United States Patent 3696409

A system for identifying a particular position on the face of a display device by simply touching the area with the finger. The area is a conductive pad connected to a relaxation oscillator whose frequency is lowered by the touch. The oscillator is therefore identifiable and a digital computer is enabled to perform a prescribed act such as new information transferred to the display device, dependent upon the location of the area touched.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
178/18.01, 341/33, 345/173
International Classes:
G06F3/033; G06F3/048; H03K17/98; H03M11/00; (IPC1-7): G08C9/02
Field of Search:
340/365C 178
View Patent Images:
US Patent References:
3437795DATA INPUT DEVICES AND SYSTEMS1969-04-08Kuljian

Foreign References:
Primary Examiner:
Habecker, Thomas B.
What is claimed is

1. A touchplate identification system for identifying one of a plurality of touchplates which has been touched by a human being, comprising:

2. A digital computer input-output system including a display device, having a faceplate, connected to the digital computer, comprising:

3. The system of claim 1, wherein the touchplates are transparent and conductive.

4. The system of claim 1, wherein the oscillators are relaxation oscillators each having a capacitor in an RC charge circuit at the input stage and wherein the associated conductor is connected to the input of the oscillator so that any added capacitance is added in parallel to the input capacitor.

5. The system of claim 1, further including a display device having a faceplate wherein the touchplates are transparent and conductive and are attached to the faceplate of the display device.

6. The system of claim 1 wherein each of the plurality of oscillators has essentially the same free-run frequency.


Management systems and teaching systems utilizing a digital computer lend themselves very well to the use of a finger-touch faceplate. It is an excellent, simple manner of communicating with a digital computer where the interface between the operator and the computer has been the subject of much effort over the years to be made simpler.

In the prior art, there have been many efforts made to recognize a particular area on the face of a cathode ray tube display for example by touching the selected area with a conductor, with a "light pen" and with the finger. Using only the finger as a pointer has presented problems which have been successfully overcome in this invention. In prior art devices, the circuitry has been extensive and complex, but most important has not been as reliable as desired. Efforts have been made to detect a position by wire grids over the face of a display device which indicate a position when intersecting wires are pressed together to contact each other. There have also been light grids used to attempt to locate where the operator's finger is placed by interrupting the circuit of a photosensitive device. Another interesting prior art approach to solving the problem is found in U.S. Pat. No. 3,382,588 in which one or more capacitors are made a part of the faceplate of the display and are intentionally of a high leakage character. When the leakage field is interrupted by the operator's finger, the capacitive reactance is changed and a bridge circuit is upset. Another prior art approach is one where, in essence, a transmitter is placed in the area of the faceplate of the display device and the finger acts as an antenna. These devices are all highly complex. Additionally, it has been found that in many of the prior art devices which utilize cathode ray tubes as the display device, that the electron beam of the cathode ray tube causes an undesirable static charge on the areas of the faceplate to be touched.

It will be seen that the invention described herein substantially reduces or eliminates these problems.


A plurality of conductive pads are attached to the face of a display device forming a finger-touch faceplate. Each pad is electrically connected to an oscillator. It has been determined that the human body is basically capacitive. Therefore when a person touches one of the conductive pads, capacity is added in parallel to the capacitance in the RC charge path of a relaxation oscillator. The frequency of the oscillator output is therefore lowered. A recognition of the fact that the frequency has been lowered and of the indentity of the oscillator with which the touched pad is associated results in an exact identification of the pad and the fact that it has been touched by the operator. This knowledge in the form of electric signals can then be transmitted to a digital computer which will respond in a prescribed way. In the preferred embodiment it is ordinarily expected that new information will be presented on the face of the display device replacing that which the operator pointed to through the transparent conductive pad.

In the preferred embodiment, the capacitor in the charge path discharges through a unijunction transistor and then charges again. Electric charge collected on the pad as a result of the electron beam of a cathode ray tube, if a cathode ray tube is used, will be conducted from the pad and discharged through the unijunction transistor, thus keeping the faceplate free of charge.

An object of this invention is to provide an economical and reliable means for selecting a given area on the face of a display device.

Another object is to electronically identify a pad that has been touched by a person.

Another object is to permit a person to touch a desired word or symbol appearing on the face of a display device and have a change in the information displayed as a result of the touch.

Still another object is to prevent the buildup of electric charge on the face of the display device.

These and other objects will become more apparent in the detailed description that follows.


FIG. 1 is a block diagram of the system.

FIG. 2 is a perspective view of the conductive pads installed on a display device and the electronics package.

FIG. 3 is a schematic diagram of the relaxation oscillators used in the invention.

Fig. 3a, fig. 3b and FIG. 3C illustrate waveforms as seen on the face of an oscilloscope at various points within the oscillator of FIG. 3.

FIG. 4 is a logic diagram of an AND circuit made up of a NAND circuit and an inverter.

FIG. 5 is a logic diagram of a typical counter used in this invention.

FIG. 6 illustrates the use of a flip-flop circuit in conjunction with a counter, as typically used throughout this invention.

FIG. 7 is a logic diagram of the select matrix of FIG. 1.

FIG. 8 is a logic diagram of the digital comparator of FIG. 1.

FIG. 9 is an analog representation of the output of eight oscillators with none of the associated pads touched and of the output of eight oscillators with one of the pads touched.


FIG. 1 is a block diagram of the system. The preferred embodiment has 32 touchplates and therefore 32 oscillators. Oscillator 1 and oscillator 32 are shown with respective outputs 33 and 64 connected to select matrix 91. Oscillators 2 through 31 and connections 34 through 63 are not shown to avoid confusion. The select matrix 91 has outputs 92, 93, 94 and 95 which terminate in AND circuit 96.

The system has a master clock 65 whose output 66 is connected to AND circuit 68 which has additional input line 67 and output line 69 which is an input to the A section 71 of the sequence counter 70. An output 72 of the A section 71 is an input to AND circuit 73, which has additional inputs 87 and 88. The output 89 of AND circuit 73 is an input to B section 90 of sequence counter 70. Output lines 74 and 85 from the B section 90 serving as inputs to select matrix 91 are shown. Output lines 75 through 84 from the B section 90 are not shown for the sake of simplicity.

Referring again to AND circuit 68, it can be seen that an additional output line 86 terminates in AND circuit 100 which has additional input line 99. AND circuit 100 has an output line 101 which is connected to P counter 98. The output of AND circuit 96 is line 97 which serves as an input to the P counter 98. Digital comparator 103 has an input 102 from P counter 98 and an input 104 from thumb-wheel threshold 105. Digital comparator 103 has an output 106 connected to output line 107 which serves as one input to AND circuit 110, the other inputs being line 108 and line 109. The output 111 of AND circuit 110 serves as the "clear" input of delay flip-flop 112.

Referring back to digital comparator 103, it can be seen that its output 106 also is connected to input line 134 of inverter 127 whose output line 128 is connected to AND circuit 130. AND circuit 130 has additional inputs 129, 131 and 132, and has an output line 133 which is a "clear" input to Q counter 115 which has an additional input 116.

With reference to delay flip-flop 112, it is apparent that the "clear" output line 114 terminates at AND circuit 119 which has another input line 117 which is an output of Q counter 115. AND circuit 119 has an output line 118 that terminates in the output gate 120 which has inputs 121 through 125 and output lines 126.

FIG. 2 illustrates a typical display device 200 having a screen 197. Attached to screen 197 are transparent, conductive touchplates 133 through 164. The touchplates are typically comprised of metal oxide, well-known in the art and cemented to the face of the display device with transparent cement, also well known. To each of the touchplates is connected a conductor. For example, conductor 165 is connected to touchplate 133, conductor 180 is connected to touchplate 148, conductor 181 is connected to touchplate 149, and conductor 196 is connected to touchplate 164. There is a conductor attached to each of the other touchplates which are not shown. Each conductor is connected to a separate oscillator. For example, conductor 165 is connected to oscillator 1, conductor 180 is connected to oscillator 32. Conductors 165 through 196 are shown generally as entering chassis 198. Chassis 198 contains printed circuit boards 199, 201, 202 and 203. All of the oscillators and other components (not shown) are mounted on the printed circuit boards 199, 201, 202 and 203. The chassis 198 need not be remote from the display device 200.

FIG. 3 is the schematic diagram of oscillator 1 of FIG. 1. It is identical to the other 31 oscillators of the preferred embodiment. Except for the value of capacitor 210 and the deletion of touchplate 133 and conductor 165, it is also identical to master clock 65 of FIG. 1. The transparent, conductive touchplate 133 is connected to the oscillator through conductor 165 at point A. Delaying capacitor 210 is connected through conductor 211 to point A and from its other plate connected to ground through conductor 212. Unijunction transistor 226 has its emitter connected through conductor 225 to paint A. Its base 2 is connected to a positive voltage source through conductor 222, conductor 220, resistor 219, conductor 218, conductor 230 and conductor 231. Its base 2 is also connected to coupling capacitor 224 through conductor 221. Its base 1 is connected to ground through conductor 227, resistor 228 and conductor 229. Variable resistor 216 is connected to the positive voltage source at one side through conductors 217, 230 and 231. At its other end, resistor 216 is connected to resistor 214 through conductor 215. The other side of resistor 214 is connected to point A through conductor 213. The combination of resistors 216 and 214, together with capacitor 210 form the delay circuit for the oscillator.

The other plate of capacitor 224 is connected through conductor 223 to point B. The base of transistor 236 is connected to the positive voltage source through conductors 235 and 234, and through resistor 233 and conductors 232 and 231. The emitter of transistor 236 is connected through conductor 238 to ground. The collector of transistor 236 is connected to the positive voltage source through conductor 237, resistor 239, conductor 240 and conductor 231. The output from transistor 238 is taken from its collector through conductor 241 to point C and out on conductor 243. Point C is kept at a specified positive voltage by diode 245 connected at its anode to point C through conductor 242 and at its cathode through conductor 244 to a positive voltage source.

FIG. 3A is a representation of the voltage waveform at point A of FIG. 3. The waveform 246 is a plot of voltage in the Y direction versus time in the X direction.

FIG. 3B is a representation of the voltage waveform at point B of FIG. 3. Waveform 247 is representative of voltage in the Y direction and time in the X direction.

FIG. 3C is representative of the voltage at point C of FIG. 3. Wave 248 represents voltage in the Y direction and time in the X direction, illustrating the pulse output of the relaxation oscillator of FIG. 3.

Throughout this discussion, the AND circuit is discussed. In practice the NAND circuit is more readily available in integrated circuits. FIG. 4 illustrates AND circuit 68 having NAND circuit 205 with inputs A, B and C and output D. When A, B and C are all "up", D is "down", mathematically expressed as a Boolean equation:

D = A . B . C

A simple inverter 206 whose input is D and whose output is E is connected to the NAND circuit 205 in series. When D is "up", E is "down" and vice versa. Expressed mathematically in Boolean form:

E = D

E = A . B . C

This is readily understood to be a design choice in the use of integrated circuits. FIG. 4 shows a simple AND circuit which is comprised of a NAND circuit and an inverter. All of the AND circuits referred to herein are of similar configuration.

FIG. 5 is a logic diagram of a well-known counter, old in the art. Flip-flops 309, 310, 311 and 312 have set inputs from circuits 301, 303, 305 and 307 respectively. They have "clear" inputs from circuits 302, 304, 306 and 308 respectively. This counter is indicated generally as A section 71 of FIG. 1. Outputs are indicated at 317, 318, 319 and 320 in ascending order. The flip-flops are provided with force-set inputs indicated as 313, 314, 315 and 316. The flip-flops also have force-clear inputs indicated at 322, 323, 324 and 325. The input signal coming from AND circuit 67 of FIG. 1 enters through amplifier 300.

FIG. 6 is a logic diagram showing the B section 90 (FIG. 1) of the sequence counter 70. The lower four stages of the B section are configured exactly the same as the four stages of the A section 71 shown in detail in FIG. 5. The uppermost stage of the B section is comprised of AND circuit 330, whose inputs are the lower section outputs 326, 327, 328 and 329, and flip-flop 333, whose inputs are AND circuit 331 and on the "set" side, AND circuit 332 on the "clear" side and terminal 334 which conditions flip-flop 333. The set output of flip-flop 333 is shown at terminal 335.

Referring now to FIG. 7, the select matrix 91 of FIG. 1 is shown in logic form. Decoder 350 has as its inputs the highest order bit (B5) on line 72 and the next highest bit (B4) on line 73 and an "enable" on line 351 which permits the decoder to operate when the A section of the sequence counter 70 of FIG. 1 is 0, 1, 14 or 15. Decoder 350 has four outputs, line 352 which is connected to multiplexer 356, line 353 which is connected to multiplexer 357, line 354 which is connected to multiplexer 358, and line 355 which is connected to multiplexer 359. The three remaining lower order bits of the B section (B3, B2 and B1) are connected to each of the four multiplexers. The inputs are shown at lines 74, 75 and 76 of multiplexer 356; lines 77, 78 and 79 of multiplexer 357; lines 80, 81 and 82 of multiplexer 358; lines 83, 84 and 85 of multiplexer 359. The output line from each of the oscillators 1 through 32 of FIG. 1 are shown as lines 33 through 64, eight being an input to each of the four multiplexers.

Each of the multiplexers has an output through which the output of a selected oscillator passes. The operation will be described later. Multiplexer 356 has an output 66, multiplexer 357 has an output 67, multiplexer 358 has an output 68 and multiplexer 359 has an output 69. The outputs are connected as the inputs to OR circuit 96 of FIG. 1 whose output is line 97. An OR circuit is simply one that is responsive to any one of a multiplicity of inputs.

The P counter 98 of FIG. 1 is made the of a pair of counters in identical configuration with the sequence counter 70 except that not as many counter stages are used. The operation will be fully described later.

The thumb-wheel threshold 105 of FIG. 1 is a simple rotary thumb-wheel switch, old in the art. In the preferred embodiment, the thumb-wheel threshold 105 has five outputs each of which is adjustable to ground or an open circuit. Line 104 of FIG. 1 represents the five outputs of thumb-wheel threshold 105. In similar fashion, line 102 of FIG. 1 represents five outputs from P counter 98.

FIG. 8 is a detailed logic diagram of digital comparator 103 of FIG. 1. It is well-known in the art and therefore will not be described in detail. Its inputs from the P counter 98 are shown as P6, P5, P4, P3 and P2. The lowest stage of P is not used in this application. Its inputs from the thumb-wheel threshold 105 and shown as TW5, TW4, TW3, TW2 and TW1. The stages are shown generally as 375, 374, 373, 372 and 371. In each stage the inputs are compared to each other and at the output 370 it can be determined whether P is less than TW, whether P equals TW or whether P is greater than TW. In the preferred embodiment, only the output indicated as P is greater than TW is used.

FIG. 9 is of interest only as a graphic illustration of an analog representation of eight of the oscillators. The output of the P counter 98 is run into a digital-to-analog converter (not shown) and the pulse train from each of the eight pads shown scanned may then be viewed on an oscilloscope as an amplitude rather than a frequency. This is old in the art and is only for purposes of illustration.

FIG. 9A shows the resultant amplitude output of eight oscillators with no touchplate (referred to as "pad" in this figure) being touched. FIG. 9B shows the pad for oscillator No. 4 being touched and its representation of frequency now as an amplitude is shown being quite low and below a threshold frequency set by the thumb-wheel threshold switches of thumb-wheel threshold 105.

The delay flip-flop 112 of FIG. 1 is identical to the flip-flop 333 of FIG. 6. It also is old in the art and need not be shown in detail. The Q counter 115 of FIG. 1 is made up of counters identical to the configuration of the sequence counter 70 and therefore need not be described in detail.

The output gate 120 of FIG. 1 is simply five AND circuits which have been described in FIG. 4. Outputs 126 of output gate 120 are available as inputs to any desired device. Typically, the application is to the buffer register of a digital computer. The output is of course not limited to such an application.


FIG. 3 is a detailed schematic diagram of the oscillators 1 through 32 of FIG. 1 as well as master clock oscillator 65 of FIG. 1. As indicated earlier, the difference between the master clock oscillator 65 and all other oscillators 1 through 32 is that the capacitor 210 is of a higher capacitance and there is no touchplate 133 or conductor 165. The relaxation oscillator here shown is of itself old in the art, particularly with reference to the master clock oscillator 65.

Simply stated, in the case of the master clock oscillator 65, adjustable resistor 216, resistor 214 and capacitor 210 form a charging path for capacitor 210. When capacitor 210 is charged to a critical value, the potential thereby appearing at paint A causes the unijunction transistor 226 to conduct providing a discharge path through the unijunction transistor 226, resistor 228, back to the other plate of capacitor 210. The discharge is rapid, dropping the potential at point A resulting in the cutoff of unijunction transistor 226. The charge cycle starts again, resulting in capacitor 210 again becoming charged to cause the conduction of unijunction transistor 226 permitting a discharge path through resistor 228. This is a typical relaxation oscillator. The charge and discharge of capacitor 210 results in the waveform of point A shown in FIG. 3A.

The output of base 2 of unijunction transistor 226 is coupled to point B via capacitor 224 and is shown in FIG. 3B as it appears at point B of FIG. 3. That waveform is inverted and amplified in NPN transistor 236. The output waveform is shown at FIG. 3C which is the collector output at point C of transistor 236. The diode 245 conducts when point C tends to go beyond +5 volts, resulting in a shaped waveform of FIG. 3C. The frequency of the pulses of FIG. 3C may be varied by varying the charge path of capacitor 210 by changing the resistance of variable resistor 216.

The oscillators 1 through 32 are each provided with a touchplate and a conductor as an input. In FIG. 3 the touchplate 133 is connected to the circuit through conductor 165.

It has been determined that the human body is basically capacitive. The capacitance varies with the size of the body but not significantly. When a person touches touchplate 133, capacitance is added in parallel with capacitor 210. This added capacitance results in a larger RC time constant in the charge path. A preselected frequency of output pulses as shown in FIG. 3C is thereby lowered with the addition of capacitance at touchplate 133. The operation as described above for the master clock oscillator 65 is identical to that of oscillators 1 through 32 except that the frequency is lowered by adding capacitance in the latter case. Recognizing the oscillator (and therefore the touchplate) and the fact that its frequency is lowered, is at the heart of this invention.

Referring to FIG. 1, sequence counter 70, P counter 98, and Q counter 115 are shown. In the preferred embodiment, the A section 71 of sequence counter 70 has four binary stages and the B section 90 of sequence counter 70 has five binary stages. The P counter 98 has six binary stages and the Q counter 115 has nine binary stages. As is well-known in the art, each binary stage can be represented by two different voltage amplitudes. An arbitrary selection can be made assigning, for example, a "one" to a voltage and a "zero" to a ground potential. If all four stages are "one" the binary number represented is 1111, but for simplicity it will be referred to in its decimal equivalent as 15. Likewise the B section when filled is 31, the P counter when filled is 63 and the Q counter when filled is 1023. The Q counter 115 is ordinarily set at 1023 because its input 116 depends upon an output from the master clock 65 and the fact that Q=1023.

The master clock 65 produces pulses as shown in FIG. 3C in a manner described above at a rate of 4×103 PPS in this preferred embodiment. These pulses are transmitted over output line 66 to the AND circuit 68. AND circuit 68 is shown in FIG. 4 with the Boolean expressions set out above. When there is a pulse out of master clock 65 on line 66 and when Q=1023 on line 67 the output of the AND circuit 68 on line 69 is active only when both of such inputs are present. Determining that Q=1023 is simply done by appropriate AND circuits logically arrayed in one of a variety of possible configurations, well known in the prior art. Another output 86 of AND circuit 68 goes to AND circuit 100 which has another input 99 requiring that A=0 in order for the output line 101 to be activated which is a "clear input" to P counter 98.

FIG. 5 shows the A section 71 of sequence counter 70 in logic detail. The signal coming in on line 69 is amplified through amplifier 300 and sets the first stage of the counter, defined as circuits 301 and 302, to receive the inputs and flip-flop 309 whose output "A1 " is shown at 317. The reception of another pulse through amplifier 300 toggles the first stage and sets the second stage whose output "A2 " is shown at 318. Another input pulse sets the first stage so that output A1 is again A1. Another pulse toggles the first and second stage and sets the third stage whose "one output" at "A3 " is shown at 319. In like manner the stages are toggled back and forth until all four are "1's". When A=15 (filled) line 72 is activated. AND circuit 73 receives an input from AND circuit 68 and a signal that the digital comparator 103 is "high" as shown on line 88. This will be explained later. Assuming that line 88 has a signal present and assuming that line 87 has a clock pulse, then AND circuit 73 passes a pulse on line 89 to the B section 90 of sequence counter 70. This is merely an extension of the A section 71.

The B section 90 is shown is greater detail in FIG. 6. The four lower order bits of the B section are identical to the configuration of the A section 71 of FIG. 5 and need not be shown. A fifth stage is supplied by a flip-flop 333. The flip-flop 333 is well-known in the prior art. It is conditioned at terminal 334 from line 336 when the lower four stages of the B section 90 are filled (B=15), then the four inputs to AND circuit 330 are activated and the last stage 333 is toggled to a "1" state.

However, prior to the toggling of any of the stages in the B section 90, when B=0, output lines 74 through 85 convey that fact to select matrix 91.

Select matrix 91 is shown in logical detail in FIG. 7. Decoder 350 is a device well-known in the prior art which simply provides the four possible combinations for two binary signals. When B=0, then B5 on input line 72 equals 0 and B4 on input line 73 equals 0. The four possible outputs are B5 . B4, B5 . B4, B5. B4 and B5 . B4. With B5 =0 and B4 =0 then line 355 is activated as an enable input to multiplexer 359. Multiplexer 359 has inputs B3, B2 and B1 on input lines 83, 84 and 85 respectively. The multiplexer 359 is a device well known in the prior art which takes three control inputs and combines those three inputs in the eight possible gating combinations to gate as an output one of the eight inputs. Additionally, the enable input on line 355 is also placed on all eight outputs 57 through 64. The eight possible combinations of B1, B2 and B3 are as follows:

B3 . B2 . B1 B3 B2 B1 B3 . B2 . B1 B3 B2 B1 B3 . B2 B1 B3 . B2 B1 B3 . B2 B1 B3 . B2 . B1

when B=0 output 64 carrying the function B5 . B4 . B3 . B2 . B1 is activated. This permits the oscillator connected to line 64 to send its pulses through the multiplexer and on line 69 to OR circuit 96, out on line 97. The selection of line 64 has been shown for illustrative purposes. Multiplexers 356 through 359 operate in identical fashion on each of the eight input lines except that the conditioning signals from decoder 350 are different. In each case, one line of any one multiplexer can be selected thereby selecting a specific oscillator.

Referring to FIG. 1, P counter 98 accepts the pulses from the selected oscillator from line 97 to P counter and begins counting the pulses from the selected oscillator. The time period during which the pulses are counted is measured by the counting of the clock pulses by the A section 71.

It has been determined that approximately 60 pulses counted in the P curve 98 (high count capacity = 63) is very workable in the preferred embodiment. The master clock 65 operates at 4×103 PPS those pulses being transmitted into the A section 71. However, various counts of A are not available for measuring the time of counting in the P counter 98. When A=0, the P counter 98 is cleared. When A=14, the output of digital comparator 103 is gated out at AND circuit 110 and AND circuit 130. When A=15, AND circuit 73 is enabled, permitting a pulse to go through line 89 into the B section 90. Therefore, the time period measured by the A section 71 is from A=2 to A=13, a count of 12 clock pulses.

T = 1/f


T = period

F = frequency

T = (1×10-3 /4

t = 0.25×10-3

total time (12 pulses) = 12T Total time = 12×0.25×10- 3 Total time = 3×10-3

Since 60 pulses is a good number for counting in the P counter, a minimum frequency is required to provide at least 60 pulses within the count period time of 3×10-3 seconds. The period (T) of one of 60 pulses equals the total time available divided by the number of pulses; therefore:

T = (3×10-3 /60)

t = 5×10-5

f = (1/5)×105 = 20×103 pps.

the oscillators must be able to provide a minimum of 20,000 pulses per second to meet the design requirements of this system.

As stated before, the oscillators 1 through 32 have been designed to produce 20×103 PPS in this preferred embodiment.

Keeping in mind that the A section 71 counts 16 pulses, the actual time spent for counting the pulses of each oscillator is 3×10-3 for the actual count and 1×10-3 seconds for housekeeping details between counts.

Actual scan time = 32×4×10-3 = 128×10-3 seconds

System Frequency = (1×103 /128) = 7.8 cycles per second

As those skilled in the art well know, these parameters are merely indicative of the preferred embodiment but are subject to wide variation dependent upon the components and specific circuit embodied.

Referring again to FIG. 1, the thumb-wheel threshold is set to represent 40 pulses. In the preferred embodiment, thumb-wheel threshold 105 has only five bits and therefore the binary equivalent of 40 cannot be set but the lowest order bit of the binary equivalent of 40 is ignored and the other bits are transmitted into digital comparator 103. Digital comparator 103 is shown in FIG. 8 and the signals from thumb-wheel threshold 105 come in at TW5, TW4, TW3, TW2 and TW1.

As the pulses from the selected oscillator are being counted in P counter 98, the count from the P counter is continually transmitted to digital comparator 103. The P counter has six bits, but only the upper five are used because of the five-bit digital comparator 103. The inputs from the P counter enter the digital comparator 103 as P6, P5, P4, P3 and P2. In FIG. 8 it can be seen that three outputs are available:




in the preferred embodiment, only the P>TW output is used. That is, when the count of the P counter, when A=14, is greater than the count of the thumb-wheel threshold 105, line 106 has a positive pulse present. Assuming that the selected oscillator had not been touched and its frequency remained therefore at 20×103 PPS, 60 pulses would have been counted in the prescribed time in the P counter, such a count being higher than the thumb-wheel threshold 105 setting of 40 resulting in a "comparator high" signal out on line 106. This condition would result in a signal present on line 107 to AND gate 110 enabled by the master clock at 109 and by A=14 at 108. The output of AND circuit 110 is 111 and is a "clear" input to delay flip-flop 112 resulting in a "clear" output on line 114 into AND circuit 119.

Referring again to output 106 of digital comparator 103, when the comparator is "high", a signal is presented through line 134 to inverter 127 which then presents a ground level to line 128 which does not permit enabling of AND circuit 130 and therefore Q counter 115 is not "clear" but remains at a count 1023. The output 117 of Q counter 115 is enabled only when Q=1023 and therefore is not enabled in this situation. Therefore AND circuit 119 does not present a signal to output gate 120 over line 118. As a result, there are no output signals permitted through the output lines 126.

Now assume a person touches the touchplate of the selected oscillator. The operation is identical to that described above except that now the capacitance of the body of the person touching the touchplate is added in parallel to the capacitance 210 of FIG. 3. As described earlier, this results in a reduced frequency output from the selected oscillator. Under such circumstances, when A=14, the count from the P counter will be less than 40 which has been set into the digital comparator 103 from the thumb-wheel threshold 105. When that occurs, the output on line 106 from digital comparator 103 will be grounded and the ground will be present on line 107 to AND circuit 110 thereby disenabling that AND circuit. However, the ground voltage presents itself on line 134 to inverter 127 which inverts that signal to a positive voltage. The positive pulse presents itself to AND circuit 130 which is further conditioned by the master clock at input 129, A=14 at input 132 and Q=1023 at input 131. Therefore, AND circuit 130 is enabled and the "clear" Q output 133 is enabled clearing the Q counter 115. The delay flip-flop 112 is still cleared at this time and therefore line 114 has a positive voltage present. Line 117 now has a positive voltage present and Q=0. Output gate 120 is enabled through line 118 and the appropriate location as indicated by the B section 90 of sequence counter 70 is transmitted to a buffer through output lines 126. The appropriate location or address from the B section 90 comes in through B1, B2, B3, B4 and B5 on input lines 121, 122, 123, 124 and 125 of output gate 120.

Q counter 115 immediately begins to count back to 1023 as a result of input 116 which requires only a pulse from master clock 65 and Q=1023. When Q=1022, delay flip-flop 112 is set through input 113. The clear output 114 is disabled, disabling AND circuit 119. If the person touching the touchplate does not remove his finger after the cycle has been completed, it is assumed that he did not intend touching the same touchplate twice. The preferred embodiment is designed to preclude the possibility of sending the same information through output lines 126 in a successive cycle. Referring to FIG. 1, AND circuit 73 is disabled because the comparator is "low" and therefore input 88 to AND circuit 73 is disabled. Therefore, the B section 90 count remains the same. As the finger is kept on the touchplate for another cycle, the digital comparator remains "low" and Q counter 115 is again cleared as described earlier. However, output line 111 from AND circuit 110 is not enabled and therefore "clear" output 114 remains disabled, AND circuit 119 remains disabled and output gate 120 remains disabled. When the operator's finger is removed and A=14, then once again the output of digital comparator 103 on line 106 will be "high", AND circuit 110 will be enabled, line 111 will be enabled clearing delay flip-flop 112. "Clear" output line 114 will be enabled conditioning AND circuit 119 to be enabled when Q=1023. When this happens, output gate 120 is once more enabled permitting information to pass through output lines 126 to the buffer.

There are many ways to implement this invention. For example, discrete components could be used throughout rather than the integrated circuits used in the preferred embodiment. There are many, many possible select matrixes available. There are also many different kinds of counters which could be used. This invention is not reliant on any one design choice.