# Patent

## Title
Compact stud finder

## Abstract

A pocket-sized object finder has a compact housing containing a battery, circuitry and a capacitor plate for detecting an object hidden behind a wall. The battery powered circuitry includes multiple signal indicators that illuminate in a serial manner when the object is being detected. The signal indicators are successively tapered and are mounted at a front side of a tapered end of the housing. The capacitor plate is disposed in the housing along its rear wall and is responsive to variations in capacitance that occur as the object finder is brought near and over the object. The compact housing defines a cavity with a width that is no more than two inches and one third its length. A removable access door, with a pocket clip, at an end opposite the signal indicators allows access to the battery within the housing cavity.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of application Ser. No. 09/538,087, filed Mar. 29, 2000; now U.S. Pat. No. 6,593,754 B1, was patented on Jul. 15, 2003, which claims benefit to provisional application Ser. No. 60/127,322, filed Apr. 1, 1999.


FIELD OF THE INVENTION
This invention relates to electronic instruments for detecting a stud or other object behind an opaque surface, such as wall board.
BACKGROUND OF THE INVENTION
Carpenters, electricians, do-it-yourselfers and others are often faced with the problem of locating the position of the wall studs behind the wall board material forming the wall surface. They are interested in hanging pictures, drilling holes and so on. However after the walls are finished and painted the location of the hidden substructure (i.e. the studs) is not visually detectable. The same is true of finding the location of hidden wooden frames in furniture and boats from the outside surface of the structure.
U.S. Pat. No. 4,099,118 issued Jul. 4, 1978 discloses an electronic wall stud sensor which is suitable for detecting a wall stud behind a wall surface. It utilizes one or more capacitor plates, a fixed frequency oscillator, a dual one-shot multivibrator, a field effect transistor, and a complicated calibration procedure. Each individual circuit must be calibrated at the time of manufacture, which is a costly procedure for mass production.
U.S. Pat. No. 4,464,622 describes a wall stud sensor similar to U.S. Pat. No. 4,099,118 but with a plurality of capacitor elements and means for detecting the presence of alternating current in the wall. Finding the presence of alternating current in walls is often not practical or possible with modern wiring methods. U.S. Pat. No. 5,352,974 describes a stud sensor similar to U.S. Pat. No. 4,099,118 but with means for storing calibration data for thick or thin walls. However, in most cases, the user will not know if the wall is thick or thin. The circuit used is complex and uses special purpose hardware. The sensor also uses a plurality of capacitor plates. Both of these devices require factory calibration.
U.S. Pat. No. 5,485,092 describes a device for investigating surface and subsurface structures. It uses four-sided conductive elongated plates and rectangular sensor plates connected together in a special arrangement. The different surfaces are charged at different rates and a differential amplifier and peak detector are used to determine information about the subsurface. It requires a complicated charging scheme and an expensive voltmeter for readout, which requires an interpretation of the results which would be difficult for an inexperienced person.
Prior art sensors were required to be a relatively large size so as to make them sufficiently sensitive for their intended purpose. Prior circuits required a relatively large sensor, and to isolate the sensor from the user's hand, which contributed to the relatively large size of the sensors.
Thus, there is a need for a low cost subsurface object locator that is easy to use, works well in the environment for which it is designed, simply and reliably identifies the location of substructure components in an efficient manner, is easy to manufacture, requires no calibration or adjustments by the factory or operator, and can be made of a relatively small size.
SUMMARY OF THE INVENTION
The invention provides a compact device capable of efficiently finding the location of hidden objects or substrata such as studs, joists and other similar objects below the surface of walls, floors and similar type structures. The device may also be used to find the location of braces, wood frames or other substructures in wooden furniture such as tables and cabinets, wooden boats and similar type structures.
In particular, the invention provides a pocket-sized object finder having a housing containing a battery, electronic circuitry and a capacitor plate. The housing has front, rear, side and end walls that define a cavity of a width no more than two inches and one third the length of the housing. The battery powered circuitry is responsive to variations in capacitance effected in the capacitor plate arising from the presence of the object. The capacitor plate is disposed in the housing along the flat rear wall, which is adapted for sliding along the wall so as to capacitively couple the capacitor plate to the object. A signal indicator visible from the front wall of the housing is activated in response to the change in capacitance and illuminates when the object is detected. A pocket clip is located at the front wall.
In one preferred form, the object finder has multiple signal indicators located at a tapered end of the housing. Successive signal indicators taper in the direction the housing is tapered and activate serially, without overlapping, at essentially a leading edge of the object and deactivate serially, again without overlapping, at essentially a trailing edge of the object.
In another preferred form, the housing has a removable access door over an access opening to a battery compartment of the cavity in which the battery is disposed. Preferably, the pocket clip is a part of the access door.
Thus, the present invention provides a compact object finder that using a small area capacitor plate allowing the over form factor of the finder to be smaller than prior finders. The small capacitor remains sufficient sensitive to detect objects at the same or even a greater depth than prior devices. The present object finder can be easily retained to a person's body either by grasping by hand, placing into a shirt or pants pocket, or by clipping it to one's clothing. The device is also easy to use and operate without manual calibration or adjustments on the part of the operator.
Other features and advantages of the invention will be apparent from the detailed description and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a first embodiment of a circuit for practicing the invention;
FIG. 2 shows various waveforms and details of operation of the circuit of FIG. 1;
FIG. 3 is a schematic diagram of a second embodiment of a circuit for practicing the invention; and
FIGS. 4A-4B are schematic diagrams illustrating a comparison of the operation of the first embodiment and the second embodiment;
FIG. 5 is a perspective view of an electrical instrument design incorporating the invention;
FIG. 6 is top plan view of the instrument of FIG. 5;
FIG. 7 is a right side plan view of the instrument;
FIG. 8 is a left side plan view of the instrument;
FIG. 9 is a bottom plan view of the instrument;
FIG. 10 is a front plan view of the instrument;
FIG. 11 is a rear plan view of the instrument;
FIG. 12 is an exploded perspective view illustrating how the sensor plate, circuit board, switch and batteries are assembled in the instrument housing; and
FIG. 13 is a schematic showing use of the instrument against a wall to detect a stud.



DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1 there is illustrated a circuit diagram of the invention. Shown on this figure is a portion of a wall structure 10, studs 11, 12 and wall board 13 to be illustrative of one way of operating the invention. In this case it is desired to locate the positions of the hidden studs 11 and 12. As shown in FIG. 1 there is a metallic sensor plate 21 connected to a CMOS oscillator 20 which produces a square (or rectangular) wave output. The circuit consists of a timer IC 22, the sensor plate and resistors. The frequency of the square (or rectangular) wave produced by the oscillator 20 is determined by the values of resistors R1 and R2 and the capacitance presented by the plate 21. When the sensor plate is above a section of the wall 13 with no studs it will cause the oscillator 20 to run at frequency f1. When the sensor is above a section of the wall 13 that has a stud below it the oscillator will have a different frequency f2.
The square (or rectangular) wave output of the oscillator 20 goes to a microprocessor circuit 30 via line 26. The microprocessor circuit 30 is programmed to measure the frequency difference f1 minus f2. The frequency difference has been found to be a reliable and consistent means of identifying subsurface objects such as studs and has been found to be relatively independent of the wall material. This makes the device self calibrating, obviating the need for any special factory calibration. If the frequency difference exceeds an amount deemed sufficient to indicate the presence of a stud, an LED is turned on.
The circuit 30 actually has four LEDs D2, D3, D4, D5 that can be activated at different amounts of frequency change. This is illustrated in more detail in FIG. 2, discussed later on. More or fewer LEDs could be used as indicators depending upon resolution and cost considerations. The circuit is powered by batteries 40 (four 1.5V pancake cells) through protective diode D1 (e.g., a 1N270 diode) and line 42. Resistor R3 is used to limit the current in the LEDs. Resistor R4 is used for a power on reset for circuit 30. Normally open switch 45 is pressed to enable power to circuit from the batteries 40 to circuit 30.
Although visual LED indicators D2-D5 are described here, it should be clear that audible indicators could be used as well. For example, different audible tones could be produced corresponding to various frequency differences encountered in scanning the wall, as the leading edge of a stud was approached, the frequency could go up, and as the trailing edge of the stud was passed the frequency could go down. In fact, there are occasions where audible indications may be better, such as in cases where the visible indicators may be hard to see.
Referring now to FIG. 2 there is illustrated the relation of the signal indicator means (LEDs) D2-D5 to the position of the sensor plate along the wall. As the sensor moves along the wall the frequency changes in accordance with curve 52. As the frequency decreases, the circuit 30 (FIG. 1) senses this change and turns on one or more of the LEDs D2-D5. The LEDs could be turned on so as to overlap in on-times or not. In the preferred embodiment, the on-times do not overlap to preserve battery power.
To use the device described, the plate 21 is placed on or in close proximity to wall 13 where there are no studs and the switch 45 is pressed. This causes circuit 30 to be activated and it will measure the first frequency f1 from the oscillator 20 and save it in memory. After this step is performed, which takes less than a second, the lowest LED D3 (green) comes on and stays on as a power indicator, while the switch 45 is pressed. This signals to the operator that the device can now be moved across the wall being probed. As the sensor is moved across the wall the circuit 30 is continuously measuring the second or subsequent frequency f2 from oscillator 20 and comparing it to the first frequency f1 by taking the frequency difference. When the difference exceeds a first threshold, the next LED up, LED D4 (amber) will be lit and LED D3 will go out. When the difference exceeds a second threshold, greater than the first threshold, the next LED D5 (amber) will be turned on and LED D4 will go out. When the difference exceeds a third threshold, greater than the second threshold and which indicates the presence of the leading edge of the stud, the highest LED D2 (red) goes on and the LED D5 goes out. LED D2 stays on as the thickness of the stud is traversed by the device. When the trailing edge of the device is reached, the LEDs go off and on in the reverse sequence. Thus, a user trying to find a stud, will mark the leading edge of the stud when LED D2 comes on, and will mark the trailing edge of the stud when the LED D2 goes off.
When a user first puts the device against a wall or other surface to be probed, there is no way of telling if it is initially placed over a stud or other subsurface object or not. The device assumes that it is not. However, if by chance it is, then the subsequently found frequency difference will be negative and unless special provision is made in the programming of the microprocessor, an error will result. It is an easy matter, however, to program the microprocessor so that if the f1-f2 frequency difference is found to be negative, it means that the device was initially placed over a stud or other subsurface object. The device could be programmed to flash the LEDs or beep a buzzer in that event to alert the user to start over, placing the device in a different initial position.
Referring to FIG. 3, there is illustrated a circuit diagram of a second embodiment of the invention. Shown on this figure is a portion of a wall structure 60, studs 61, 62 and wall board 63 to be illustrative of one way of operating the invention. In this case, it is desired to locate the positions of the hidden studs 61 and 62. As shown in FIG. 3, there is a metallic sensor plate 71 connected to a CMOS oscillator 70. The frequency of the oscillator 70 is determined by IC 72, the values of resistors R1 and R2 and the capacitance presented by the plate 71.
The capacitance of the plate 71 is determined by the surrounding medium including the wall material, the studs, the circuit and the person holding the device. It is desirable to reduce the stray capacitance as much as possible since this will improve the sensitivity of the plate 71. The capacitance of plate 71 is influenced considerably by the operator and the housing of the device.
Capacitance is related to its potential with respect to other objects. If an additional plate 75 is introduced in the vicinity of plate 71 with the same potential as plate 71, it will reduce the “stray” effects. This improves the sensitivity of the plate 71 and allows it to sense further into the wall.
The potential of plate 71 changes as the oscillator 70 operates. In a typical situation it may vary from 0 to 5 volts in amplitude. Hence the guard plate 75 must have its potential vary in the same way. This is accomplished by using a buffer amplifier 78, with a gain of one, which has the voltage of the sensor plate 71 at its input and produces a near exact replica of it at its output, which is connected to plate 75 via line 77. Hence plate 75 is driven at the same potential as plate 71.
Referring to FIG. 4A, a side view of a sensor plate 100 is shown to illustrate how a sensor with a single plate operates. The sensor plate 100 is connected to the oscillator 103, which causes its potential to vary. The electrical E-field lines 102 are free to go in any and all directions.
FIG. 4B illustrates how the second embodiment described above operates. In FIG. 4B, a sensor plate 110 is connected to an oscillator 116 and a guard plate 114 is driven from amplifier 118 so it has the same potential as the sensor plate. The E-field 112 is now prevented from going in the direction of the guard plate 114. This is because both plates are at the same potential and by electrical laws there can be no E-field between conductors of the same potential. With fewer E-field lines, there is less capacitance of plate 110. Hence it will be more responsive to dielectric changes in the direction opposite to the guard plate 114. The guard plate 114 may be somewhat larger than the sensor plate 110 so as to extend beyond the edges of the sensor plate 110, which redirects the E-field lines emanating from the edges of the plate 110 in the direction toward the surface being probed.
Referring back to FIG. 3, the remainder of the circuit of FIG. 3 acts in the same way as the first embodiment of FIG. 1. When the sensor plate is above a section of the wall 63 with no studs it will cause the oscillator 70 to run at frequency f1. When the sensor is above a section of the wall 63 that has a stud below it the oscillator will have a different frequency f2. The output of the oscillator 70 goes to a microprocessor circuit 80 via line 76.
The microprocessor circuit 80 is programmed to measure the frequency difference f1 minus f2. As in the first embodiment, this can be done by any suitable means. For example, the microprocessor circuit 80 will typically include a counter. The counter can be programmed to count the number of times the oscillator output signal to the microprocessor goes high in a certain period, which yields a measure of the frequency of the oscillator output. If the frequency difference between the first measured frequency and the subsequently measured frequencies exceeds an amount deemed sufficient to indicate the presence of a stud, an LED is turned on.
The circuit 80 actually has four LEDs D2, D3, D4 and D5 that can be activated at different amounts of frequency change. This is illustrated in more detail in FIG. 2. More or fewer LEDs could be used as indicators depending upon resolution and cost considerations.
The circuit is powered by batteries 90 through protective diode D1 and line 92. Resistor R3 is used to limit the current in the LEDs. Resistor R4 is used for a power on reset for circuit 80. Switch 95 is pressed to enable power to circuit from the battery 90 to circuit 80.
The microprocessor is capable of detecting very small changes in the frequency of the oscillator, which improves the sensitivity of the device and permits making the device relatively small. FIGS. 5-13 illustrate the design of an electrical instrument, which may include either of the two previously described circuits. As illustrated, this instrument is generally pen-light sized, able to easily fit into a breast pocket. In the preferred embodiment, the device is approximately 1 inch wide, {fraction (19/32)} inches thick (not including the pocket clip) and 5{fraction (9/16)} inches long. The housing 105 is provided with a pocket clip 109, integrally molded as part of the battery cover 107, to help hold the device in a user's breast pocket, since the device is small enough to fit, being at least three times longer than it is wide, and in the case of the embodiment disclosed, being over five times longer than it is wide. To be of material benefit over prior art devices, it is preferred that the locator be less than two inches wide, which is more than accommodated by the preferred embodiment, since it is only 1 inch wide.
The instrument in FIGS. 5-13 has been labeled with reference numbers as if it includes the first circuit, of FIG. 1. If so, the metal sensor plate 21 can be provided on the bottom side of a printed circuit board 101, and fixed in the housing, for example by an adhesive, so the exposed surface of the plate is against bottom wall 103 of housing 105 so as to minimize any air gap between the plate 21 and the surface being probed. The plate 21 may be provided as the copper layer commonly provided as part of an ordinary printed circuit board. If the device is made to include the second circuit of FIG. 3, the circuit board 100 can be provided with metal (typically copper) layers on both sides, with the layer on side 106 being the guard plate 75, and the layer 71 being on the lower side, as is layer 21. The lower side sensor plate may be etched so as to make it smaller than the upper side guard plate. The other circuits, i.e., the oscillator, microprocessor, LED and buffer amp (if applicable) circuits, are provided on circuit board 115, which is secured in the housing 105 as far away as possible from the sensor plate 21.
The top end of the housing 105 tapers in width to a blunt point 111, to give an operator a better approximation of the center of the device. Transparent or translucent windows 113 centered laterally on the front surface of the housing 105 are aligned with the respective LEDs D2-D5 and also taper in width toward the top to a sharper point, also to help the operator locate the center of the housing, and therefore the edge of a stud or other subsurface object.
Variations and modifications to the preferred embodiments described will be obvious to persons skilled in the art without deviating from the spirit of the invention. Therefore, the invention should not be limited to the preferred embodiments described, but should be defined by the claims which follow.

## Claims

1. A pocket-sized object finder, comprising:
a battery; 
electronic circuitry for detecting an object hidden behind a wall, the circuitry being powered by the battery and including a signal indicator that illuminates when the object is detected; 
a capacitor plate coupled to the circuitry, the circuitry being responsive to variations in capacitance effected in the capacitor plate arising from the presence of the object and activating the signal indicator in response thereto; and 
a housing having a width and length and a front wall from where the signal indicator is visible and where a pocket clip is located, the front wall being joined to a rear wall by side and end walls to define a cavity having a width of no more than two inches and no more than one third the length of the housing, the cavity containing the battery, the circuitry and the capacitor plate, wherein the capacitor plate is disposed in the cavity along the rear wall, the rear wall being flat and adapted for sliding along the wall so as to capacitively couple the capacitor plate to the object.

2. The object finder of claim 1, wherein the signal indicator is located at a tapered end of the housing.

3. The object finder of claim 2, wherein the circuitry includes multiple signal indicators disposed in the front wall of the housing, wherein successive signal indicators taper in the direction the housing is tapered.

4. The object finder of claim 3, wherein the signal indicators activate serially without overlapping at essentially a leading edge of the object and deactivate serially without overlapping at essentially a trailing edge of the object.

5. The object finder of claim 1, wherein the housing has a removable access door over an access opening to a battery compartment of the cavity in which the battery is disposed.

6. The object finder of claim 5, wherein the pocket clip is a part of the access door.