlled circuit on and off or for other purposes, the knob 482 of this embodiment has a conductive metal core 484 with the cylindrical side wall of the core being covered by a layer of electrical insulation 486. The base member 483 of the control device 481 and the clasp 40 which holds the knob on the base member are also formed of conductive metal. A conductor 488 extends from the base member through the transparent cover plate 489 to which it is affixed and along the back of the cover plate to connect the conductive core 484 of the knob with the input of a touch sens- mg circuit 487. The touch sensing circuit 487 may be of any of the known forms which produce a switching signal, for such purposes as turning lamps on and off for example, in response to touching of a conductor by an operator. The operator may selectively initiate a switching signal by touching the exposed surface 491 of the conduc- tive knob core 484 at the front of the knob. Insulation 486 prevents unwanted production of switching signals when the operator grasps the sides of the knob 482 in order to turn the knob. The image display devices of the previously described circuit controls are of the form known as flat panel displays wherein orthogonally extending arrays of x busbars and y busbars enable activation of different combinations of image pixels at different times to create changing lm- ages. Some newly developed display of this kind are flexible and can have a curved configuration. The term panel display as used herein and in the appended claims should be understood to refer to displays which have a curved shape as well to displays having a planar configu- ration. Referring to FIG. 57, the image display device 492 which provides changeable graphics 493 in proximity to a turnable knob 494 of any of the previously described circuit control devices can be of the cathode ray tube type commonly used as computer monitors, in television receivers or for other purposes. The knob and base assembly 496 of any of the previously described types is affixed to the screen 497 of the cathode ray tube 492 or to a transparent overlay plate 498 which is superimposed on the screen. The flat panel displays of the previously described embodiments are of the type in which grids of busbars are used to energize particular image pixels in an array of pixels as needed to form a desired image. Referring to FIG. 58, the flat panel displays 516 can be of the known segmented electrode type in which electrical energization of electrodes 517 within the display produces images corresponding to the configuration of the electrodes. Such electrodes can, for example, be configured and positioned to provide changeable calibration marks for a rotary knob 519. As is understood within the art such a display can be used to image any desired numeral from 0 to 9 by selective energizing of electrodes 518 which are electrically isolated from each other and which are arranged in the form of a slanting squared number 8. Circuit control devices using magnets and Hall effect sensors to track the rotational movement of rotary knobs have been previously described with reference to FIGS. 40 to 43D. It is often preferable that control devices of this type be very compact. Under some conditions, it may also be desirable to concentrate the magnetic field at the location of the Hall effect sensors while minimizing field strength at locations outside of the knob and base of the control device as other nearby electronic components may be adversely affected by a strong magnetic field. FIGS. 59 and 60 illustrate a Hall effect utilizing control device 521 which meets these criteria. Referring jointly to FIGS. 59 and 60, the control device 521 of this example has a circular base member 522 which is affixed to the face of a transparent cover plate 523 of an electronic image display screen 524 in the previously described manner. A turnable hollow knob 526 snap engages on the base member 522 in the manner previously described with reference to the control device of FIGS. 5 and 6. Referring again to FIGS. 59 and 60, a pair of stationary Hall effect sensors 527A and 527B are secured to base member 522 within the hollow knob 526 and a permanent magnet 528 is contained within the knob. The electrical circuit and programming of the control device 521 may be similar to that depicted in FIG. 42 and 42A and which has been previously described with reference to the embodiment of FIGS. 40 to 42A. Referring again to FIGS. 59 and 60, a combination of features enables the control device 521 to be very compact. First, the diameter of the knob 526 may be mini- mized as the magnet 528 is at a centered location relative to the rotary axis 529 of the knob and is situated directly over the sensors 527A and 527B. The magnet 528 is preferably disk shaped although it may have other configurations if desired. The sensors 527A and 527B are mutu- ally perpendicular and extend in an orthogonal relationship with the rotary axis 529 and an end of one sensor 527A abuts a middle region of the other sensor 527B. Thus the sensors jointly have a T-shaped configuration. Magnet 528 has north and south magnetic poles N and S which are spaced apart in a direction which is at right angles to the rotary axis 529 of knob 526 as depicted in FIG. 61 which is a diagrammatic side view of the magnet. This causes the magnetic field at the field sensing face of each sensor 527A and 527B to vary as the knob 526 is turned notwithstanding the fact that the magnet 528 remains centered over the sensors as the knob is turned. The magnet 528 may be fixedly embedded in the roof of the knob 526 but is preferably slidable within a conforming cavity 520 in the knob. A thin disk 525 of plastic, such as Teflon (TM) for example, having a low coefficient of friction is situated between the magnet 528 and the sensors 527A and 527B to establish a slight spacing of the magnet from the sensors. A compressible body 530 of resilient material, such as silicone sponge for example, is disposed within cavity 520 at the opposite end of the magnet 528 to urge the magnet against the disk 525. FIG. 62 is a side view and FIG. 63 is a top view of an alternate magnet 528A which has a more complex set of poles N- j , N2, S-| and S2 that is particularly advantageous if there are electrical components in the vicinity of the knob 526 that might be adversely affected by a magnetic field. In particular, there are two north pole regions and two south pole regions within the magnet 528A. A first north pole N- | and a first south pole S- j are in the lower half of the magnet 528A and at opposite sides of the rotary axis 529 of the magnet. The other north pole N2 and south pole S2 are in the top half of the magnet 528A with the other north pole N2 being directly over the south pole S- | and the other south pole S2 being directly over the first north pole - | . This four pole magnet 528A concentrates the magnetic flux in the region which extends around the periphery of the magnet and thereby reduces magnetic field intensity outside of the control device. Referring again to FIGS. 59 and 60, one or more mag- netic field concentrators 531, formed of ferromagnetic material such as iron for example, may be used to distort the field of the magnet 528 in a manner which reduces field intensity at locations outside of the knob 526 and base member 522 and/or which intensifies the field at the location of the sensors 527A and 527B. In this embodiment, the field concentrator 531 is a ferromagnetic disk which is situated between the sensors 527A, 527B and base member 522 in position to attract magnetic flux that would otherwise be more widely dispersed in space. Magnetic field concentrators may be situated at other locations, such as between magnet 528 and the compressible material 530 for example, to reduce magnetic flux intensity at locations outside of the control device. FIGS. 64 and 65 illustrate a modified arrangement of the magnet 528A and sensors 527A and 527B which also enables a very compact construction of the control device 521A. The control device 521A of FIGS. 64 and 65 has a base member 522 affixed to the cover plate 523 of an electronic image display screen 524 and a turnable knob 526A snap engaged thereon which components may be similar to the corresponding components of the embodiment of FIGS. 59 and 60. In the embodiment of FIGS. 64 and 65, the magnet 528A is a rod of circular cross section having one end embedded in the knob 526A and which extends towards base member 522 within the knob along the rotary axis 529 of the knob. Referring to FIGS. 66A and 66B, the -north magnetic pole N of magnet 528A extends along the magnet 528A at one side of the rotary axis 529 and the south pole S extends along the opposite side of the magnet. The Hall effect sensors 527A and 527B are again secured to the base member 522 and in this embodiment are jointly arranged in a V-shaped configuration. The magnetic field sensing faces 533 of the sensors 527A and 527B are directed towards the magnet 528A. In this example the two sensors 527A and 527B extend at right angles to each other although other anglings of one sensor relative to the other are also possible. The right angling is preferred as it produces a 90° phase difference in the output signals of the two sensors as has hereinbefore been de- scribed with reference to FIG. 43. The knob 526A of this example has an outer shell 534 formed of ferromagnetic material. This largely confines the magnetic field of magnet 528A to the interior of the knob and increases the strength of the field at the loca- tion of the sensors 527A and 527B. Except as herein described the control device 521A of FIGS. 64 and 65, including the electrical circuit and programming, may be similar to the corresponding elements of the embodiment of FIGS. 59 and 60. FIGS. 67 and 68 depict still another variation of the Hall effect utilizing control device 521 B which can have a very compact construction. The turnable knob 526, base member 522, magnet 528 and electronic image display screen 524 are all similar to the corresponding components of the embodiment of FIGS. 59 to 63 as previously described. A disk shaped ferromagnetic magnetic flux concentrator 531 is again adhered to the surface of the base member 522 that faces magnet 528. In this variation the two Hall effect sensors 527A and 527B are disposed between the flux concentrator 531 and magnet 528 in a stacked relationship wherein the sensor 527A extends at right angles to sensor 527A. Both sensors 527A and 527B face towards magnet 528. The first sensor 527A is adhered or otherwise affixed to the flux concentrator 531 and the second sensor 527B is adhered to the first sensor. The control device 521 B may otherwise be similar to the previously described control device 521 of FIGS. 60 to 63. The previously described Hall effect utilizing control devices of FIGS. 40 to 43D and FIGS. 59 to 68 have a magnet which moves when the knob of the control device is turned and which thereby causes a change in the output signals of the Hall effect sensors that enables detection of the current rotational orientation of the knob. FIGS. 69, 70 and 71 depict still another Hall effect utilizing control device 521 C in which there is no movement of a magnet relative to a Hall effect sensor. The base member 522 and electronic image display screen 524 of the control device 521 C may be similar to the corresponding components of the previously described control device of FIGS. 59 to 63. The turnable knob 526C of control device 521 C which snap engages on base member 522 may also be similar to the corresponding component of that previously described control device except insofar as there is no magnet situated within the knob and the knob has an internal tubular projection 537 which is centered on the axis of rotation 529 of the knob and which extends towards the base member 522. The projection 537 has a diameter which is smaller than the diameter of the surface 538 of base member 522 that faces the projection. At least the projection 537 portion of the knob 526C is formed of non-magnetic material. First and second Hall effect sensors 527A and 527B extend outward from the peripheral region of base member surface 538 and are at locations which are spaced outward from knob projection 537 and which have an angular spacing of 90° from each other about the rotary axis 529 of the knob. The sensors 527A and 527B face towards the rotary axis 529 and are affixed to the base member 522 by adhesive or other means. First and second magnets 539A and 539B are secured to the base member surface 538 at angular locations relative to rotary axis 529 that correspond to the angular locations of the sensors 527A and 527B about the axis, the magnets being within knob projection 537. The output signals of the Hall effect sensors 527A and 527B are caused to vary in response to turning of the knob 526C by a circular magnetic shield 541 which encircles knob projection 537 and which rotates therewith. Shield 541 is formed of ferromagnetic material and is positioned to extend between the sensors 527A and 527B and the magnets 539A and 539B. The expanse of the shield 541 in a direction parallel to the rotary axis of knob 526C progressively diminishes from a maximum at one end to a minimum at the opposite end. FIG. 71 depicts the shield 541 in a flattened out form as it appears prior to the time that it is formed into its circular shape. Referring again to FIGS. 69 and 70, the shield 541 reduces the strength of the magnetic field at the face of each sensor 527A and 527B to an extent that is dependent on the current rotational orientation of the shield about the rotary axis 529 of the knob. Turning the knob 526C in one direction progressively increases the amount of shielding material that is between each magnet 539A and 539B and its sensor 527A or 527B and turning the knob in the opposite direction progressively decreases the degree of shielding of each sensor. Thus the output signal from each sensor 527A and 527B has a different value at each successive angular orientation of the knob 526C and serves to identify the current setting of the knob and to track rotary motion of the knob. The electrical circuit of the control device 521C for varying a control signal to a controlled circuit in response to operation of the device may be similar to that previously described with reference to FIG. 42 except that the microprocessor 111 of FIG. 42 is conditioned to perform a different set of operations which are shown in flowchart form in FIG. 71A. The control device 521 C of FIGS. 69 and 70 can have only a single sensor 527A and a single magnet 539A if it is not necessary that the control device produce an output signal throughout a full 360° of revolution of the knob 526C. An indeterminate output signal is produced when the abutment of the two ends of the tapered shield 541 is between the sensor and magnet if there is only a single sen- sor and magnet. Use of the two sensors 527A and 527B and two magnets 539A and 539B makes the control device operable throughout a full 360° turning of the knob 526C. FIG. 72 depicts a modification of the circuit control device of FIGS. 69 to 71 wherein the magnetic shield func- tions as a flux concentrator 538 rather than as a shield. The flux concentrator 538 has the same configuration as the previously described magnetic shield except that it is of larger diameter and is adhered to the inner surface 539 of the side wall 541 of the turnable knob 526C. In this modification, the flux concentrator of enlarged diameter encircles both the magnets 539A and 539B and sensors 527A and 527B. The control device 521D of FIG. 72 including the electrical circuit of the device may otherwise be similar to the previously described control device of FIGS. 69 to 71. Referring to FIG. 72, the flux concentrator 538 attracts magnetic flux into the locations of the sensors 527A and 527B that would otherwise be dispersed elsewhere. The degree of this flux concentration at the locations of the sensors 527A and 527B and thus the strength of the sensor output signals is dependent on the 9/08260 rotational position of the knob 526C as there is a different expanse of ferromagnetic material behind each sensor 527A and 527B at each different rotational position of the knob. The previously described embodiments of the invention each have a base member and turnable knob which contain knob position sensing means for producing output signals that identify the current rotational orientation of the knob or which enable identification of the rotational ori- entation by an external circuit. In each of the previously described embodiments, the output signals are delivered to circuits and/or electrical connectors which are situated at the marginal region of the electronic display screen through thin conductors which extend along the transparent cover plate of the display screen within the image display area of the screen. Referring now to FIG. 73, the output signals can be transmitted from the knob 526 and base member 522 assembly to a circuit 542 at the marginal region 543 of the image display screen 524 by means which do not require the presence of output signal conductors within the image display area of the screen. In particular the output signals are transmitted to the marginal region 543 of the display screen in the form of electromagnetic energy signals which in this specific ex- ample are infrared energy signals 544. For purposes of example the control device 521 shown in FIG. 73 is a Hall effect utilizing control device which may be similar to the control device previously described with reference to FIGS. 59 to 63 except as is herein de- scribed. The control device may also be of any of the other previously described forms which variously sense the rotational position of a turnable knob by varying an electrical resistance or capacitance, by radio frequency sensors, by photoelectric means or by other means. The base member 522 of the control device 521 is seated in a conforming opening 546 in the transparent cover plate 523 which overlays the display screen 524. An infrared source 547, such as an infrared light emitting diode for example, is embedded in the base member 522 and is positioned to direct infrared energy 544 to the marginal region 543 of the display screen 524 along a path which is within the transparent cover plate 523 and parallel to the surfaces of the cover plate. The infrared en- ergy 544 is directed towards an infrared detector 548, such as an infrared phototransistor for example, which is embedded in the marginal region 543 of the screen 524. Infrared source 547 is a component of a transmitter circuit 549 that is within the base member 522 and infrared detector 548 is a component of a receiver circuit 551 which is within the marginal region 543 of the screen. Referring now to FIGS. 74 and 74A, the transmitter circuit 549 has a multiplexer 552 controlled by a microprocessor 553. The multiplexer 552 receives the knob po- sition signals which in this particular example are produced by the Hall effect sensors 527A and 527B in the previously described manner and repetitively transmits the current sensor signal values to the microprocessor 553 through an analog to digital converter 554. Microproces- sor 553 delivers the digitized signal values to a serial transmitter 556 which is the driver circuit for the infrared source 547. Thus the infrared source 547 emits sequences of pulses of infrared energy that encode the current sensor signal values in digital form. The infrared detector 548 converts the coded infrared signals back into digital electrical signals and applies them to a signal processing microprocessor 557 through a hysteπsis circuit 558. The signal processing microprocessor 557 is programmed to produce a circuit control sig- nal in the manner hereinbefore described with reference to microprocessor 111 of FIG. 42. In an alternate arrangement, with reference again to FIG. 74, the functions of signal processing microprocessor 557 can be performed by the microprocessor 553 of the transmitter circuit 549. Referring again to FIG. 73, electrical power for operating the electronic components which are within the base member 522 and knob 526 assembly can be transmitted to the base member through thin conductors or transparent conductors which extend from the marginal region 543 of the display screen 524 to the base member along a surface of the transparent cover plate 523 in the manner which has been previously described. Alternately a battery can be provided within the base member 522 and knob 526 assembly. The particular control device 521 which is depicted in FIG. 73 does not require conductors or a battery for this purpose. Power for operating electronic components of the base member 552 and knob 526 assembly is produced by a photovoltaic cell 559 of the kind which converts light to electrical energy. The cell 559 is situated at the sur- face of base member 522 that faces display screen 524 in position to receive light that is produced by the display screen at the location that is overlaid by the cell. The image display screens of the previously described embodiments of the invention have features which require that the knob and base member assembly be at a particular location on the screen that is predetermined at the time of manufacture of the control device. In most of the previously described embodiments, for example, signal and/or power conductors extend along the screen cover plate to the predetermined location of the knob and base member. The embodiment of FIGS. 73 and 74 which uses infrared beams to transmit signal data has an opening in the display cover plate at the predetermined location of the base member into which the base member extends. FIGS. 75 and 76 depict an embodiment of the invention which allows a user to situate the knob 526 and base member 522 at any desired location on the face of the display screen transparent cover plate 523 and to remove or relocate the knob if desired. For purposes of example the control device 521 shown in FIGS. 75 and 76 is again a Hall effect utilizing control device similar to the control device previously described with reference to FIGS. 59 to 63 except as is herein described. Alternately, the control device of FIGS. 75 and 76 may be of any of the other previously described forms which sense the rotational position of a turnable knob by varying an electrical resistance or capacitance, by radio frequency sensors, by photoelectric means or by other means . With reference to FIGS. 75 and 76, the base member 522 of this embodiment is affixed to the transparent cover plate 523 by means which enable selective affixing and detaching of the base member, such means being double sided adhesive tape 561 in this particular example. Other affixing means can used such as, for example, a soluble glue, a suction cup, hook and loop fabric of the type sold under the trademark Velcro or a magnet secured to the base member if the display screen is provided with a back panel formed of ferromagnetic material. Thus the base member 522 and knob 526 assembly can easily be removed from the display screen 524 and, if desired, be reattached at another location 562. Electrical power for operating the internal circuits of the control device 563 is provided by a battery 564 contained within the base member 522. Position signals indicative of the angular orientation of the knob 526 are sent from the knob 526 and base member 522 assembly to the signal processing microprocessor 111 in the marginal region of the display screen or elsewhere by radio frequency transmission. For this purpose a transmitcing circuit 566 which includes a radio transmitter 567 and broadcasting antenna 568 are embedded in the base member 522. A receiving circuit 569 which includes a radio receiver 571 and a receiving antenna 572 are situated in the marginal region 543 of the display screen 524. Referring now to FIG. 77, the transmitting circuit 566 of this example includes a multiplexer 573 which is cycled by a microprocessor 574. Multiplexer 573 repetitively transmits the knob position signals which are pro- duced by the Hall effect sensors 527A and 527B to the microprocessor 574 through an analog to digital converter 576. Microprocessor 574 delivers the digitized signal values to the radio transmitter 567 in serial form and the output of the transmitter is applied to the broadcasting antenna 568. Radio receiver 571 of the receiving circuit 569 outputs the digitized signal values to a serial data detector 577. Detector 577 transmits the signals to the signal processing microprocessor 111 through a binary serial m- terface 578 which is of the RS-232 type in this example. The signal processing microprocessor 111 is programmed to produce a circuit control signal m response to turning of the rotary knob in the manner hereinbefore described with reference to microprocessor 111 of FIG. 42. Referring again to FIGS. 75 and 76, the display screen controller circuit 579 provides calibration marks 581 , labels and/or other changeable graphics adjacent to the location of the knob 526 that pertain to operation of the control device. The location or x-y coordinates of the knob 526 can be supplied to the display screen controller circuit by any of several different procedures. This information can be programmed into the controller circuit 579 after a knob location has been selected or reselected. A cursor can be moved to the location of the knob 526 in order to enter the coordinate information if the display screen 524 is of a type, such as a computer monitor screen for example, that produces an operator movable cursor image and which has pre-existing cursor location detection means. In the present example, the location of knob 526 is detected by infrared light sources 582 and infrared detectors 583 in the manner that is used in touch panel controls, such as in bank automatic teller machines for example, to sense the location at which an operator's finger touches a panel. A column 584 of the infrared sources 582 is contained within the marginal region 543 of the display screen 524 and extends along one side of the screen. Each source 582 of the column 584 is positioned to direct a beam 586 of infrared towards an associated detector 583 of a column 587 of detectors 583 which column extends along the opposite side of the display screen 524 at the marginal region 543 of the screen. A row 588 of the infrared sources 582 and an opposite row 589 of the detectors 583 extend at right angles to the column 584 of sources. Each source 582 of the row 588 directs an infrared beam 589 towards an individual one of the detectors 583 of the row 589 of detectors, the infrared beams 589 being at right angles with the infrared beams 586. The infrared sources 282 and detectors 583 create a rectilinear grid of infrared beams 586, 589 just in front of the display screen 524 and which is parallel to the display screen. Successive ones of the sources 282 are spaced from each other by a distance which is less then the diameter of the base member 522 of the control device which base member is formed of infrared opaque material. Thus placement of the knob 526 and base member 522 at any selected location on the display screen 524 interrupts a unique combination of the intersecting infrared beams 586 and 589. Detectors 583 are connected to a coordinate de- termining circuit 590 of the known form that is used in touch panel controls and which n the present invention is used to transmit signals to the display screen controller circuit 579 that identify the location at which the graphics 581 are to be displayed. While the invention has been described with reference to certain specific embodiments for purposes of example, many variations and modifications are possible and it is not intended to limit the invention except as defined in the following claims. Claims 1. A control device for an electrical circuit which control device has a base member and a turnable knob coupled thereto, the knob being turnable to any of a plurality of different angular orientations to change an electrical condition in the circuit, wherein the improvement comprises: said base member being affixed to an electronic image display device which has an image display area at which instantly changeable images are displayed, said base member and knob being in front of said image display device and wherein at least a portion of said knob overlays the image display area of said image display device. 2. The control device of claim 1 wherein said image display device has a transparent cover plate at said image display area thereof, said base member being affixed to said transparent cover plate. 3. The control device of claim 1 further including a control signal producing circuit, said control signal producing circuit having electronic components which are contained within at least one of said base member and said knob in front of said image display device. 4. The control device of claim 3 wherein said turnable knob is detachable from said base member and wherein at least a portion of said components of said control signal producing circuit are embedded within sa d turnable knob and are detachable from said base member therewith. 5. The control device of claim 3 wherein said turnable knob is detachable from said base member and wherein at least a portion of said components of said control signal producing circuit are attached to a removable and replaceable carrier which is fitted into said base member between said turnable knob and said image display device. 6. The control device of claim 5 wherein said electronic display device has a transparent cover plate to which said base member is affixed, further including a plurality of electrical conductors extending along a surface of said cover plate for connecting said control device to a controlled circuit, said base member having a floor which faces said cover plate and which has a plurality of passages which extend towards said cover plate, said carrier having a plurality of conductive pins which extend into said passages to connect said electronic components with said electrical conductors. 7 . The control device of claim 5 wherein said electronic display device has a transparent cover plate to which said base member is affixed, further including a plurality of electrical conductors extending along a surface of said cover plate for connecting said control device to a controlled circuit and at least a pair of resilient conductive contacts which are connected to separate ones of said electrical conductors, said resilient conductive contacts having first ends which are secured to said base member and second ends configured and positioned to clasp said carrier. 8. The control device of claim 1 wherein said image display device is a panel display of the form having internal x-busbars which extend in orthogonal relationship with y-busbars and which activate different combinations of image pixels at different times to provide changeable images . 9. The control device of claim 1 wherein said image display device is a cathode ray tube form of display device . 10. The control device of claim 1 wherein said image display device has a transparent cover plate at said image display area thereof, said base member being affixed to said transparent cover plate, further including at least a pair of electrical conductors which extend along a surface of said transparent cover plate from said base member to an edge of said image display area for connecting said control device to said electrical circuit, and wiper contact means for electrically connecting said electrical conductors with components which are secured to said turnable knob, said wiper contact means being contained within said turnable knob and said base member in front of said image display device. 11. The apparatus of claim 1 further including an image display device controller coupled to said image display device and which causes said image display device to present indicia at said image display area including indicia which are adjacent to said turnable knob at spaced apart locations around the perimeter thereof. 12. The control device of claim 1 wherein said control device produces an initial sequence of control signal values at successive settings of said turnable knob, further including signal processing means for converting said initial sequence of control signal values to a differing sequence of assigned control signal values to compensate for deviations of said initial sequence of control signal values from a desired sequence of control signal values. 13. The control device of claim 1 wherein said image display device has a transparent cover plate at said image display area thereof and wherein said base member is affixed to said transparent cover plate and wherein at least one opening extends into the transparent cover plate at the location of said circuit control device, said base member having a protrusion which extends into said opening . 14. The apparatus of claim 13 wherein a plurality of said control devices are attached to said transparent cover plate and wherein said transparent cover plate has a plurality of said openings including at least one opening at the location of each of said plurality of control devices, each of said control devices having at least one protrusion which extends into an opening at the location of the control device, said protrusions and openings having a keyed configuration wherein particular ones of said protrusions are fittable only in particular ones of said openings . 15. The apparatus of claim 13 wherein an electrical conductor extends along a surface of said transparent cover plate to connect said control device with a controlled circuit and wherein said protrusion is an electrical connector pin which connects an electronic component of said control device with said electrical conductor. 16. The apparatus of claim 13 wherein said trans- parent cover plate has a control signal conductor extending along a back surface of said cover plate and wherein said opening extends through said transparent cover plate, said protrusion being formed at least in part of electrically conductive material and being electrically connected to said control signal conductor and to a control signal producing circuit within said knob and base member. 17. The apparatus of claim 1 wherein at least a portion of said turnable knob is formed of electrically conductive material, further including a touch sensing circuit electrically connected to said electrically conductive material and which produces a circuit control signal in response to an operator touching said turnable knob . 18. The apparatus of claim 1 wherein said turnable knob turns about an axis of rotation and wherein said base member forms a circular chamber situated between said turnable knob and a front surface of said image display device, further including a circular discontinuous electrical resistance situated in said chamber and which is centered on said axis of rotation, a first wiper contact carried by said turnable knob in position to contact said resistance and to travel therealong as said knob is turned, a second contact secured to said turnable knob and being electrically connected to said first wiper contact, and at least a first and a second conductor extending along said transparent cover plate, said first conductor being connected to said electrical resistance to apply voltage thereto and said second conductor being electrically connected to said second wiper to transmit a selectively variable voltage to an electrical circuit. 19. The apparatus of claim 18 wherein said base member is of annular configuration and wherein said electrical resistance is a trace of resistive material bonded to said front surface of said transparent cover plate. 20. The apparatus of claim 18 wherein said base member is cup shaped and has a base region which is adjacent to said front surface of said image display device, said electrical resistance being a trace of resistive material bonded to base region within said base member . 21. The apparatus of claim 18 wherein an electrically conductive surface extends around said axis of rotation within said chamber in spaced apart relationship with said electrical resistance and wherein said second contact is a wiper contact positioned to contact said conductive surface and to travel along said conductive surface as said knob turns, said second conductor being electrically connected to said conductive surface. 22. The apparatus of claim 1 wherein said turnable knob has an internal chamber, further including an electrical resistance disposed in said chamber and extending in parallel relationship with an axis of rotation of said turnable knob, said resistance being secured to said transparent cover plate, a threaded rod extending along said axis of rotation within said internal chamber and being secured to said turnable knob, said threaded rod being formed of electrically conductive material, an internally threaded wiper sleeve disposed on said threaded rod and which engages the threads of said rod, said internally threaded sleeve having a projection which contacts said electrical resistance, a wiper guide secured to said transparent cover plate and extending into said internal chamber of said knob and being positioned to prevent rotation of said wiper sleeve when said knob is rotated whereby said wiper sleeve travels along said electrical resistance when said knob is turned, at least one first conductor secured to said transparent cover plate and being electrically connected to said electrical resistance to apply voltage thereto and a second conductor secured to said transparent cover plate for receiving an output voltage from said wiper sleeve. 23. The control device of claim 1 further including an electrically conductive body carried by said turnable knob at a location which is offset from an axis of rotation thereof, a pair of spaced apart electrical coils disposed at fixed locations which are offset from said axis of rotation, each of said electrical coils being an inductive component of the resonant circuit of a separate one of a pair of electrical oscillators, and a digital data processor conditioned to detect changes in the ratio of the frequencies of said oscillators produced by movement of said conductive body and to generate an electrical circuit control signal that varies in response to changes of the ratio. 24. The control device of claim 23 wherein said image display device has a transparent cover plate at said image display area thereof, said base member being affixed to said transparent cover plate, and wherein said electrical coils are secured to the back surface of said transparent cover plate. 25. The control device of claim 23 wherein said electrical coils are secured to a circuit board which is situated behind said image display device. 26. The control device of claim 23 wherein said electrical coils have an angular spacing from each other of about ninety degrees relative to said axis of rotation. 27. The control device of claim 23 wherein said electrical oscillators are disposed within said base member. 28. The control device of claim 1 further including an electrically conductive body disposed within said base member at a location which is offset from an axis of rotation of said knob, a pair of spaced apart electrical coils carried by said knob at locations which are offset from said axis of rotation, each of said electrical coils being an inductive component of the resonant circuit of a separate one of a pair of electrical oscillators, and a digital data processor conditioned to detect changes in the ratio of the frequencies of said oscillators produced by movement of said conductive body and to generate an electrical circuit control signal that varies in response to changes of the ratio. 29. The control device of claim 28 wherein the region containing said conductive body and said electrical coils is substantially enclosed by electrically conductive shielding material. 30. The control device of claim 1 wherein said image display device has a transparent cover plate at said image display area thereof, said base member being affixed to said transparent cover plate, further including at least one elongated first capacitor plate disposed between said transparent cover plate and said turnable knob and which curves around an axis of rotation thereof, said first capacitor plate having a width which progressively changes along at least a portion thereof, a second capacitor plate secured to said turnable knob in spaced apart relationship to said first capacitor plate in position to be traveled along said first capacitor plate as said knob is turned, said second capacitor plate being sized to extend along only a portion of the first capacitor plate, said first and second capacitor plates being part of the resonant circuit of an electrical oscillator, and a frequency detector coupled to said oscillator which outputs a circuit control signal which changes in response to changes of the frequency that is detected by the frequency detector. 31. The control device of claim 30 wherein said first capacitor plate is a trace of electrically conductive material bonded to said front surface of said transparent cover plate. 32. The control device of claim 30 wherein said first capacitor plate is a trace of electrically conductive material bonded to said base member. 33. The control device of claim 30 further including a curved band of electrical conductor extending alongside said first capacitor plate in spaced apart relationship therewith and being electrically connected to said oscillator, said second capacitor having a wiper contact positioned to travel along said band of electrical conductor as said knob is turned. 34. The control device of claim 30 further including a third capacitor plate extending in the plane of said first capacitor plate in spaced apart relationship therewith and which also curves around said axis of rotation of said turnable knob, said third capacitor plate having a width which progressively changes along at least a portion of thereof and wherein said third capacitor plate becomes progressively wider in the same direction that said first capacitor plate becomes progressively wider, said first and third capacitor plates being electrically connected to said oscillator to form the capacitive component of the resonant circuit thereof, and wherein said second capacitor plate faces both said first capacitor plate and said third capacitor plate in spaced apart relationship therewith. 35. The control device of claim 1 further including a light source and a photoelectric light detector which are contained within said base member and turnable knob, said photoelectric light detector being positioned to detect light which travels from said light source to said photoelectric light detector along a light path, and means for altering a characteristic of the detected light in response to turning of said turnable knob to enable detection of the angular orientation of said turnable knob. 36. The apparatus of claim 35 wherein said light source directs light towards a surface of said turnable knob and wherein said means for altering a characteristic of the detected light includes a mirror disposed on said surface in position to reflect light from said light source to said photoelectric light detector, said mirror having a degree of reflectivity that progressively changes at successive locations along a circular path on the mirror that is centered on the axis of rotation of the turnable knob. 37. The apparatus of claim 35 wherein said light source directs light towards a surface of said turnable knob, further including a mirror disposed on said surface in position to reflect light from said light source to said photoelectric light detector, and wherein said means for altering a characteristic of the detected light includes a light modulating disk having a degree of light transmissivity that progressively changes at successive locations along a circular path on the disk that is centered on the axis of rotation of the turnable knob and which is intersected by said light path. 38. The apparatus of claim 35 having a pair of said photoelectric light detectors and wherein said light source directs light towards a surface of said turnable knob, further including a mirror disposed on said surface in position to reflect light from said light source to each of said photoelectric light detectors, and wherein said means for altering a characteristic of the detected light includes a light modulating disk having opaque areas that alternate with light transmissive areas along a circular path on the disk that is centered on the axis of rotation of the turnable knob and which is intersected by said light path, said opaque areas and light transmissive areas being configured to transmit light to both of said photoelectric light detectors at a first angular setting of said turnable knob and to transmit light only to a first of said photoelectric light detectors at the following angular setting of said turnable knob and to transmit light to neither of said photoelectric light detectors at the next angular setting of said turnable knob and to transmit light only to a second of said photoelectric light detectors at the following setting of said turnable knob as said knob is turned in one angular direction. 39. The apparatus of claim 35 further including an opaque light barrier extending between said light source and said photoelectric light detector. 40. The apparatus of claim 35 further including image display device controller means for causing said image display device to present an image of a dark area at the region of said image display area that is behind said light source and said photoelectric light detector. 41. The apparatus of claim 35 wherein said base member is affixed to a front surface of a transparent cover plate of said image display device and wherein said light source and said photoelectric light detector are situated at a back surface of said transparent cover plate. 42. The control device of claim I wherein said image display device has a transparent cover plate at said image display area thereof, said base member being affixed to said transparent cover plate, further including a magnet secured to said turnable knob at a location which is offset from an axis of rotation thereof, at least a first Hall effect sensor and a second Hall effect sensor disposed between said turnable knob and said transparent cover plate which Hall effect sensors are spaced apart from each other, and a signal processor electrically coupled to said Hall effect sensors and being conditioned to produce a circuit control signal that varies response to variations of the resistances of said Hall effect sen- sors caused by rotary motion of said turnable knob and magnet . 43. The control device of claim 42 further including a third Hall effect sensor disposed between said turnable knob and said transparent cover plate and being electπ- 9/08260 cally coupled to said signal processor, said third Hall effect sensor being offset from said axis of rotation of said turnable knob and being spaced apart from said first and second Hall effect sensors. 44. The control device of claim 1 further including a magnet secured to said turnable knob at a location which is offset from an axis of rotation thereof and a magnetic pad which generates an output signal in response to rotary movement of the magnet, said magnetic pad being behind said image display device. 45. The apparatus of claim 1 wherein said turnable knob has an opening into which said base member extends, said base member having a circumferential groove situated within said opening and said turnable knob having a slot in a side wall thereof which bounds said opening, further including an elongate loop of resilient spring material disposed in said opening, said loop having a first end which is secured to said side wall of said turnable knob and a second end which extends into said slot, said loop being proportioned to snap engage in said groove as said turnable knob is forced into said base member. 46. The apparatus of claim 1 wherein one of said base member and said turnable knob has an opening into which a peripheral portion of the other of said base member and said knob is fitted, a portion of said opening being narrower than said peripheral portion whereby said turnable knob snap engages on said base member. 47. The apparatus of claim 1 wherein said turnable knob has a base with a circular opening therein that is centered on an axis of rotation of said turnable knob and wherein said base member has an annular peripheral portion 99/08260 which is seated in said opening and which has a diameter conforming to the diameter of said opening, further including a post extending along said axis of rotation within said turnable knob and having a flanged end which extends towards said transparent cover plate, and a socket engaging said flanged end of said post and being secured to said transparent cover plate. 48. The apparatus of claim 1 wherein said image display device is a panel display having segmented electrodes which are energized to produce changeable images. 49. A control device for providing operator selected control signals to a controlled electrical circuit comprising: an image display device having a transparent cover plate with an image display area thereat at which information pertaining to operation of the control device is displayed, a base member affixed to said cover plate of said image display device at a location which is within said image display area, a knob engaged on said base member and being manually turnable about an axis of rotation, and a control signal producing circuit which produces a signal indicative of the angular orientation of said knob about said axis of rotation, said control signal producing circuit having components which are contained within said knob and base member. 50. A method of processing control signals produced by an electrical circuit control device of the type in which manual movement of a first component along a series of different settings causes a second component to output a circuit control signal that progressively changes during the course of the movement to identify the successive settings of the first component, comprising the steps of: detecting and storing the original value of said control signal at each of a series of different settings of said first component, designating an assigned value for the control signal at each of said series of different settings of said first component which assigned values may differ from the original values, storing each assigned value in association with the corresponding original value, thereafter responding to movement of said first component by converting detected original values of said control signal to the associated assigned values. 51. The method of claim 50 including the further steps of correcting for the effects of wear within said electrical circuit control device by: repositioning said first component at each of a series of said different settings thereof, detecting changed original values at any of said different settings of said series, changing the original value that is stored in association with the assigned value at each setting of said first component at which a changed original value s detected. 52. The control device of claim 1 further including at least one magnet secured to one of said base member and said knob, first and second Hall effect sensors secured to the other of said base member and said knob and being at locations which are within the magnetic field of said magnet, and a signal processor electrically coupled to said Hall effect sensors and being conditioned to produce an electrical circuit control signal that varies in response to variations of the resistances of said Hall effect sensors caused by turning of said knob. 53. The control device of claim 52 wherein said knob turns about an axis of rotation thereof and wherein said magnet is positioned to be intersected by said axis of rotation with at least one of the magnetic poles of the magnet being offset from the axis of rotation. 54. The control device of claim 53 wherein said magnet has a circular disk shape and is substantially centereα on said axis of rotation. 55. The control device of claim 53 wherein said magnet has a north magnetic pole that is offset from said axis of rotation in a first direction and a south magnetic pole that is offset from said axis of rotation in a second opposite direction. 56. The control device of claim 53 wherein said magnet has first and second north magnetic poles and first and second south magnetic poles and wherein said magnetic poles are each offset from said axis of rotation of said knob, said first north magnetic pole and said first south magnetic pole being at one side of said axis of rotation and said second north magnetic pole and second south magnetic pole being at an opposite side of said axis of rotation, said first north magnetic pole and first south magnetic pole being closer to said Hall effect sensors than said second north magnetic pole and said second south magnetic pole. 57. The control device of claim 53 wherein said Hall effect sensors have magnetic field sensitive faces which extend parallel relationship with said axis of rotation of said knob and wherein said first Hall effect sensor extends at right angles to said second Hall effect sensor and has an end which faces a mid-section of said second Hall effect sensor, said Hall effect sensors being situated between said magnet and said base member of said control device. 58. The control device of claim 53 wherein said first Hall effect sensor overlays said second Hall effect sensor, said Hall effect sensors having magnetic field sensitive faces that are parallel to each other and which extend at right angles to said axis of rotation of said knob in angled relationship to each other. 59. The control device of claim 53 wherein said Hall effect sensors have magnetic field sensitive faces which face towards said axis of rotation of said knob and which are angled relative to each other and in parallel relationship with said axis of rotation of said knob and wherein said magnet extends along said axis of rotation and has north and south magnetic poles that are at opposite sides of said axis of rotation. 60. The control device of claim 52 further including a magnetic flux concentrating body of ferromagnetic material disposed within the knob and base membere member and being positioned to concentrate magnetic flux therein. 61. The control device of claim 52 wherein said knob is formed at least in part of ferromagnetic material. 62. The control device of claim 52 wherein said knob has an interior chamber into which said base member extends, said magnet being seated within said knob, said Hall effect sensors being within said interior chamber of said knob and being secured to said base member. 63. The control device of claim 1 wherein said knob turns about an axis of rotation that intersects said base member, further including at least one magnet and at least one Hall effect sensor secured to one of said knob and said base member, said magnet and Hall effect sensor being spaced apart in a radial direction relative to said axis of rotation of said knob, an annular body of ferromagnetic material secured to the other of said knob and said base member, said annular body of ferromagnetic material being centered on said axis of rotation and being positioned to extend within the magnetic field of said magnet, wherein the expanse of said body of ferromagnetic material in a direction parallel to said axis of rotation progressively changes at successive locations around the circumference of said body of ferromagnetic material. 64. The control device of claim 63 wherein successive segments of said annular body of ferromagnetic material pass between said magnet and said Hall effect sensor as said knob is turned. 65. The control device of claim 63 wherein one of said magnet and said Hall effect sensor has a first spacing from said axis of rotation and the other of said magnet and said Hall effect sensor has a second spacing from said axis of rotation that is greater than said first spacing, wherein said annular body of ferromagnetic material has a radial dimension that exceeds said first spacing and is smaller than said second spacing whereby successive portions of said annular body of ferromagnetic material pass between said magnet and said Hall effect sensor as said knob is turned. 66. The control device of claim 63 wherein said Hall effect sensor has a first spacing from said axis of rotation and said magnet and said Hall effect sensor has a second spacing from said axis of rotation, and wherein said annular body of ferromagnetic material has a radial dimension that exceeds both said first spacing and said second spacing. 67. The control device of claim 1 wherein said knob and base member form a circuit housing and wherein said image display device has a transparent cover plate at said image display area and wherein said base member of said control device extends into an opening in said transparent cover plate, further including a position signal producing circuit disposed within said circuit housing and which produces a signal which varies in response to turning of said knob, a light source producing a light beam which encodes said signal, said light source being secured to said base member in position to direct said light beam into said transparent cover plate along a light path which is within said transparent cover plate and which extends from said opening to a location at an edge of said transparent cover plate, a light detector disposed at a marginal region of said image display device and being positioned to view said location and to receive light which has traveled along said light path and which outputs said signal in electrical form. 68. The control device of claim 1 wherein said knob and base member form a circuit housing, further including a signal producing circuit disposed within said circuit housing and which produces an electrical signal that varies in response to turning of said knob, a radio transmitter disposed in said housing in electrical connection with said signal producing circuit and which broadcasts said signal -in radio frequency form, a signal receiving circuit situated away from said image display area of said image display device which signal receiving circuit includes a radio receiver which converts the radio frequency signal back into an electrical signal. 69. The control device of claim 68 wherein said image display device has a marginal region which adjoins said image display area, said radio receiver being situated within said marginal region of said image display device. 70. The control device of claim 68 further including means for attaching said base member to said image display device at any selected location within said image display area thereof. 71. The control device of claim 70 wherein said means for attaching said base member to said image display device enables detachment of said base member and knob therefrom. 72. The control device of claim 68 wherein said image display device includes a display controller conditioned to cause display of an image that pertains of operation of said control device, further including position sensing means for producing a position signal which identifies the location of said base member and knob on said screen and wherein said display controller positions said image within said image display area in response to said position signal. 73. The control device of claim 72 wherein said image display area has first and second opposite edges and third and fourth opposite edges which extend between said first and second opposite edges and wherein said position sensing means includes a row of light detectors extending along said first edge and another row of light sources extending along said third edge, said position sensing means further including a row of light sources extending along said second edge and being positioned to direct light to the detectors at said first edge along a plurality of parallel light paths and another row of light sources extending along said fourth edge and being positioned to direct light to the detectors at said third edge along another plurality of parallel light paths, and wherein said position sensing means detects the location of said base member and knob by monitoring said light detectors to detect blockage of particular ones of said light paths. 74. The control device of claim 1 further including a continuous annular band of electrically resistive material situated between said turnable knob and said image display device, a wiper contact secured to said turnable knob and wherein a portion of said wiper contact bears against said continuous annular band of electrically resistive material and wherein said portion of said wiper contact travels along said annular band of electrically resistive material in contact therewith as said knob is turned, an electrical voltage source having a positive terminal and a negative terminal, one of said terminals being electrically connected to said wiper contact and the other of said terminals being electrically connected to said annular band of electrically resistive material at a first fixed location thereon, a first analog to digital signal converter having an input connected to said annular band of electrically resistive material at a second fixed location thereon which second fixed location is spaced apart from said first fixed location, a second analog to digital signal converter having an input connected to said annular band of electrically resistive material at a third fixed location thereon which third fixed location is spaced apart from both of said first fixed location and said second fixed location, wherein said first and second analog to digital signal converters output a pair of location signals one of which has a value determined by the current voltage at said second fixed location and the other having a value determined by the current voltage at said third fixed location, and a digital data processor which repetitively receives the pairs of location signals and which stores a plurality of different output signal values each being associated with a different pair of location signal values and which outputs a control signal having the value that is associated with the current pair of location signal values. 75. The control device of claim 74 wherein said first, second and third fixed locations are situated at 120° angular intervals along said annular band of electrically resistive material. 76. A control device for an electrical circuit which control device comprises: a base member and a movable member attached thereto which movable member is manipulated by an operator to change a setting of said control device, said base member being affixed to an electronic image display screen at a location which is at least partially within an image display area thereof, a display screen controller coupled to said image display screen and being conditioned to cause display of an image which conveys information pertinent to operation of said control device, a signal producing circuit situated at said location and which requires input of an electrical voltage, and a photovoltaic cell for converting light to electrical energy, said photovoltaic cell being disposed within said base member and being connected to said signal producing circuit to supply said electrical voltage thereto, said photovoltaic cell being positioned to receive light which is produced by said image display screen.

Claims: AMENDED CLAIMS [received by the International Bureau on 03 December 1998 (03.12.98); original claim 1 amended ; remaining claims unchanged (1 page)] 1. A control device for an electrical circuit which control device has a base member and a turnable knob coupled thereto, the knob being turnable to any of a plurality of different angular orientations to change an electrical condition in the circuit, wherein the improvement comprises: said base member being affixed to an electronic image display device which has an image display area at which changeable images are displayed, said base member and knob being in front of said image display device and wherein at least a portion of said knob overlays the image display area of said image display device. 2. The control device of claim 1 wherein said image display device has a transparent cover plate at said image display area thereof, said base member being affixed to said transparent cover plate. 3. The control device of claim 1 further including a control signal producing circuit, said control signal producing circuit having electronic components which are contained within at least one of said base member and said knob in front of said image display device. 4. The control device of claim 3 wherein said turnable knob is detachable from said base member and wherein at least a portion of said components of said control signal producing circuit are embedded within said turnable knob and are detachable from said base member therewith.

Inventors: JAEGER DENNY, TWAIN KENNETH M, JAEGER, DENNY, TWAIN, KENNETH, M.
Applicants: INTERTACTILE TECH CORP, INTERTACTILE TECHNOLOGIES CORPORATION
Application year: 1998
Main classification: G09G5
IPC codes: G09G5, H01H9