United States Patent
`
`[19]
`
`Serrano
`
`[54] ELECTRICALLY INTEGRATED TOUCH
`INPUT AND OUTPUT DISPLAY SYSTEM
`
`[75]
`
`Inventor:
`
`Juan de J. Serrano, Seymour, Conn.
`
`[73] Assignee: General Electric Company,
`Louisville, Ky.
`
`[21] Appl. No.: 69,001
`
`[22] Filed:
`
`Aug. 23, 1979
`
`Int. Cl.3 ................................................ G06F 3/14
`[51]
`[52] US. Cl. ................................ 340/712; 340/365 C;
`340/365 VL
`[58] Field of Search ............ 340/712, 365 C, 365_VL,
`340/756, 765
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,207,905
`9/1965 Bray .................................... 340/712
`3,322,485 5/ 1967 Williams .............. 340/784
`
`
`9/1973 Barkan et al. ............... 340/712
`3,757,322
`7/1976 Challoner et a1. .............. 340/712
`3,971,013
`
`4,017,848 4/1977 Tannas, Jr. ...................... 340/712
`
`5/1978 Bagley ................. 340/712
`4,078,257
`
`...... 340/712
`4,112,429 9/ 1978 Tsuha et al.
`..
`
`...... 340/712
`4,121,204 10/1978 Welch et al.
`..
`1/1980 Holz .................................... 340/712
`4,186,392
`
`Primary Examiner—Marshall M. Curtis
`Attorney, Agent, or Firm—Bernard J. Lacomis; Radford
`M. Reams
`
`[57]
`
`ABSTRACT
`
`A user touch input and output display system which
`combines elements of a capacitive attenuator type touch
`
`[11]
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`[45]
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`4,290,061
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`Sep. 15,‘ 1981
`
`switch and a display into a single system. In one em-
`bodiment type, one of the electrodes of the display
`performs double duty as a rear pad, for example a trans-
`mitter pad, of a capacitive attenuator type touch switch.
`The other rear pad necessary fer operation of a capaci-
`tive attenuator touch switch may either be within the
`same chamber or envelope as the display electrodes, or
`may be outside the chamber. The display may for exam-
`ple be a gas discharge display, and the shared electrode
`does double duty as the display anode and a touch
`switch transmitter pad. Alternatively, the display may
`be a liquid crystal display. In embodiments where the
`other rear pad is within the chamber, to avoid noise
`interference to the touch switch caused by the gaseous
`discharge,
`it is preferable to operate the system in a
`display mode and in a touch switch mode at different
`times. During the touch switch mode, the display anode
`which also serves as the touch switch transmitter pad is
`supplied with an effectively AC signal. During the
`display mode, the display anode may be supplied with
`either a pulsating or a steady DC voltage. In another
`embodiment type, a liquid crystal display is driven by an
`AC voltage source having a high series impedance, and
`a display electrode is capacitively coupled to a touch
`pad. When a user touches the pad, sufficient signal is
`shunted to ground to drop the voltage across the dis-
`play electrode below that necessary to maintain visible
`energization.
`
`30 Claims, 11 Drawing Figures
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`US. Patent
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`Sep. 15, 1981
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`US. Patent
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`Sep. 15, 1981
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`Sheet 3 of3
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`4,290,061
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`A76. 9
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`

`ELECTRICALLY INTEGRATED TOUCH INPUT
`AND OUTPUT DISPLAY SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to input/output systems
`which conveniently combine or
`integrate a touch
`switch for data entry and a display device for data out-
`put.
`It has long been recognized that in many control
`applications it is desirable to combine a display such as
`an indicator light, with a switch actuating element es-
`sentially superimposed over the display. One common
`approach is to provide a display lamp or the like and a
`more or less transparent switch superimposed over the
`lamp. Advantages of such an approach include immedi-
`ate feedback to the user at the physical location and
`moment of contact, savings in required panel area
`where input and output devices can be colocated, and
`potential savings in cost where one .device serves the
`functions of two devices at less than the total cost of
`two devices.
`Examples of devices wherein data entry switches and
`display devices are integrated in various manners and to
`various degrees are provided by the following US. Pat.
`Nos.
`3,207,905—Bray;
`3,757,322—Barkan
`et
`al.;
`4,017,848—Tannis,
`Jr.;
`4,078,257—Bagley;
`4,112,429—Tsuha et al and 4,121,204—Welch et al. In
`each of the devices disclosed in these patents, the data
`input switches and the display output elements are gen-
`erally separately identifiable devices with merely a
`physical connection between the two.
`Other integrated entry/display Systems have been
`disclosed wherein there is electrical interaction between
`the data input elements and the display output elements.
`One example of this general approach is a technique
`wherein an electrically conductiVe area (such as a
`NESA spot) is applied to the outer envelope of a cold
`cathode gaseous discharge lamp such as a neon lamp.
`An exciting voltage just below that which is necessary
`to trigger conduction is applied to the lamp electrodes.
`With the display device thus just, ready to trigger, an
`external electric field disturbance occasioned by the
`proximity of a finger to the NESA spdt triggers con-
`duction in the display. In addition to merely providing
`a visible indication,
`the conduction may be used to
`signal other circuitry that the spot has been touched.
`Thus a single device serves as both a switch and a dis-
`play.
`Another example of an integrated entry and display
`device with electrical interaction between the display
`and a touch switch, and one which is particularly rele-
`vant in the context of the prese'nt‘invention, is disclosed
`in the Challoner et al U.S. Pat. No. 3,971,013. The Chal—
`loner et al device includes an ACdriven gas discharge
`panel display, and at least one conductive touch key
`located proximate the display to pick up an AC voltage
`from a stray field generated by the gas discharge panel
`display. The Challoner et a1 device further includes an
`electrical connection to the conductive touch key for
`the purpose of detecting a drop in the AC voltage level
`of the key which occurs due to the proximity of an
`operator’s finger.
`SUMMARY OF THE INVENTION
`
`It is an object of the present invention'to provide an
`effective integrated entry/display system where an
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`electrical interaction between a display device and a
`data input touch switch is employed to advantage.
`It is another object of the invention to provide such
`an integrated entry/display system wherein a savings in
`operating circuitry as well as physical structure of the
`entry/display device may be realized through the shar-
`ing of common elements.
`Briefly stated, and in accordance with one aspect of
`the invention, there is provided an integrated entry/dis-
`play system including a dielectric panel and a display
`having at least one electrode adjacent one side, for
`example a rear side, of the dielectric panel. In the illus-
`trated embodiments, the display is shown both as a gas.
`discharge display, and as a liquid crystal display. Also
`adjacent the one side of the dielectric panel, but electri-
`cally insulated from the one display electrode, is a ca-
`pacitive attenuator touch switch rear pad. A capacitive
`attenuator touch switch front pad is~adjacent the other
`side (front side) of the dielectric panel and arranged for
`capacitive coupling through the panel with both the one
`display electrode and the touch switchvrear pad. Suit-
`able circuitry is provided for operating the display by
`applying a suitable signal to the one display electrode;
`and additionally for operating the one display electrode,
`the touch switch rear pad and the touch switch front
`pad all together as a capacitive attenuator touch switch
`by applying an effectively AC exciting voltage to one of
`the electrodes on the one side (rear side) of the dielec-
`tric panel'and sensing the resultant signal capacitively
`coupled .to the other of the electrodes on the rear side of
`the dielectric panel. To achieve capacitive attenuator
`touch switch operation, the circuitry operates to detect
`a decrease in the coupled signal when the touch switch
`front pad is touched.
`’
`Thus, the one display electrode serves dual functions
`as an element on the display and as an element of the
`touch switch. Preferably, this display element is the
`anode of a gas discharge display-and also serves as the
`transmitter pad of a capacitive attenuator touch switch.
`In the event the display is a gas discharge display, it
`may have a single anode and a plurality of cathodes, for
`example seven cathodes arranged in a configuration
`known as a seven segment display for the purpose of
`displaying numerical digits.
`.
`The capacitive attenuator touch switch rear pad may
`be either positioned-inside the same sealed, gas-filled
`envelope or chamber as the electrodes for the display,
`or it may be located outside the gas-filled envelope.
`This is largely a matter of choice of fabrication tech-
`nique, but in cases where the touch switch rear pad is
`outside of the gas-filled chamber, the problem of mini-
`mizing interference of the gas discharge display with
`the capacitive input touch switch is minimized.
`In accordance with another aspect of the invention,
`in cases where operation of the display interferes with
`the touch switch, as likely would be the case where the
`touch switch rear pad is positioned within the gas—filled
`chamber which, when the display is operating, contains
`ionized gas, the capacitive attenuator touch switch is
`utilized for presence detection at times when informa-
`tion is not being displayed and the display is therefore
`quiescent.
`In accordance with another aspect of the invention,
`applicable in particular to a liquid crystal display, the
`display is driven by an AC voltage source having a high
`series impedance. One of the display electrodes is ca-
`pacitively coupled to a touch pad such that when the
`touch pad is. touched sufficient signal
`is shunted to
`
`5
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`

`4,290,06l
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`3
`ground to drop the display voltage below that neces-
`sary to maintain visible energization.
`The present invention therefore effectively integrates
`both a capacitive input touch switch and a display into
`a single system, making possible savings both in panel
`area and in driving circuitry.
`BRIEF DESCRIPTION OF THE DRAWINGS
`While the novel features of the invention are set forth
`with particularly in the appended claims, the invention,
`both as to organization and content, will be better un-
`derstood and appreciated, along with other objects and
`features thereof, from the following detailed descrip—
`tion, taken in conjunction with the drawings, in which:
`FIG. 1 is a front panel View of a prior art seven seg-
`ment gas discharge display device as it appears to a user;
`FIG. 2 is a cross-sectional view taken along line
`11—11 of FIG. 1;
`FIG. 3 is a cross—sectional view of elements of a ca-
`pacitive attenuator type touch switch;
`FIG. 4 is a view taken along line IV—‘IV of FIG. 3
`showing the capacitive attentuator type touch switch as
`it appears from the front of a panel;
`FIG. 5 is a cross-sectional View of an embodiment of
`the present invention which advantageously combines
`elements of a prior art gas discharge display and a prior
`art capacitive attenuator-type touch switch;
`FIG. 6 is a cross-sectional view of another embodi-
`ment of the invention illustrating one particular ap-
`proach to minimizing the problem of noise interference
`to the capacitive touch switch portion of the device
`caused by operation of the display portion of the device;
`FIG. 7 is an electrical schematic diagram showing the
`entry/display device of FIG. 5 connected to suitable
`operating circuitry;
`FIG. 8 illustrates electrical waveforms during opera-
`tion of the circuitry of FIG. 7;
`-
`FIG. 9 is a view similar to FIG. 6, but illustrating an
`embodiment wherein the display portion of the device
`is a liduid crystal display;
`FIG. 10 is a cross-sectional view of another embodi-
`ment including a liquid crystal display, with a slightly
`different principle of operation for touch input; and
`FIG. 11 is an electrical schematic diagram of the
`equivalent circuit when the embodiment of FIG. 10 is
`touched.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`Referring first to FIGS. 1 and 2, there is shown by
`way of example a typical prior art gas discharge display
`10 of the type which may comprise a portion of a sys-
`tem embodying the present invention. While the partic-
`ular display 10 illustrated is of the seven segment type
`adapted for displaying numerals, it will be appreciated
`that the precise form of display is not at all important
`with respect to the present invention, and a display
`having only a single visible element may as well be
`employed. Further, it will be understood that the pres-
`ent invention is not limited to any particular type? of
`display, and the gas discharge display 10 is shown only
`for purposes of convenient example.
`The gas discharge display 10 includes a transparent
`dielectric panel 12, which preferably is glass. Spaced
`below one side 14 (the rear side) of the panel 12 is a
`dielectric sheet 16 which preferably also is glass for
`convenience in fabrication, but actually need not be
`transparent. Between the transparent dielectric panel 12
`
`4
`and the dielectric sheet 16 is a gas-filled envelope or
`chamber 18 suitably sealed around the edges thereof by
`means of a conventional glass frit seal 20.
`Within the gas-filled chamber 18, adjacent the one
`side 14 of the dielectric panel 12, is a display electrode
`22, which typically is an anode element to which a
`relatively positive potential is applied. The electrode 22
`is transparent. To achieve transparency, the electrode
`22 may, for example, comprise either a relatively thin
`coating of a conductive substance such as tin oxide or
`wire mesh. A lead 24 is provided for connection to the
`display electrode 22, and extends through the dielectric
`sheet.
`Also within the gas-filled chamber 18 are a plurality
`of cathode electrodes 26a-26g which define the visible
`portion of the display and which are spaced from the
`anode electrode 22. The cathode electrodes 260—26g
`have individual leads 28. As is well known in the display
`art, the gas-filled chamber 18 is filled with a suitable
`ionizable gas or mixture of gases such as neon or kryp-
`ton, at relatively low pressure. When a sufficiently high
`voltage is applied between the anode electrode 22 and
`one or more of the cathodes 260—2655 a visible glow
`occurs in the immediate vicinity of energized cathode
`electrodes. By selectively connecting the individual
`cathodes 260—26g to a source of negative potential,
`various energization and thus display patterns can be
`achieved, as is well known in the art.
`Referring now to FIGS. 3 and 4, essential elements of
`a conventional prior art capacitive attenuator-type
`touch switch 30 include a dielectric panel 32, which
`may or may not be transparent and preferably is glass, a
`touch pad 34 on the front surface 36 of the panel 32, and
`a pair of rear pads 38 and 40 on the rear surface 42 of the
`panel 32. The physical arrangement is such that there is
`capacitive coupling of the front pad or touch pad 34
`through the dielectric panel 32 with both of the rear
`pads 38 and 40.
`,
`For the operation of the capacitive attenuator touch
`switch such as the prior art touch switch 30, one of the
`rear pads, for example the rear pad 38, is designated the
`transmitter (T) pad, and the other rear pad, read pad 40
`in this example, is designated the receiver (R) pad. In
`operation, an effectively AC driving signal is applied to
`the transmitter pad 38 via a connecting lead 44, and a
`level detection circuit is connected to the receiver pad
`40 via a connecting lead 46. So long as the touch pad 34
`is not touched, signals are coupled through a first capac-
`itor comprising the transmitter pad 38 and the touch
`pad 34 and through a second capacitor comprising the
`touch pad 34 and the receiver pad 40, to appear on the
`lead 46. When the touch pad 34 is touched, a portion of
`this signal is shunted to. ground through the body of the
`user, resulting in a detectable decrease in signal coupled
`to the receiver pad 40 and thus available at the lead 46.
`By “an effectively AC signal” is meant a voltage signal
`which changes with time, and this may be either a true
`AC signal which oscillates about zero volts, or a pulsat-
`ing DC signal which, after being capacitively ecupled,
`is in effect an AC signal insofar as the circuitry is con—
`cerned.
`‘
`. As has been recognized, advantages of such capaci-
`tive attenuator touch switches include high reliability
`due to the absence of mechanical switching contacts,
`good appearace, insulation of the user from direct elec-
`trical connection (DC-coupled) with circuitry, and easy
`cleanability.
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`Referring now to FIG. 5, there is shown the panel
`associated portion 50 of one embodiment of an inte—
`grated entry/display system in accordance with the
`present invention. As is explained below, in the embodi-
`ment of FIG. 5, a single electrode 52 serves both as an
`electrode of a display and as a capacitive attenuator
`touch switch rear pad. In the particular embodiment
`illustrated and described by way of example herein, the
`electrode 52 serves as the anode of a gas discharge
`display, and as such may be compared to the anode 22
`of FIGS. 1 and 2. The electrode 52 also serves in the
`particular embodiment illustrated as the transmitter pad
`of a capacitive attenuator type touch switch, and as
`such may be compared to the transmitter pad 38 of
`FIGS. 3 and 4. It will be apparent that this c0nfigura-
`tion permits a saving in physical structure. In addition,
`as is described in detail below with particular reference
`to FIG. 7, a saving resulting from commonality of driv-
`ing circuitry results.
`In addition, the integrated entry/display system por-
`tion 50 of FIG. 5 includes a dielectric panel 54 which is‘
`at least partly transparent, with the electrode 52 adja-
`cent one side 56 (rear side) of the dielectric panel 54. As
`mentioned above, the electrode 52 serves both as one
`electrode of a display, and also as one rear pad of a
`capacitive attenuator touch switch.
`Also adjacent the one side 56 (rear side) of the dielec-
`tric panel 54 is another capacitive attenuator touch
`switch rear pad 58 which is electrically insulated from
`the electrode 52.
`To complete the display section of the system portion
`50, a dielectric sheet 60 is spaced from the rear side 56
`of the panel 54, and sealed by a glass frit seal 62 to form
`a sealed gas-filled chamber 64. Additionally, conven-
`tional display cathodes 66d, 66c, 66g, 66b and 660 are
`carried by the dielectric sheet 60 and are within the
`gas-filled chamber 64 such that when a voltage is ap-
`plied between the display anode electrode 52 and any of
`the cathodes 66, the gas in the vicinity of the cathode
`glows to provide a visible display. A suitable lead 68 is
`provided for the electrode 52, suitable leads 70 for the
`cathodes 66, and a suitable lead 72 for the touch switch
`rear pad 58 are all provided.
`To complete the touch switch portion of the inte-
`grated entry/display device 50 of FIG. 5, a capacitive
`attenuator touch switch front pad 74 is adjacent the
`other side 76 (front side) of the dielectric panel 54 and
`arranged for capacitive coupling through the panel 54
`with both the one display electrode 52 and the touch
`switch rear pad 58.
`A complete system in accordance with the present
`invention also includes suitable circuitry for operating
`the portion 50 of the entry/display system. While one
`example of suitable circuitry is described below with
`particular reference to FIG. 7, a general functional
`description of the circuitry with reference only to FIG.
`5 follows. In particular, the circuitry operates the dis-
`play section by applying a suitable signal to the one
`display electrode 52. In the case of a gas discharge
`display, this signal may be either a steady positive DC
`potential, or may be an alternating or pulsating DC
`signal, preferably at a frequency no higher that 4 kHz.
`In such operation, the voltage applied to the display
`anode electrode 52 is positive with respect to the volt-
`age of the cathodes 66, and is typically in the order of
`180 volts, although series current-limiting resistance is
`required, as is known in the art, due to the negative
`resistance characteristic of a gaseous discharge.
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`The circuitry also provides capacitive attenuator
`touch switch operation by employing the same elec-
`trode 52 which serves as the anode of the display as one
`of the rear pads-of a capacitive attenuator type touch
`switch, and employing the rear pad 58 as the other rear
`pad. Preferably, the electrode 52 serves as the transmit-
`ter pad of the touch switch, with the rear pad 58 then
`' serving as the receiver pad. More specifically, an effec-
`tively AC exciting voltage is applied to the electrode
`52, and the resultant signal which is coupled through a
`first capacitor comprising the electrode 52 and the front
`pad 74 and through a second capacitor comprising the
`front pad 74 and the rear electrode 58 is sensed for the,
`purpose of detecting a decrease in the coupled signal
`when the touch switch front pad 74 is touched, as in
`conventional capacitive attenuator type touch switch
`operation.
`One potential problem with the embodiment of FIG.
`5 where a gas discharge display is involved is that it is
`difficult to achieve simultaneous operation in both the
`display mode and the capacitor attenuator touch switch
`mode for the reason that the touch switch rear pad 58
`comprising the touch switch output element is within
`the electrically noisy environment of the chamber 64
`containing ionized gas. While in accordance with the
`invention, one approach to this particular problem is a
`multiplexing technique wherein the system panel por-
`tion 50 is operated in the display mode and in the capac-
`itor attenuator touch switch mode at different times,
`another approach is illustrated in FIG. 6. The FIG. 6
`embodiment differs from the FIG. 5 embodiment in that
`a touch switch rear pad 58' is outside of the gas-filled
`chamber 64, but still adjacent the one side 56 (rear side)
`of the dielectric panel 54 and still arranged for capaci-
`tive coupling with the touch switch from pad 74. This
`approach greatly alleviates the coupling of noise into
`the rear pad 58, and permits the simultaneous operation
`in the touch switch mode and the display mode, so long
`as the display portion of the system panel portion 50 is
`operated with an effectively AC signal on the anode
`element 52, which signal may be pulsating DC.
`While the embodiment of FIG. 6 includes elements of
`a high voltage gas discharge display, other display types
`may equally well be employed, As one particulary ex-
`ample, elements of a liquid output display may be in-
`cluded instead; This latter approach is illustrated in
`FIG. 9, which is described in detail hereinafter.
`In FIG. 7, is shown one example 80 of circuitry suit-
`able for the present invention where the display is a gas
`discharge display. The particular circuitry illustrated in
`FIG. 7 implements the multiplexing technique referred
`to above. It will be appreciated however that the cir-
`cuitry 80 is only one example, and that numerous modi-
`fications are possible. It will further be appreciated that,
`although the circuitry of FIG. 7 is shown connected to
`the panel portion of the integrated entry/display system
`embodiment of FIG. 5, that it may as well be employed
`with the embodiment of FIG. 6. Additionally, while the
`embodiment of FIG. 7 implements one particular mode
`of operation, specifically a multiplexing technique,
`other operational modes are quite possible. Examples of
`such other operational modes are mentioned below
`following the detailed description of FIGS. 7 and 8.
`It is assumed that the circuitry 80 of FIG. 7 is con-
`nected to a suitable external utilization device (not
`shown) which may be any type of system for which
`inputs and outputs are required. It is further assumed
`that the external utilization device has an output for
`
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`controlling when the display is to operate and what
`numeral is to be displayed, an input responsive to the
`touch switch portion of the integrated entry/display
`system of the invention, and a control output to select
`either a display mode or a touch switch mode of opera-
`tion. As was mentioned above, with the particular em-
`bodiment of FIGS. 5 and 7 wherein the touch switch
`rear pad 58 is located within the gas—filled envelope, it is
`highly preferable that the display portion and the capac-
`itor touch switch portions of the system be operated at 10
`different times.
`In FIG. 7, a switching subcircuit 82 controls the
`application of voltage from a +180 volt source 84 to the
`electrode 52 which serves both as the display anode and
`as a transmitter pad of a capacitive attenuator-type 15
`touch switch. The switching subcircuit 82 includes a
`PNP switching transistor 86 having its emitter con—
`nected to the +180 volt source 84, and its collector
`connected to the lead 68 of the electrode 52. The tran-
`sistor 86 collector is also connected through a pull- 20
`down resistor 88 to circuit ground 90. The base of the
`transistor 86 is connected to a logic level translating
`network comprising resistors 92 and 94 connected in
`series between the +180 volt source 84 and the collec-
`tor of an NPN switching transistor 96, with the base of 25
`the switching transistor 86 specifically connected to the
`junction between the resistors 92 and 94. The emitter of
`the NPN transistor 96 is connected to circuit ground 90,
`and an input line 98 is connected to the base of the
`transistor 96.
`Thus when the voltage on the input line 98 is low,
`both of the transistors 96 and 86 are cut off, and the
`electrode 52 is essentially at circuit ground 90 potential.
`When the line 98 has a logic high voltage, than both
`transistors 96 and 86 conduct, and the voltage on the 35
`electrode 52 is substantially +180 volts.
`In FIG. 7, the display cathodes 66 are connected by
`their individual leads, for example the exemplary leads
`75 and 70’, through current limiting resistors 100 and
`102 to the collectors of representative NPN switching 4O
`transistors 104 and 106. The emitters of the switching
`transistors 104 and 106 are connected to circuit ground
`90, and the bases of these transistors are connected to
`the outputs of a conventional BCD to Seven Segment
`Decoder 108 which receives a BCD INPUT from the 45
`external utilization device (not shown). The exemplary
`transistors 104 and 106 thus comprise as individual con-
`trol elements for selectively connecting corresponding
`display cathode elements 66 to a relatively negative
`potential.
`Connected to the touch switch rear pad 58 which
`serves as a receiver pad is a receiver subcircuit 110
`which serves to output a logic high signal at a Touch
`Switch Signal Output terminal 112 when the touched
`pad 74 is not being touched and touch switch mode 55
`operation is enabled, and to output a logic low signal on
`the Touch Switch Signal Output terminal 112 when the
`touch pad 74 is touched. Specifically, the receiver sub—
`circuit 110 includes an AC buffer amplifier 114 having
`its input connected to the lead 72 from the rear pad 58. 60
`A high resistance 116 is connected between the ampli-
`fier 114 input and circuit ground 90 to stabilize the
`system. A rectifing diode 118 is connected in series
`between the output of the amplifier 114 and a parallel
`RC output network comprising a resistor 120 and a 65
`capacitor 122. This output network serves to rectify and
`smooth the output of the amplifier 114 to produce
`steady output logic levels.
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`Additionally included in the circuit 80 of FIG. 7 is a
`20 kHz multivibrator 124 for generating a pulsating DC
`signal which, after level conversion, produces the effec-
`tively AC signal which is supplied to the electrode 52
`when it is operating as a touch switch transmitter pad.
`While the multivibrator 124 illustrated produces a
`square wave output frequency of 20 kHz, this is not at
`all critical, and the frequency may for example be any-
`where within the range of from 10 to 100 kHz and still
`provide proper operation of the touch switch.
`The output of the 20 kHz multivibrator 124 is con-
`nected to an input of a NAND gate 126 having its out-
`put connected to the line 98 which drives the base of the
`switching transistor 96. To enable the NAND gate 126
`to pass pulses when the touch switch mode of operation
`is desired, a CONTROL INPUT terminal 128 which
`receives a mode control signal from the external utiliza-
`tion device (not shown) is connected to an input of the
`NAND gate 126. Thus when the control input terminal
`128 is at logic high, the NAND gate 126 is enabled to
`pass inverted pulses from the multivibrator 124. Con-
`versely, when the CONTROL INPUT terminal 128 is
`low,
`the NAND gate 126 remains inactive (output
`high), turning on both the transistors 96 and 86 to apply
`a steady +180 volts to the display electrode 52.
`In order to ensure that no conduction occurs within
`the gas-filled chamber 64 during capacitive touch
`switch mode operation, the CONTROL INPUT termi—
`nal 128 is also connected through an inverter 130 to an
`ENABLE input 132 of the BCD to Seven Segment
`Decoder 108. The BCD to Seven Segment Decoder 108
`functions to drive appropriate ones of the display cath—
`ode switching transistors, such as the representative
`transistors 104 and 106, only when the ENABLE input
`132 is high.
`With reference now also to FIG. 8 waveforms, in the
`operation of the circuit 80 of FIG. 7, whenever the
`CONTROL INPUT terminal 128 is low, the display
`mode of operation is selected. In the display mode of
`operation, the NAND gate 126 is inactive, so that its
`output (line 98) is high, both transistors 96 and 86 are
`conducting, and a steady +180 volts DC (0 Hz) is ap-
`plied to the electrode 52, which in this mode of opera-
`tion functions as a display anode. With the CONTROL
`INPUT 128 low, the inverter 130 is activated so that the
`ENABLE input 132 of the BCD to Seven Segment
`Decoder 108 is high, and the BCD to Seven Segment
`Decoder 108 Drives the switching transistors 104 and
`106 and thus the display cathodes 66 in response to
`signals on the BCD INPUT.
`For the touch switch mode of operation, the CON~
`TROL INPUT terminal 128 is high, and the NAND
`gate 126 is thus enabled to pass inverted pulses (line 98)
`from the 20 kHz multivibrator 124, and the voltage on
`the electrode 52 alternates at a 20 kHz rate between
`+ 180 volts and circuit ground potential. In this mode of
`operation, the electrode 52 is operating as the transmit-
`ter pad of a capacitive attenuator touch switch. With
`the CONTROL INPUT terminal 128 high, the inverter
`130 is inactive, the ENABLE input 132 of the BCD to
`Seven Segment Decoder 108 is low, and both transis-
`tors 104 and 106 remain off regardless of the signal on
`the BCD INPUT.
`The receiver subcircuit 110 senses a voltage on the
`touch switch rear pad 58, which functions as a receiver
`pad, to produce a positive (logic high) output voltage
`on the Touch Switch Signal Output terminal 112 when-
`ever the touch pad 74 is not being touched and a signal
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`is therefore capacitively coupled from the transmitter
`pad 52 through the dielectric panel 54 to the touch pad
`74, and from the touch pad 74 again through the dielec-
`tric panel 54 to the receiver pad 58. When the touch pad
`74 is touched, the signal available at the receiver pad 58 5
`is attenuated. Insufficient signal reaches the amplifier
`114, and the Touch Switch Signal Output terminal 112
`goes low.
`~
`In FIG. 8, there is no specified time scale indicating
`the duration of the touch switch mode when the CON- 10
`TROL INPUT terminal 128 is high and the duration of
`the display mode when the CONTROL INPUT 128
`terminal is low. The relative durations of, and the ratios
`between, the operation of these two mo