text: referring more particularly to the drawings , fig1 is an overall view of a touch panel unit 12 which has a faceplate 14 . in addition , a series of photoemitters such as light emitting diodes are located , for example , at one side 18 of the faceplate to direct beams of infrared light across the faceplate 14 to photodetectors such as phototransistors located at the opposite edge 20 of the faceplate 14 . similarly , an additional set of light emitting diodes may be provided to direct illumination from the lower edge 22 of the faceplate upwardly to phototransistors located at the upper edge 24 of the faceplate . preferably the pairs of light emitting diodes and phototransistors are energized sequentially to scan across the faceplate both in the horizontal direction and in the vertical direction such that they intersect to form a grid pattern . the light from each light emitting diode is directed across the faceplate to impinge on the opposing phototransistors locating the coordinates of where a finger 26 may be pointing , for example , either to a switch point or a portion of a display . of course , the interruption of one or more of the beams in each direction , serves to locate the position of the finger 26 . a set of four circuit boards 32 , 34 , 36 and 38 may serve to mount the light emitting diodes and the phototransistors , along with their associated electronic circuitry . in practice , the opposing pairs of light emitting diodes and phototransistors are switched on and off concurrently , with sequential energization of the successive pairs of phototransistors and light emitting diodes serving to scan across the face of the faceplate in both the horizontal and vertical directions . in practice , the output from the phototransistors may be connected in parallel to detection and processing circuits , with the particular phototransistor from which a pulse is being received being identified by the timing of the switching circuitry which sequentially turns on the paired light emitting diodes and phototransistors . reference will now be made to the circuit diagram of fig2 which includes a circuit illustrating the principles of the present invention . more specifically , in fig2 a phototransistor 42 is shown connected by switch 44 to a phototransistor output lead 46 . a plurality of additional switches 48 are also shown , and are representative of a series of parallel semiconductor switches which sequentially gate a series of parallel phototransistors to their conducting states , simultaneously with the energization of the opposite paired light emitting diode 40 which directs light onto the phototransistors 42 . the sequential energization of the switch 44 and the many switches indicated at 48 , and their precise timing , identifies for the system which phototransistor is generating ( or not generating ) an output pulse 52 of fig3 . the current associated with this pulse 52 flows through the load resistor 49 which has one end connected to a reference voltage - v . this causes the voltage level at the end of the load resistor 49 to rise producing the output pulse 52 . fig3 shows an idealized positive going output pulse such as might be recovered from the phototransistor 42 when the beam is not interrupted . however , as mentioned above , because the phototransistor output lead 46 extends a considerable distance across the printed circuit board , for example printed circuit boards 34 and 36 of fig1 a great deal of electromagnetic noise is picked up and added to the phototransistor output signal . this is indicated for example in fig4 of the drawings where the phototransistor signal 52 may be relatively small , in the order of ten to fifteen millivolts , while the noise signals 54 may be substantially greater and may for example be even 100 to 150 millivolts . in order to substantially cancel out these undesired noise signals , a differential operational amplifier 56 is used with the signal from the phototransistor output lead 46 fed through a high pass filter circuit , which includes load resistor 49 , series capacitor 58 and resistor 60 , to the negative (-) input terminal of the amplifier 56 ; and a compensating signal equivalent to the noise signal components picked up on the noise compensating pickup lead 62 is coupled to the positive (+) input terminal of operational amplifier 56 through a high pass filter circuit which includes load resistor 65 and series connected capacitor 64 and resistor 66 . the compensation pick - up lead 62 indicated schematically at 62 in fig1 is a long electrical conductor extending for substantially the full length of the circuit boards 34 and 36 , commensurate in extent with the phototransistor output circuit 46 , as indicated at the left in fig2 . the lead 62 can be in the form of a wire , a printed circuit , plating , planar etch , etc . the end of the lead 62 has a load termination circuit 63 equivalent to a phototransistor 42 and a switch 48 coupled to it . this equivalent load circuit 63 can be in the form of a capacitor and a diode connected in parallel with one another . accordingly , the noise signals picked up on lead 62 results in current flow through load resistor 65 which will be substantially the same as the electromagnetic noise signals on phototransistor output lead 46 . as a result of the two equivalent and balanced input circuit branches to the operational amplifiers 56 , these two sets of noise signals will substantially cancel one another out , while leaving only the desired characteristics of the phototransistor output signal 52 to be amplified and shaped by the time it arrives at test point tp - 2 as shown in fig5 and 6 . it is also noted that the high pass filter including series capacitor 58 , and resistor 60 and the load resistors connected in shunt to the capacitor discriminates against the relatively slowly changing levels of ambient light which will be received by the photodiodes and which might otherwise vary the output signal from the operational amplifier 56 . for example , in one embodiment the high pass filter would pass 4 % of the signal at 400 cps ( cycles per second ), 40 % at 2000 cps , and 90 % at 6000 cps . the circuit formed by capacitor 64 , resistor 66 , and load resistor 65 in the compensation pickup lead branch is equivalent to the high pass filter . attention is also directed to the diode 72 in the feedback loop of the operational amplifier 56 . when the signal received by the differential amplifier (-) input terminal is negative relative to a reference level , the diode is forward biased , the feedback resistance is very low , and the gain at the operational amplifier 56 is very low whereupon the negative input signal is not amplified . however when the input to the (-) input terminal of operational amplifier 56 is positive , the diode 72 is back biased raising the feedback resistance and the gain of the operational amplifier 56 whereupon the input signal is amplified . a first test point , designated tp - 1 in fig2 is the point at which noisy signals such as those shown in fig4 may be observed . similarly the representative signal of fig5 at test point tp - 2 was clamped by diodes 75 and 77 ( which do not conduct negative pulse signals or signals of more than + 0 . 7 volts such as might be caused by switching transients and noise ) and was amplified by operational amplifiers 56 and 74 . the actual signal with some slight residual uncompensated noise , is shown in fig6 . the diode 75 also serves to provide a path of conduction when the feedback diode 72 is forward biased . the resistor 79 connected to clamping diode 77 serves to terminate the input to the (+) terminal of operation amplifier 56 and to aid the circuit in comming up to operation when the power is turned on . incidentally both of the operational amplifiers 56 and 74 as shown in fig2 may , for example , be of the types known as tl072 or ls353 operational amplifiers , and which are available from a number of manufacturers . incidentially , the comparators 80 and 82 to be discussed below may , for example , be of type lm393 . the high and low voltages for the system may , by way of example but not limitation , be either the commonly available plus and minus five volts or plus and minus twelve volts . however it has been determined that the higher the voltages , within limits , the more effective the circuit becomes . the comparators 80 and 82 each have one input terminal (-) connected to receive the amplified output pulse from operational amplifier 74 and the other input terminal (+) coupled to receive reference signals vref - 1 and vref - 2 respectively . thus an output pulse from comparator 80 is fed to the d input terminal of flip - flop 76 after it is processed and limited by the clamping diode 81 and pull up resistor 83 . incidently , the flip - flops 76 and 78 may , for example , be of the type ls74 . similarly the output of comparator 82 is fed to the d input terminal of flip - flop 78 after it is processed and limited by clamping diode 85 and pullup resistor 87 . in operation the hysteresis portion of the circuit of fig2 which includes flip - flops 76 and 78 evaluates the pulse level on the output lines from comparators 80 and 82 during gate pulse or clock pulse led and stores it as data depending upon the levels of the signals . for example if the levels of the signal on these two output lines are both higher in amplitude than the reference voltages vref - 1 and vref - 2 ( noninterrupted beam ) such as at time t 1 in fig6 or both lower in amplitude than the referenced voltages vref - 1 and vref - 2 ( interrupted beam ) such as at time t 3 that state of the data is stored in the flip - flops 76 and 78 and used as the output data q and q on output lines 92 and 94 . if however the state of two signals from comparators 80 and 82 are not both low or both high relative to the reference voltages vref - 1 and vref - 2 , such as at time t 2 ( as might occur if a beam is only partially interrupted ), the states of the flip - flops typically stored by an external data receiver ( not shown ) from the preceding scan or beam would typically be used . similarly a hysteresis circuit can be implemented with a single j - k flip - flop 98 such as an ls 109 in the manner illustrated in fig7 . in this embodiment , if the inputs to the j input and the k input to the flip - flop 98 are both low , the output q is low . if however the signal to the j and k inputs are both high the output q is high . however if the signal of the j input is low and the signal to the k input is high the output signal q remains in its stored state from the preceding beam . moreover if the signal of the j input is high and the input signal to the k input is low the flip - flop cannot toggle and the output q will remain in the same state it was in from the preceding beam . consequently , if the finger should move only slightly , such as might occur as a result of a slight finger tremor , and result in pulses such as the times t 2 , t 3 , t n + 1 and t n + 2 shown in fig6 the state of the circuit of fig2 will indicate or , in the case of fig7 will remain stable in response to such slight irregular movements . at the same time the circuit will be able to rapidly detect the desired finger movements . of course such transient conditions could also be caused by partial eclipsing of the beam by a finger , low level uncompensated electromagnetic noise signals , and slight variations in the level of ambient light . in addition while the photodetector circuit of fig2 shows one type of circuit it is possible to use another type illustrated in fig8 such as load resistors 100 and 102 coupled between the collector of each phototransistor 42 and in common to the lead 62 . the emitters of phototransistors 42 are switched to ground to select the desired beam . these load resistors 100 and 102 would be tapped with a reference voltage + v : such as 5 volts , with the last load resistor tapped to ground . the collectors of each of the phototransistors 42 are ganged to the lead 46 so that a negative going pulse is produced in response to an unblocked beam when the switches 48 are sequentially closed . while the circuit of fig2 has been described as generating and processing positive going phototransitor output pulses 52 it is possible to handle negative going output pulses 52 that would be produced by the circuit of fig8 by reversing the polarity of clamping diodes 75 and 77 and feedback diode 72 . in summary , in the appended drawings and the foregoing detailed description , one preferred embodiment of the invention has been described . it is to be understood that minor variations in the implementation of the invention are contemplated . for example , alternative forms of output circuitry , and circuits for the differential combining of the signal and compensating noise voltage signals , may be utilized ; and the invention is applicable to other systems including parallel output photosensitive elements . accordingly , the present invention is not limited to that precisely as shown in the drawings and described hereinabove .