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Proximity Switch

This project, a proximity switch from 1990, shows how to make a robust touch switch that operates through inches of glass.

Capacitive switches are very effective when used as touch switches through glass but what happens if the glass is too thick and you can't use this method?

The answer is in this design and it is to use infrared instead of capacitive measurement. Of course this brings it's own problems i.e. reflected light and ambient sunlight that must be rejected.

An Important aspect of the design is that the circuit works out the steady state reflection error so that large reflections that stop other systems from working will not stop this design from operating correctly.

Executive Summary of the Proximity Switch

A reflection-type photoelectric proximity detector and switch is capable of detecting the incremental and binary proximity of a finger through several layers of glass, cancelling out extraneous reflections and stray sources of radiation.

The circuitry includes a modulated infrared emitter, an infrared detector, an optical barrier, an infrared filter, a bandpass filter, a signal rectifier, a biasing circuit to remove unwanted detected modulated signal, an amplifier, an incremental proximity output, a comparator, a detection reference, and a binary proximity detect output.

In one embodiment a multitude of photoelectric proximity switches are installed on the inside of a store window, detect finger proximity from outside the window, and control electric appliances inside the store window. From outside the window, a user can select what is displayed inside the window.

Background of the Proximity Switch

This design relates to proximity detectors and switches, and in one embodiment it is concerned with a touch detector for mounting inside a store window, enabling a user outside the window to control appliances and/or displays inside the store.

A number of different types of proximity detectors and switches have been in use or disclosed. Switch panels that use the reflection of infrared radiation to detect finger touch are disclosed in Sauer's U.S. Pat. No. 4,340,813. Short range reflective controls such as Micro Switch (a division of Honeywell) Model FE7B use the reflection of modulated infrared radiation to detect objects.

Fukuyama et al. U.S. Pat. No. 4,306,147 discloses a reflection-type photoelectric switching apparatus which detects the presence of an object in a mechanically adjustable sensing area. Aromat (a member of Matsushita Group) Model MQ-W3A-DC12-24V photoelectric sensor detects the presence of an object in an electrically adjustable sensing area using optical triangulation.

Philipp U.S. Pat. No. 4,879,461 discloses a reflective-type detection system which senses the presence of an object against a background using a general technique of nulling to negate the effects of stray (e.g. ambient) light by adding the complement of the detected signal to a summing point in the circuit to cancel the signal generated by stray light.

In a system using a modulated emitter, this requires the generation of an amplitude and phase regulated modulated signal as a complementing signal. As explained below in the present system a filtered and rectified version of a detected signal is nulled with a constant (DC) level signal.

Reflecting-type infrared switches use the reflection of infrared light off an object to detect the presence of the object. Such sensors are in common use in industry to detect the presence of an object without having mechanical contact with the object, leaving the object undisturbed. These sensors lack mechanical contacts which wear and soil, giving them long lifetimes.

Reflective-type infrared switches have the potential to detect finger proximity through glass. A preferred application of a reflecting-type infrared switch is a touch switch that allows the control of electric devices located inside a store window from outside. Locating all system components inside the store window affords them safety from the elements of weather and vandals and eliminates the need to drill through building walls and glass. The operation of electric devices could occur during or after store hours.

Detecting finger proximity through one or more glass panes that might include air gaps in an environment that can include a multitude of radiation sources and reflections increases the difficulty of the task. Reflective-type infrared switches that use modulated emitters and infrared and electronic filters minimize the effect of extraneous radiation outside the bandwidth of the infrared and modulation frequencies. Extraneous radiation sources would include sunlight, artificial window and street lighting, and automobile headlights.

A common detection practice used to detect the presence of an object with a reflecting-type infrared switch using a modulated emitter is to optically and electronically filter the reflected signal and compare it to a threshold which is applied against a composite signal which may include reflections from other objects in the sensing region.

This technique does not take into account the presence and effect of large extraneous reflections which can occur in the preferred window application described above. The present design addresses this situation by including a means of applying a threshold to an amplified version of the detection signal after the extraneous steady-state signal has been cancelled.

Another detection practice used by Fukuyama and Aromat adjusts the sensing area to exclude extraneous objects from a detection range. This is not adequate in the preferred application since extraneous objects (glass panes) are in intimate proximity to the object to be detected (finger).

Proximity detectors are used in industry to measure, for example, the web tension and accumulating roll diameter of winding fabric, proximity of microprobes from the surface of electronic parts, product width or position determination, and distance measuring on automatic vehicles. Typically ultrasonics are used in applications that resolve within a centimeter.

A reflective-type proximity detector offers the possibility of measuring displacement to a much finer resolution, albeit over a much shorter working distance, typically under 30 centimeters.

Reflecting-type infrared proximity detectors operate on principles similar to reflective-type proximity switches, having instead an analog signal output indicating the strength of the reflected signal, which increases as the object gets closer to the detector. One difficulty encountered by reflective-type proximity detectors is the tradeoff of sensitivity to incremental movements and saturation from total reflection.

This situation is similar to the case of the reflecting-type infrared switch where there is a small change in reflection (due to incremental movement) in the presence of a large background reflection (due to total reflection).

It is therefore a purpose of the design to create a reflective-type proximity switch that can detect the touch of a human finger, discriminated from stray radiation in a multitude of environments including the preferred application mentioned above. It is a further purpose of the design to produce an output which is relative and which increases as an object comes closer to the detector, removing the effect of total reflection to allow greater amplification of incremental movement.

Summary of the Proximity Switch

In accordance with the present design, a proximity detector and switch is relatively simple in construction and circuitry, and highly accurate and sensitive in use. The principal circuitry can be used for detecting presence of an object to produce a binary output, i.e. as a switch, or a continuous output for determining the proximity of an object.

A principal object of the design is to provide a relatively simple and cost efficient system having a means of removing the effect of large background type or steady-state reflections to allow greater amplification of changes in reflection. A related object is to detect small incremental changes of reflection in the presence of large steady-state reception which may be from reflection or direct transmission or both.

Another object of the design is to produce an output that increases and decreases as an object incrementally moves respectively closer to and farther away from the detector.

Another object of the design is to provide a reflective-type proximity switch capable of binarily detecting finger proximity.

Another object of the design is to produce a binary output that indicates when an object to be detected is within a zone of proximity.

A further object of the design is to detect finger presence or proximity through one or more panes of glass that may include an air gap, by cancelling out the radiation reflected by the glass.

Another object of the design is to provide visual feedback to indicate the switch state of a touch sensitive switch.

Another object of the design is to provide a simple means to calibrate the switch during installation.

Another object of the design is to control electric devices with a photoelectric proximity switch, and particularly to allow a user outside a window to control displays of information or other visual material inside the window.

Another object of the design is to provide a system which will permit a computer or modem to control visual feedback to indicate when to touch the switch.

Another object of the design is to prevent unintended touches or objects moving past the switch from activating the output of the switch.

Another object of the design is to include several touch switches, serial communication with a computer, touch response delay, relays to control electric devices with the touch switches, visual feedback to indicate switch state, calibration state, and where and when to touch the switch, in a self contained system.

To achieve an output indicating proximity of an object, a photoelectric proximity detector according to the design includes a means for transmitting a modulated infrared beam, means for mechanically blocking stray radiation, means for optically selecting radiation in the spectral band of the transmitted beam, means for converting a received infrared modulated beam into an electric signal, means for electrically selecting signals at the modulation frequency, and means for electronically converting the modulation signal into a proximity level.

The system further includes means for electronically compensating for extraneous steady-state infrared modulated radiation picked up by the detector, and means for electronically amplifying the compensated proximity level.

To further achieve the goal of a photoelectric proximity switch or touch switch, the following elements are added to the photoelectric proximity detector described above: a means for electronically converting the compensated proximity level into a binary proximity state, and means for electronically delaying the binary proximity output by an adjustable delay.

The system thus produces a signal which can be used for touch detection, for communicating the binary proximity state to electric devices, computers and modems. These may be used to control displays of information, thus giving a user control of the displays by touch selection.

In one preferred embodiment of the design, an array of infrared light emitting diodes (IRLEDs) emit a beam of pulse modulated infrared radiation outwardly toward a sensing area. The IRLEDs are modulated together to produce a single-phase beam from the array. The beam is reflected off objects in the sensing area, and this reflected radiation is received and converted into an electric signal by an infrared phototransistor (IRPT).

The IRPT preferably includes an infrared filter integral in its construction, to optically block out-of-band radiation. An opaque tube is fitted around the IRPT to mechanically block off-axis stray radiation, effectively narrowing the sensing area.

The signal from the IRPT goes through a bandpass filter which electronically selects signals at the modulation frequency, and a half wave rectifying circuit converts this AC signal into a DC proximity level. An important feature of the design is that, after the described processing to produce a DC proximity level, a bias signal is subtracted from the proximity level to compensate for extraneous steady-state modulated radiation. This can be from glass surfaces, for example, in a through-the-window touch detector.

The compensated proximity level signal can be amplified and the result output as a proximity signal of the detector device. It can also be used to produce a binary output, to operate as a switch.

To implement the device as a photoelectric proximity switch or touch detector, the amplified compensated proximity level signal is converted into a binary output by comparison to a detection threshold. The comparison is made with hysteresis to prevent a jittery output when the two signals are near in value.

In a preferred implementation, the binary output of the comparison is delayed using a charging circuit to ignore quick unintended touches caused for example by a finger brushing over the photoelectric proximity switch.

In the specific embodiment of a touch detector, such as for enabling a user outside a store window to make touch selections to operate display equipment inside the store window, visual indications and visual feedback to the user are desirable. To this end, the proximity detector and switch of the design includes a visual means for indicating the location of the touch area, a further visual indication of the switch output, and a request for switch touch.

In a preferred embodiment of the design, the touch area or touch areas are indicated by illuminated light conducting acrylic target rings which circumscribe the IRLED arrays and the IRPT at each touch sensor, thereby circumscribing the touch area with a ring of light. This target ring preferably flashes to request a touch and to indicate that switch is detecting touch.

For calibration of the touch sensor or proximity device, i.e. to adjust the magnitude of the biasing signal which subtracts out steady-state extraneous signal detected by the detector, a bicolored LED is used to indicate the bias state, which is the sign of the relative magnitude of the compensation level as compared to the proximity level (without presence of a finger or other object to be detected) before the summation of the two levels is performed.

Upon installation of a touch detector, an installer adjusts the magnitude of the biasing signal to equal the magnitude of the uncompensated proximity level by observing the bicolored LED, which illuminates green when the bias signal is greater than the proximity level, red when the bias signal is less than the proximity level, and extinguishes when the bias signal is equal to the proximity level.

Once the bias signal is set, the sensitivity of the touch switch is adjusted by setting the gain of the amplifier, which amplifies the compensated proximity level signal, so as to produce a detection output when the object to be detected is in proximity to the touch sensor.

To carry out the purpose of communication with a user, such as in the environment of a store window, the proximity switch of the design includes a means for relaying the switch state to electrical devices including a computer connected to a display device, and optionally including a modem.

The display generated by the computer, in response to touch selections of the user, provides part of the visual feedback to the user. Thus, in addition to flashing illuminated target rings, a request for user selection can also be made by the display of information generated by a computer, such as a textual message.

Similarly, confirmation of user selection by the flashing of the target ring of the particular switch touched, while all remaining unselected switches revert to steady illumination of their target rings, can also be indicated by the computer display--for example, by graphically highlighting what has been selected.

In preferred embodiments of the design the binary switch state output of a touch switch can drive a relay that can turn on other electrical devices in response to the user's touch selection. Examples are video disk players, toy trucks, fans and light bulbs, as alternatives to strictly displaying information on a screen as described above.

A serial communication device can be included to enable a computer to interrogate the state of each touch sensitive switch, arm and reset a latch that stores the photoelectric proximity switch's touch state, and control the flashing of the target rings. Multiple switch panels, each containing a plurality of touch switches, can be in communication with one computer through a serial communication scheme wherein each unit is given a unique address and only responds to commands that contain its address.

Modem communication is achieved through use of a serial interface that connects to a telephone line. FIG. 1A is a front view of an optical assembly of a photoelectric proximity detector and switch in accordance with the design.

Figure 1a : Is a front view of an optical assembly of a photoelectric proximity detector and switch in accordance with the invention for the proximity switch

Figure 1b : Is a cross sectional view of the optical assembly shown in fig. 1a for the proximity switch

Description of the Proximity Switch

FIG. 1a and FIG. 1b show optical assembly components of the photoelectric proximity detector and switch 5 of the design. An array of IRLEDs 6 transmit modulated infrared light in beams represented by lines 7 and 8 through a plurality of glass panes 9 and 10 which include an air gap 11.

Such insulated double-pane glass is often used in store windows. As indicated by a line 12, some of the beam reflects back into an infrared phototransistor (IRPT) 13 by reflection off glass surfaces (which can include both inner and outer surfaces).

Some of the beam, as indicated at 14, is reflected back into the IRPT 13 by a finger 15 in proximity to the top or outer glass pane 10. The IRPT 13 is furnished with a black IR filter package (not shown). An opaque tube 16 preferably is included to sheath or shroud the IRPT 13, thus restricting the acceptance angle of the IRPT 13 and minimizing the detection of off-axis illumination such as represented by a ray 17 from the sun S.

Even if the sun's rays are filtered out of the system by the IR filter and by electrical components described below, the IR component of the sun's spectrum that passes through the IR filter can be strong enough to saturate the detector 13 and thus prevent good readings If the sun were in a position S' substantially aligned with the axis of the IRPT 13, allowing enough of the sun's rays 20 to pass axially through the opaque tube 16 and saturate the IRPT 13, the finger 15 would interrupt these rays 20, allowing the detection of the finger 15.

A light conductive target ring 18 in one preferred embodiment, which may be made of acrylic, is illuminated by an outer array of a plurality of visible LEDs 19, emitting constant or blinking light to indicate the status of the photoelectric proximity switch 5.

Figure 2 : Is a block diagram indicating a preferred implementation of circuitry for the device of the invention, with outputs for both compensated proximity detection level and for binary switch functions for the proximity switch

View larger image here.

FIG. 2 is a block diagram showing the signal processing used in a preferred embodiment of the photoelectric proximity detector and switch. An oscillator 21, typically 1 Khz, drives a plurality of IRLEDs 6 through a drive circuit 22. An infrared phototransistor (IRPT) 13 converts reflected (and in some cases scattered and direct-transmitted) infrared radiation into an electric signal that is buffered at 24 and filtered by a bandpass filter 25 with a center frequency of the modulation oscillator 21.

The bandpass filtered signal is half-wave rectified at 26 and ripple filtered at 27 to produce a proximity signal 28 proportional to the amplitude of the modulated infrared radiation received by IRPT 13.

A bias signal 29 is subtracted from the proximity signal 28 by a summing circuit 30. This has the effect of cancelling out any steady-state extraneous signals caused by reflections, scattering and direct path leaks between the IRLEDs 6 and the IRPT 13. The resulting biased signal at 31 is amplified by an amplifier 32 and then further ripple filtered at 33, producing a compensated analog proximity signal 35 that indicates the incremental proximity of an object 45 (which can be the finger 15) in the field of view common to the IRPT 13 and IRLEDs 6. The signal 35 can be fed to an appropriate form of readout, or its value can be used to control electric devices or displays as desired.

If the system of the design is to function as a touch switch or object-present switch, components 36-43 in FIG. 2 are used to implement this.

When the compensated proximity signal 35 is greater than a detection reference 36, the output of a comparator 37 goes high, starting a time delay 38. If the output of the comparator 37 remains high for the duration of the time delay's period, the time delay 38, at the end of the time delay period, turns on a relay 40 through a relay driver 39, to control an electric device 43 (e.g. a video disk player and connected display).

If the comparator 37 output falls before the completion of the time delay period, the time delay 38 resets and does not produce an output. In a preferred embodiment a serial communication device 41 transmits the time delay's output 50 to a computer 44 and modem 42. The computer 44 through the serial communication device 41 controls the flashing of the target ring LEDs 19 though a control line 54 by allowing the output of a low frequency oscillator 53 to pass through a NAND 55 and into a driver 56.

Thus, the target ring LEDs 19 are on or flashing when the control line 54 is respectively low or high.

Figure 3 : Is a schematic circuit diagram representing one embodiment of the proximity detector and switch of the invention for the proximity switch

View larger image here.

FIG. 3 is a circuit schematic showing one possible implementation of the photoelectric proximity detector and system. It should be understood that this schematic is merely an example of an implementation of the functions of the block elements of FIG. 2. This implementation uses discrete parts and operational amplifiers to implement block elements. It is understood that other means may be used to implement block elements including the use of microprocessors and custom integrated circuits.

In the diagram of FIG. 3, the oscillator 21 generates pulses that drive a transistor 60 on and off, turning on and off the IRLEDs 6 at the modulation frequency (typically 1 Khz). The infrared phototransistor 13 receives radiation from objects that reflect and scatter illumination from the IRLEDs 6 and produces a corresponding current proportional to the radiation that passes through its built-in infrared filter.

Resistor 64 converts this current into a voltage which is applied to an operational amplifier 65 configured as a buffer (24 in FIG. 2). This buffered signal is filtered by a multiple feedback bandpass filter constructed of elements 66-70 (25 in FIG. 2). Operational amplifier 73 and resistors 71 and 72 amplify the filtered output of operational amplifier 70, making the filter operate at unity gain.

Resistors 74 and 75 produce the center operating voltage for the bandpass filter, stabilized by capacitor 76.

An embodiment of a half wave rectifier is indicated at 26. Capacitor 77 AC couples the output of the bandpass filter 25, presenting to operational amplifier 81 a modulated signal centered around 0 volts. Diode 79 shorts the negative portion of the bandpassed signal, half wave rectifying the signal. Resistor 78 increases the impedance of the half wave rectifier, and resistor 80 provides a path to ground for operational amplifier 81.

The combination of resistor 82 and capacitor 83 and operational amplifier 85 filters the ripple of the half wave rectified signal, with resistor 84 providing a ground path for operational amplifier 85. A bias voltage from potentiometer 91 applied through resistor 90 to operational amplifier 88 is subtracted from the filtered signal 28 by the summing circuit created with operational amplifier 88.

Resistors 86, 87, 89, and 90 are of equal value to produce a unity gain summer 30. Potentiometer 91 is adjusted to cancel any steady-state component of signal 28, thus serving as the bias signal 29 indicated in FIG. 2. Bias voltage exists at C.

The compensated signal 31 is amplified by operational amplifier 94, with the gain set by the ratio of the resistance of variable resistor 93 to resistor 92. The amplified signal 122 is buffered by operational amplifier 95 and ripple filtered by the RC combination of resistor 96 and capacitor 97, with resistor 98 providing a ground path for operational amplifier 100.

For calibration of the system, bicolored LED 145 illuminates green when current flows from operational amplifier 142 through current limiting resistor 144 into operational amplifier 143. This occurs when the proximity signal 28 is smaller than the bias voltage 141 at C in the diagram, driving the output of operational amplifier 142 high and operational amplifier 143 low, both configured as comparators.

The bicolored LED 145 illuminates red when the current flows in the opposite direction, occurring when the proximity signal 28 is larger than the bias voltage 141.

Calibration of the circuit and system can be performed by first setting the bias voltage 141 (via the potentiometer 91) to cause a zero summation when no finger or other object to be detected is present.

The zero summation, or null state, is indicated when the bicolored LED 145 is off, i.e. neither green nor red. In practice it is possible for the bicolored LED 145 to alternate quickly from one color to the other while still in a null state, since the comparators 142 and 143 that drive the bicolored LED 145 do not include hysteresis, producing an apparent mixing of the two colors, typically perceived as orange, which is also an acceptable indication of a null state.

Calibration is completed by adjusting the gain of the amplifier (32 in FIG. 2) via variable resistor 93, until a touch output at 50 is detected when a finger is placed in proximity to the touch switch.

The ripple filter output 35 is the proximity detection level (35 in FIG. 2) for the photoelectric proximity detector. If binary touch detection is desired, the detection level is compared by operational amplifier 103 to a detection reference (36 in FIG. 2) voltage formed by resistors 130 and 131, stabilized by capacitor 132. Resistor 102 provides positive feedback to operational amplifier 103, adding hysteresis to the comparator (37 in FIG. 2) for stability.

The system can be calibrated to read presence of touch at a desired distance from the detector device, by holding a finger at the desired distance and setting the gain of the amplifier (32 in FIG. 2) via variable resistor 93 to cause a position output at this position threshold.

The binary "touch" output 50 is buffered by a CMOS buffer 106, whose high output activates the time delay (38 of FIG. 2) by charging capacitor 109 through variable resistor 108. When the voltage on capacitor 109 exceeds the input level threshold of buffer 110, the output of buffer 110 goes high, turning on transistor 112 through resistor 111, turning on relay 40 which activates an electric device 43.

Diode 115 clamps the negative voltage spike generated by relay 40 when its coil is deenergized. The time delay introduced by charging capacitor 109 is set by the value of the resistance of variable resistor 108. Diode 107 quickly discharges capacitor 109 as soon as the binary touch state 50 goes low, resetting the time delay (38 of FIG. 2).

Figure 4 : Is an elevation view in section showing an example of a system for enabling a user to control displays and other electrical items inside a store window, by touching the outside of the window glass to activate proximity detectors/switches of the invention mounted just inside the store window for the proximity switch

FIG. 4 shows a typical application of the photoelectric proximity system using a plurality of photoelectric proximity switches installed in a store window. All the system components (150-154, 160, 161) are located at the inside 158 of a window 156 (which may be single or multi-glazed), away from direct contact from the outside 157, eliminating the need for drilling holes through the building wall 159 or glass 156 and protecting the equipment from weather and vandals.

The IRLEDs (6 of FIG. 1A) and IRPTs (13 in FIG. 1A) of the photoelectric proximity switches (5 in FIG. 1A) are located in a touch panel 150, mounted on the inside of the glass pane window 156 and connected by a control cable 151 to a control unit 152 which contains the electronics of the photoelectric proximity switch.

Binary state outputs of the control unit 152, carried by cable 160, operate search and play functions of a video disk player 153 which displays a visual output on a monitor 154 though a cable 161. This electrical equipment is shown only as an example of display equipment. Other devices, for example toys, videotape movies, lamp fixtures, etc. can be operated and demonstrated to the user 155, through the user's own selections.

In operation, the person 155 outside the store touches the store window 156, coming within detection range of the photoelectric proximity switch panel 150, controlling, for example, the video disk player's search and play functions. The person could browse through a store's merchandise catalog, view houses listed with a realtor's office or vacation locations from a travel agent, preview video movies or view other information or electric products.

Such display information can be stored on a video disk, but computer disk storage can alternatively be used for information and demonstrational displays.

Figure 5 : Is a schematic circuit diagram representing one embodiment of a dual threshold range comparator used in one embodiment of the invention to detect the presence of an object within a proximity zone for the proximity switch

FIG. 5 is a circuit schematic showing one possible implementation of a dual threshold range comparator 200 that can be used in place of, or in addition to the comparator 37 in FIG. 2 in order to indicate whether an object is inside a certain zone (neither too close nor too far). The comparator 200 produces a binary "in zone" and "out of zone" output 201 corresponding to a proximity level (35 in FIG. 2) value respectively being either within the bounds of two adjustable thresholds 202 and 203 or outside these thresholds.

If the compensated proximity level 35 of FIG. 3 applied to comparison line 204 is greater than the upper threshold reference 202 set by potentiometer 205 or lower than the lower threshold reference 203 set by potentiometer 206, the open collector output of a comparator 207 or 208 respectively pulls the combined comparator output line 201 low.

If the compensated proximity level 35 of FIG. 3 is lower than the upper threshold reference 202 and higher than the lower threshold reference 203, then the output of both comparator 207 and 208 are open, allowing output line 201 to be pulled high by resistor 209. A National Semiconductor LM339 is an example of an open collector comparator suitable for 207 and 208.

Figure 6 : Is a front view of a touch panel containing three photoelectric proximity switches with illuminating target rings in accordance with the invention for the proximity switch

FIG. 6 is a front view of a touch panel 301 containing the optical components (5 in FIG. 1A) of three photoelectric proximity switches. Above each proximity switch 5 is a nomenclature indicator 302, which in one embodiment is a light conducting acrylic rod illuminated from the rear by an LED with an opaque label 303 adhered to the top surface of the nomenclature indicator 302, used to identify each switch 5.

Illuminated target rings 18 indicate touch state of each switch 5, encircling the switch elements composed of a sheathed centrally located IRPT 13, surrounded by a cluster of four IRLEDs 6.

The following is a list of key components which may be used to implement the photoelectric proximity detector and switch as shown in the embodiment of FIG. 3.


IRPT 63 SFH 303F Siemens

IRLED 61 SFH 484 Siemens

Oscillator 21 LM555 National Semiconductor

Operational amp. 65, LM324 National Semiconductor

70, 73, 81, 85, 88, 94,

95, 100, 103, 142, 143

Buffer 106 74C902 National Semiconductor

Transistor 112 2N2222 Motorola

Transistor 60 TIP106 Texas Instruments

Bicolored LED XC5491 Senior Electronics, Taiwan ______________________________________

As used herein and in the claims, the term "light" is intended to include infrared as well as the visible range of light. Other forms of light than infrared, i.e. light in the visible band, can be used for certain applications when desired. Further, although the design is described as using light energy in the preferred embodiment, other forms of radiation and in fact non-radiation energy can be used, under the broad principles of the design.

For example, a system for proximity detection or switching which does not involve a glass barrier could use an ultrasonic form of energy to be reflected off objects desired to be detected, while still employing the principles of the design particularly in regard to processing the reflected signal to a clean DC amplitude level before compensating for extraneous or background signal.

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