Abstract:
An optical-interrupter provides a mechanically integrated electric light source and electric light sensor positioned across a gap to transmit a light beam across the gap that may be interrupted with an opaque vane. The optical-interrupter uses conventional LEDs for both the light source and a light receiver. An integrated circuit comparator may be used to provide an adjustable threshold for the determination of whether the light beam is blocked.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application 61/230,922 filed Aug. 3, 2009 and hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to optical sensors and, in particular, to optical interruption sensors that may detect blockage of a beam of light. 
       BACKGROUND OF THE INVENTION 
       [0003]    Optical interruption sensors, or “photo-interrupters”, are well known in the art and normally provide an opposed light transmitter, typically a light emitting diode (“LED”), and a light receiver (typically a phototransistor). The light transmitter and receiver may be placed in opposition as supported within a housing. The housing provides windows to permit a beam of light to pass from the light transmitter across a slot to the light receiver. 
         [0004]    A light blocking vane, for example, a spoke on a rotating wheel or the like, may move into and out of the slot to block or permit passage of the light beam. In this way, detection of the light beam at the light receiver can be used to detect relative movement of the vane with respect to the housing. When the vane is on a rotating wheel, for example, interruption of the light beam detected at the light receiver provides an indication of rotational speed of the wheel. 
         [0005]    The optical-interrupter may include an internal biasing circuit for controlling the brightness of the light transmitter and a detector circuit providing an electrical output (typically a binary signal) indicating whether the light beam is blocked or unblocked based on a pre-established threshold applied to an electrical signal from the light receiver. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides an extremely low cost photo-interrupter in which the light transmitter and light receiver are both light emitting diodes. The use of two LEDs provides lower-cost than an LED and phototransistor pair both because of the economy of scale of purchasing similar parts and because of the lower cost of LEDs with respect to photo transistors. 
         [0007]    Specifically, the present invention provides a photosensor-photodetector assembly providing a support defining an unobstructed light path through a gap from a first to a second position, and a solid-state light transmitter and solid-state light receiver attached to the support in the first and second positions so that light from the solid-state light transmitter may be received by the solid-state light receiver. The gap is sized to permit an optical blocking element to pass into and out of the gap to block the light from the solid-state light transmitter to the solid-state light receiver and the solid-state light transmitter and solid-state light receiver are both light emitting diodes. 
         [0008]    It is thus a feature of at least one embodiment of the invention to reduce the cost of this standard component to permit its use in a wider variety of products. 
         [0009]    The light emitting diodes may be selected to produce light of identical light frequency when operated as light emitters. 
         [0010]    It is thus a feature of at least one embodiment of the invention to provide a natural matching between the band gaps of the two devices for improved conversion efficiencies. 
         [0011]    The light emitting diodes may, in one embodiment, emit visible red light. Other colors may also be used, including infrared and other visible colors. 
         [0012]    It is thus a feature of at least one embodiment of the invention to make use of a common LED type having low cost and high efficiency. 
         [0013]    The light emitting diodes may be electrically, optically, and physically identical. 
         [0014]    It is thus a feature of at least one embodiment of the invention to permit economies of scale in part purchases and to simplify stocking of parts for manufacture. 
         [0015]    The photosensor-photodetector assembly may further include biasing circuitry connected to the solid-state light transmitter to provide a predetermined light output from the solid-state light transmitter. Alternatively or in addition, the photosensor-photodetector assembly may further include detection circuitry connected to the solid-state light receiver providing a two-state output having a first state when the solid-state light receiver receives a first intensity of light provided by unobstructed light from the solid-state light transmitter as biased by the biasing circuitry and having a second state when the solid-state light receiver receives a second intensity of light less than the first intensity of light. 
         [0016]    It is thus a feature of at least one embodiment of the invention to simplify the assembly of the device into products. 
         [0017]    The detection circuitry may include a comparator and an electrical reference, wherein the comparator receives at one input an electrical signal from the solid-state light receiver having first and second values for the first and second intensities of light the comparator, and receives at another input a reference output value from the electrical reference between the first and second values. 
         [0018]    It is thus a feature of at least one embodiment of the invention to permit flexible adjustment of the switching threshold of the device in a way that is internal and invisible to the user and thus that facilitates the use with LEDs having less precisely characterized optical characteristics. A given LED badge may be tested and the threshold set eliminating the need for precisely characterized parts. 
         [0019]    The electrical reference and the biasing circuit may provide the predetermined light output and the reference output value, respectively, as a function of a supply voltage to the photosensor-photodetector assembly. 
         [0020]    It is thus a feature of at least one embodiment of the invention to permit operation at a variety of operating voltages without precise voltage regulation. 
         [0021]    The comparator may be an integrated circuit differential amplifier. 
         [0022]    It is thus a feature of at least one embodiment of the invention to use an operational amplifier to provide for well-defined and robust switching from an LED used as a light detector. The present inventors have determined that the cost savings on the detector permit the use of this more sophisticated circuitry. 
         [0023]    The comparator provides an open collector output. 
         [0024]    It is thus a feature of at least one embodiment of the invention to permit use of an operational amplifier with a single-ended, low voltage power supply appropriate for an optical-interrupter application. 
         [0025]    The support may be a flexible printed circuit board held within a housing bending the flexible printed circuit board into a U-shape with the first and second Positions located at ends of the U-shape. 
         [0026]    It is thus a feature of at least one embodiment of the invention to provide a simple method of incorporating the light transmitter and light receiver together with other circuitry in a low-cost package. 
         [0027]    Alternatively, the support may be a rigid planar printed circuit board cut into a U-shape with the first and second positions located at ends of the U-shape. 
         [0028]    It is thus a feature of at least one embodiment of the invention to permit the use of low-cost rigid printed circuit board materials. 
         [0029]    The light emitting diodes may provide a maximum light emitting direction parallel to a mounting surface of the light emitting diode abutting the support surface. 
         [0030]    It is thus a feature of at least one embodiment of the invention to simplify the integration of the LEDs into a rigid printed circuit board material support. 
         [0031]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a perspective view, in phantom, of a photo-interrupter of the present invention showing attachment of complementary surface mount LEDs to a flexible circuit board within a housing of the photo-interrupter; 
           [0033]      FIG. 2  is a simplified diagram of the transmitter and receiver of the present invention as may detect an interrupting vane; 
           [0034]      FIG. 3  is a schematic of a circuit used in the photo-interrupter of the present invention processing the signal from an LED light receiver to provide a switched output signal; and 
           [0035]      FIG. 4  is a perspective view in phantom of an alternative housing for the optical-interrupter. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]    Referring now to  FIG. 1 , an optical-interrupter  10  per the present invention may, in one embodiment, provide a housing  12  having two upwardly extending turrets  14  and  16  opposed across a slot  18 . The housing  12  may be constructed at least on part of a transparent thermoplastic material so that light may pass through the opposed facing walls of the turrets  14  and  16 . 
         [0037]    Inside the housing  12 , a flexible printed circuit board  20  having a generally T-shaped outline includes wing portions  22 , the center of which may fit beneath the slot  18  with the wing portions  22  bent upward so that ends of the wing portions  22  pass into the turrets  14  and  16  respectively. 
         [0038]    These ends of the wing portions  22  may support surface-mount LEDs  26  and  27  respectively on their facing surfaces. Internal structure of the turrets  16  and  14  guides the wing portions  22  so that the LEDs  26  and  27  are aligned in facing configuration opposed across the slot  18  and along optical axis  30 . As so positioned, light may pass from LED  26  along the optical axis  30  to be received by LED  27  with an axis of highest intensity light emission from LED  26  aligned with the corresponding axis of LED  27 . 
         [0039]    Electrical traces of the printed circuit board  20  connect the LEDs  26  and  27  to various components  32  on the surface of the printed circuit board  20  as will be described below. Terminal pins  34  attached to the printed circuit board  20  permit the connection of the optical-interrupter  10  to other devices, for example a home appliance where mechanical motion must be monitored, for example, to determine the rotation of the spin basket in a washing machine. The terminal pins  34  permit electrical communication between the components  32  within the optical-interrupter  10  and corresponding circuitry of the external device. For this purpose, the terminal pins  34  pass through the housing  12  to be received by a connector (not shown) leading to the external device incorporating the optical-interrupter  10 . 
         [0040]    The housing  12  may include mounting holes  35  or the like for attaching the housing  12  to the device with which the optical-interrupter  10  will be used. 
         [0041]    Referring now to  FIG. 2 , LED  26  may be configured as a light transmitter, emitting light  36  along axis  30  while LED  27  may be configured as a light receiver, receiving and detecting light  36  along axis  30 . A vane  40  may periodically pass into slot  18  across axis  30  to block light  36 , and out of slot  18  away from axis  30  to permit passage of the light  36  thereby changing the amount of light received and detected by the LED  27 . 
         [0042]    Generally, LEDs differ from other photo detectors such as photo diodes and phototransistors by their construction and, often, by the materials used in the LEDs. LEDs may use direct band gap semiconductor materials (as opposed to indirect band gap semiconductor materials) in forming a PN junction. Typically the N-doped material of the PN junction is attached to an opaque support, being either or both of a heatsink or electrical conductor, and light is emitted from the P-doped material. The PN junction is normally encased within a light transparent material such as a plastic that is either clear or tinted to match the color of the emitted light. The support for the PN junction of the LEDs  26  and  27  may be associated with a reflector element. 
         [0043]    In this embodiment, the LEDs  26  and  27  are surface mount devices and the emission or reception direction of the LEDs  26  and  27  along optical axis  30  is generally perpendicular to the mounting surface of the flexible printed circuit board  20  underneath the LEDs  26  and  27 . 
         [0044]    Both LEDs  26  and  27  may be identical electrically, mechanically, and optically (for example, being the same manufacturer part number). In one embodiment, the LEDs  26  and  27  will be designed to produce a red light of the same frequency (e.g., ˜625 nm wave length) if they were biased for use as light emitters. Other colors may also be used, including infrared and other visible colors. 
         [0045]    Referring now to  FIG. 3 , LED  26  may be given conventional LED biasing by means of a biasing resistor  42  communicating with a positive supply voltage  44  received through one of terminal pins  34 . The biasing resistor  42  controls the current flow through the LED  26  from anode to cathode to control an emission of red light from the LED  26 . As will be understood, for a given voltage provided to the terminal pins  34 , the resistor  42  will define the current through the LED  26  and hence its illumination intensity. With slight changes in the supply voltage  44 , the illumination intensity will also change. 
         [0046]    The emitted light from the LED  26  is received by LED  27  whose conductivity is affected by the amount of received light. An electrical biasing of the LED  27  is provided by the inverting input of a comparator  50  which rests at a positive voltage with respect to ground. This positive voltage generates a current passing through resistors  53  to the anode of LED  27 , the cathode of which is grounded. 
         [0047]    Resistor  46  and capacitor  48  may be placed in parallel across the LED  26  to provide a low pass filter improving the signal quality developed by LED  26  by shunting to ground high frequency noise components and by limiting the switching speed of the photo-interrupter to reduce false triggering. 
         [0048]    The comparator  50  compares the voltage at its inverting input, as will be determined by the light received by the LED  27 , to a threshold voltage applied to the non-inverting input of comparator  50 . This threshold voltage is established by a conventional resistor voltage divider  52  and will determine the switching point of the comparator  50  and thus the threshold sensitivity of the LED  27  to light. The voltage reference provided by the resistor divider  52  will also change with changes in the supply voltage  44 , so as to offset the change in illumination intensity of the LED  26 . 
         [0049]    The threshold voltage may be flexibly adjusted appropriately by changing the relative values of the resistors of the resistor voltage divider  52  which spans the power and ground lines and provides the threshold voltage at the junction of its two resistors. In this way, the threshold voltage may be adjusted for different batches of LEDs  26  and  27  accommodating the fact that their light output and sensitivity may not be well characterized by the part number. In an alternative embodiment, the voltage threshold may be floating and based on the average signal level from LED  27 , for example obtained from a low pass filter connected to the anode of diode  27  (not shown). 
         [0050]    A resistor  51  may be attached between the output of the comparator  50  and the non-inverting input of the comparator  50  to provide for hysteresis reducing false triggering and promoting stability in the comparator  50 . A suitable comparator may be the LM393 comparator, being a low power, low offset voltage comparator implemented as an integrated circuit providing a differential amplifier configuration, and commercially available from National Semiconductor of Santa Clara, Calif., as well as others. This comparator provides an open collector output and may operate with a single ended voltage supply of as little as 2 V. 
         [0051]    A pull-up resistor  54  may attach between the output of the comparator  50  and the positive voltage source  55 , the latter of which may be different from or the same as supply voltage  44  to provide a pull up voltage for the output of the comparator  50 . By using the pull-up resistor  54  on an open collector output of comparator  50 , the voltage of the voltage source  55  may be selected independently from the supply voltage  44  so that the output signal amplitude may be flexibly set by the device attached to the optical-interrupter  10  in the manner of a conventional optical-interrupter not having a comparator but using the collector of a phototransistor. 
         [0052]    When light passing between the LEDs  26  and  27  is interrupted, the voltage at the negative input of comparator  50  will change, crossing the threshold established by the resistor voltage divider  52  and causing a switching of the output of the comparator  50 , generating a square wave at an output terminal  60  connected to the output of the comparator having a frequency corresponding to a frequency of the interruption by the vane  40 . 
         [0053]    A filter capacitor  41  may be placed between the supply voltage  44  and ground to provide for improved voltage stability and the resistance to electrical noise. A nanofarad stabilizing capacitor  57  may be placed on the output of the comparator  50  for high frequency stability according to techniques known in the art. 
         [0054]    Referring now to  FIG. 4 , in an alternative embodiment, the flexible printed circuit board  20  may be replaced with a conventional rigid printed circuit board  62  constructed, for example, of epoxy and glass fiber. This planar circuit board may nevertheless provide a slot  18  by being cut into a U-shape having parallel extending legs  64  flanking the slot  18 . In this case, the LEDs  26  and  27  may be side-looking LEDs which emit and receive light along axis  30  generally parallel to the surface of the printed circuit board  62 . This is in contrast to the embodiment of  FIG. 1  in which the optical axis  30  is perpendicular to the surface of the flexible printed circuit board  20 . 
         [0055]    Per conventional LED design, the LEDs  26  and  27  may include an internal PN junction  67  having an N-channel material mounted directly to an opaque cathode conductor to emit light through the P-junction material along optical axis  30 . 
         [0056]    Terminal pins  34  may be mounted on the printed circuit board  62  to connect by traces to the other components  32 . A corresponding U-shaped housing  68  may be constructed for receiving the printed circuit board  62  and the mounted components  32  and may provide for transparent windows  70  on portions of the housing  68  that lie along the optical axis  30  across the slot  18 . As before, mounting holes  35  may be provided for mounting the housing  68  and the assembled printed circuit board  62  to a device. The printed circuit board  62  may be held within the housing  68  by a cover (not shown) enclosing the housing  68  about the printed circuit board  62 . 
         [0057]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.