EENERECRIEE
`US005087825A
`[11] Patent Number:
`{45} Date of Patent;
`
`5,087,825
`Feb. 11, 1992
`
` .
`
`United States Patent
`Ingraham
`
`1:5)
`
`[56]
`
` .
`
`
`
`4,174,517 11/1979 Mandel ecsecsssesssessenee 340/310 A
`[54] CAPACITY RESPONSIVE KEYBOARD
`4,210,822
`7/1980 Wem secccoesevevesesser 307/116
`4,211,959
`7/1980 Deavenport
`ose 315/362
`[75]
`Inventor: Ronald D. Ingraham, Quincy,’ Mich.
`
`7/1980 Conner...........
`.
`4,213,061
`
`9/1980 Gibson etal. ..
`4,220,815
`178/18
`[73] Assignee: Nartron Corporation, Reed City,
`
`..
`"340/500
`4,223,301
`9/1980 Grimes et al.
`Mich.
`
`4,237,386 12/1980 Instance ...scscccsssssetsecseccneeees 307/116
`:
`
`4,237,421 12/1980 Waldron ..scsccssccsssssccceeneees 307/116
`[21] Appl. No.: 480,293
`
`scssssssssssseesneeee 323/349
`4,246,533
`I/1981 Chiang...
`[22] Filed:
`Feb. 15, 1990
`
`3/1981 Besson wesscccssseessseeees
`. 307/116
`4,257,127
`
`4/1981 WeIT ...ssssssssssssscceronees
`.. 307/638
`4,264,831
`[51] Unt, CLS wees HO01H 47/00; HO1H 35/00
`
`9/1981 WET cessssccssscscssesscssnerereenerens 307/116
`4,289,972
`{52} U.S. C1. cece cccsseeeeeeees 307/132 EA; 307/116;
`
`4,289,980 9/1981 McLaughlin..........
`. 307/308
`307/132 R; 200/511; 361/280
`9/1981 Eichelbergeret al.
`wa 341/33
`4,290,052
`[58] Field of Search ...........0..0 307/116, 125, 132 R,
`
`4,293,987 10/1981 Gottbrecht.............
`wave 29/25.42
`307/132 E, 132 EA, 132 M, 139; 200/5 A, 511,
`.. 219/10.55 B
`4,304,976 12/1981 Gottbrecht.....
`$12, 600, DIG. 1; 361/280; 178/18; 341/22, 33
`
`4,308,443 12/1981 Tucker etal.......
`wee 219/10.493
`
`oes 318/55
`4,323,829 4/1982 Whitney et al.
`References Cited
`
`4,360,737 11/1982 Leopold ..........
`.. 307/116
`4374381
`2/1983 Neetal.
`.
`340/711
`U.S. PATENT DOCUMENTS
`4,380,040 4/1983 Posset......
`361/278
`2,783,308 2/1957 Rug cssscsssessssssssssecsecsssseseseeses 379/167
`4,394,643 7/1983 Williams .
`wa. 341/33
`.. 250/213 A
`3,040,178
`6/1962 Lyman etal.
`
`
`8/1983 Frame occssssssssccsssssssccceeneeees 361/288
`4,400,758
`8/1965 Atkins etal. ..
`we 361/179
`3,200,304
`
`(List continued on next page.)
`..
`.» 361/179
`3,200,305
`8/1965 Atkinset al.
`
`8/1965 Atkins etal. ..
`3,200,306
`
`Assistant Examiner—Jeffrey A. Gaffin
`5/1966 Atkinsetal. ..
`331/111
`3,254,313
`
`Attorney, Agent, or Firm—Price, Heneveld, Cooper,
`3,275,897 9/1966 Atkinset al.
`..
`"361/181
`
`DeWitt & Litton
`3,549,909 12/1970 Adelson sccsecnsone
`307/631
`
`
`3,641,410 2/1972 Vogelsberg sesscsn-n
`... 388/830
`ABSTRACT
`(57)
`3/1972 Vogelsberg...
`"3187466
`3,651,391
`A capacity responsive keyboard which eliminates the
`.. 315/208
`5/1972 Bellis .........
`3,666,988
`
`
`necessity for metal films on the touch surface includes
`3/1974 HUrst ..---coseercessserenssecsnnnercnes 178/18
`3,798,370
`
`;
`;
`;
`4
`3,846,791 11/1974 Foster.......
`. 341/33
`an insulated substrate having a plurality of electrically
`3399713 8/1975 Barkan et al.
`.
`
`Conductive plate members mounted thereon and a di-
`3,911,215 10/1975 Hurst et al...
`“"" y7g/ig
`
`electric memberpositioned overthe plate members. A
`6/1976 Alexander.........
`"340/527
`3,965,465
`
`
`....3,984,757 10/1976 Gott et al. 323/326-Piece of compressible, electrically-conductive foam is
`
`
`315/312
`4,016,453 4/1977 Moenning .
`compressed between each plate memberand the dielec-
`
`.. 307/116
`6/1977 Holz..........
`4,031,408
`tric memberto eliminate air from the electrical path
`
`
`4,056,699 11/1977 Jordan..........
`.- 200/5 A
`between the dielectric member and the plate members.
`
`1/1978 Talmageet al.
`...--s.sssssseene eAte
`4,071,689
`A grid of conductor segments separates the plate mem-
`von 307/11
`4,090,092
`5/1978 Serrano .
`bers defining a guard band to reduce interference be-
`
`4,101,805 7/1978 Stone on. cecceeescceeseteneenees 315/74
`:
`:
`:
`A1O1,886 7/1978 Grimes et al.
`ceccsccssessseereen 165/14
`tween the switches. The guard band is connected with
`4,119,864 10/1978 Petrizio........
`.. 307/116
`One power supply line of a switching circuit which
`4,123,631 10/1978 Lewis .......
`"300/600
`produces outputs in response to capacity changes
`4,136,291
`1/1979 Waldron..............
`.-. 307/308
`caused by a user touching the dielectric member.
`4,145,748 3/1979 Eichelberger etal. ..
`wee 307/125
`4,152,629
`5/1979 Raupp ........een
`wee 318/362
`4,159,473 6/1979 Senk ....ccescseereee
`... 340/565
`4,161,766
`7/1979 Castleberry etal. osc. 361/280
`
`
`
`18 Claims, 2 Drawing Sheets
`
`1
`
`APPLE 1025
`
`APPLE 1025
`
`1
`
`

`

`
`Page 2
`
`5,087,825
`
`U.S. PATENT DOCUMENTS
`
`4,413,252 11/1983) Tyler et al. ccc 341/33
`w. 200/5 A
`4,439,647
`3/1984 Calandrelloetal.
`
`4,476,463 10/1984 Neg etal. vce
`vee 340/732
`
`w.. 173/170
`4,493,377
`1/1985 Giintheret al.
`
`veces 341/33
`1/1985 Smith
`4,495,485
`7/1985 Hilsum et al. wus 340/635
`4,529,968
`8/1985 Khatri... ccccccsccssssecsssseeeees 307/38
`4,535,254
`4,550,310 10/1985 Yamaguchi et al.
`.
`.. 341/33
`4,561,002 12/1985 Chui ou. eecseccsseeees
`.. 341/26
`4,567,470
`1/1986 Yoshikawa et al.
`cesses 341/33
`4,584,519 4/1986 Gruodis............
`wee 323/245
`4,614,937 9/9186 Poujois .....ccccsccccsseccsssesesssees 341/33
`4,651,133 3/1987 Ganesan et al. oe 341/26
`
`
`
`
`3/1988 Ingraham ........cccsssseseeses 307/116
`4,731,548
`3/1988 Grabneretal.
`.. 361/280
`4,731,694
`
`.. 200/517
`4,736,190 4/1988 Fiorella .......
`4/1988 Brown ..sesscsessosssseeeecateeseess 340/712
`4,740,781
`4,743,895
`5/1988 Alexander .....-.csssccsecsees 340/712
`4,758,735
`7/1988 Ingraham ....
`. 307/116
`4,831,279
`5/1989 Ingraham ....
`307/116
`4,855,550
`8/1989 Schultz, Jr. .
`200/600
`4,894,493
`1/1990 Smith etal. .
`200/5 A
`4,901,074 2/1990 Sinn etal. ...
`341/22
`4,920,343 4/1990 Schwartz ........
`341/33
`4,924,222
`5/1990 Antikidis etal.
`. 200/600
`§,012,124 4/1991 Holloway «nsec: 307/116
`-
`Primary Examiner-—A. D. Pellinen
`
`
`
`
`
`2
`
`

`

`U.S. Patent
`
`Feb. 11, 1992
`
`Sheet 1 of 2
`
`5,087,825
`
`1 F
`
`FIG.2
`
`28
`
`
`
`PARV¥74.¥/7ARL2
`RSSORAESe
`
`
`IG.
`
`3
`
`

`

`U.S. Patent
`
`Feb. 11, 1992
`
`Sheet 2 of 2
`
`5,087,825
`
`12
`
`47
`
`54
`
`PHASE
`
`B
`
`45
`
`.
`
`51
`
`(ae
`Ott)
`Lo 53
`
`57
`
`rc
`
`!a
`
`N
`
`ainWh
`
`13
`18
`
`I{
`wie
`Thts
`
`Wa
`
`20
`
`13
`
`18
`
`20
`
`r
`
`18
`
`ot.
`wT
`o
`3S3-_
`
`x
`
`
`
`COMPUTER
`
`t
`A
`D
`
`62
`
`64
`
`63
`
`115
`VAC
`
`69
`
`ame
`NEUTRAL
`
`4
`
`

`

`1
`
`CAPACITY RESPONSIVE KEYBOARD
`
`3,087,825
`
`10
`
`20
`
`25
`
`BACKGROUNDOF THE INVENTION
`This invention relates to switches that respond to the
`change in capacity from a user touching a surface por-
`tion of the switch. The invention is especially adapted
`for a keyboard made up ofa plurality of such capacity
`responsive switches.
`Touch-responsive switchesin the form of “capacitive
`glass” are common in the appliance field. This special
`glass has conductive elements fired on each side in a
`mannerthat formsa pair of series-connected capacitors
`having input and output terminals on the glass surface
`facing away from the user. A commonplate of the two
`capacitors is formed on the outer glass surface facing
`the user. A high frequency oscillator applies pulses to
`the capacitors. A switching circuit connected with the
`capacitors interprets the presence of pulses as a no-input
`condition. When a user touches the capacitor plate on
`the outer surface of the glass, the high frequency pulses
`are shunted to ground through the user. The switch
`circuit interprets the absence of pulses as a positive
`input condition and responds by actuating an output
`device. Such a switch system is illustrated in U.S. Pat.
`No. 4,308,443, issued to Tuckeretal.
`Such “capacitive-glass” switch systems are not truly
`capacity responsive but require physical contact by the
`user with the electrically-conductive commonplate of 30
`the series capacitors. This requires a metal film, such as
`tin oxide, be deposited on the outer surface of the glass.
`Such metalfilm is subject to abrasion by repeated clean-
`ing and may even abrade to the point of erratic opera-
`tion. A further problem with such system is that a fail-
`ure of the high frequency oscillator causes a no-pulse
`condition, to which the switching circuit may detrimen-
`tally respond by energizing one or more outputs.
`In my U.S. Pat. Nos. 4,731,548 and 4,758,735 I dis-
`close touch control switch circuits which respond to a
`changein the capacity-to-ground of a plate member as
`a result of contact by the body of a user. Such circuits
`do not require a metal film on the surface portion con-
`tacted by the user so the problem of metal film abrasion
`is avoided. Further, the circuits disclosed in my patents
`are not subject to the catastrophic failure of erroneous
`output switching caused by the failure of an oscillator.
`SUMMARYOF THE INVENTION
`
`Thepresent invention is directed to a keyboard sys-
`tem incorporating a plurality of capacity responsive
`switches in a keyboard assembly that may be used with
`a control circuit that embodies the principles of my
`aforementioned patents. The present invention may be
`embodied in a capacity responsive keyboard system
`having a planar substrate with a plurality of electrically
`conductive plate members mounted thereto and insu-
`lated from each other. A substantially rigid planar di-
`electric member overlies the conductive members and
`has a surface facing the conductive members. A plural-
`ity of flexible electrically conductive transmission mem-
`bers extend between the dielectric memberand individ-
`ual conductive members. The transmission members are
`sized to provide airtight interfaces with the dielectric
`memberand the associated plate member. Circuit means
`connected with the plate members respond to capacity
`between the plate members and ground for producing a
`
`55
`
`65
`
`2
`signal indicative of a change in capacity between plate
`members and ground.
`Another problem overcomeby the present invention
`is the tendency of capacity responsive switches that are
`closely positioned in a keyboard system to inadvertently
`become actuated even though the user is touching an
`adjacent switch. This problem is overcome by a key-
`board system which embodies the present invention and
`in which an electrically conductive guard bandis pro-
`vided on the substrate between adjacent plate members.
`The guard band is connected with a reference voltage
`of the control circuit to isolate the capacity change
`caused by the user to only one switch. These and other
`objects, advantages and features of this invention will
`become apparent upon review of the following specifi-
`cation in conjunction with the drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG.1 is an exploded perspective view of a capaci-
`tive responsive keyboard embodying the invention;
`FIG. 2 is a side elevation of the keyboard in FIG. 1,
`and
`FIG. 3 is a schematic diagram of a control circuit
`useful with the invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`Referring now specifically to the drawings, and the
`illustrative embodiments depicted therein, a capacity
`responsive keyboard system 10 includes a touchplate
`assembly 12 and a control circuit 14 connected with.
`touch plate assembly 12 (FIG.3). FIG.3 illustrates a
`plurality of input portions 13 of touch plate assembly 12
`and a control circuit 14 that actuates a single load 69
`such as an electric motor. However,it should be under-
`stood that the keyboard system 10 mayinclude a greater
`or lesser number of input portions 13 than illustrated,
`depending on the number and variety of modes in
`which load 69 may be controlled. Additionally, more
`than one load 69 may be controlled. Touch plate assem-
`bly 12 includes a substrate 16 on which a plurality of
`electrically conductive plate members 18 are mounted
`on one surface thereof. Substrate 16 is an insulator and
`plates 18 are spaced apart in order to insulate plates 18
`from one another and from ground. Also positioned on
`substrate 16 is a guard band, generally shown at 20.
`Guard band 20 is a grid of conductor segments 22 ex-
`tending between adjacentpairs of plate members 18. All
`conductor segments 22 are physically and electrically
`interconnected to define a plurality of spaces 24 with
`one plate member 18 positioned centrally within each
`space 24. Components of control circuit 14 may be
`positioned on the side of substrate 16 opposite plate
`members 18 and guard band 20 (FIG.2).
`A planar dielectric member 26 is spaced from sub-
`strate 16 facing plate members 18. Dielectric member 26
`is made from a non-porous insulating material such as
`polycarbonate or glass. A plurality of flexible, electri-
`cally conductive transmission members 28 are sand-
`wiched between a surface 32 of dielectric member 26
`and substrate 16. Each transmission member 28 has a
`cross section approximately the size of plate members
`18 andis positioned to overlie one ofthe plate members
`18. An indicia layer 30 may be adhered to surface 32 of
`dielectric member 26 which faces substrate 16. The
`purpose ofindicia layer 30 is to provide an indication of
`the function of each input portion 13.
`
`5
`
`“
`
`5
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`

`

`5,087,825
`
`3
`Touch plate assembly 10 is adapted to providing a
`capacity interface with a user. When a user touches
`outwardly-facing surface 34 of dielectric member 26,
`the capacity-to-ground for the corresponding plate
`member 18 is increased substantially, as illustrated by
`capacitor 42 in FIG. 3. Because the dielectric constant
`of air is much greater than that of dielectric member26,
`it has been found to be necessary to exclude substan-
`tially all of the air from the electrical path between plate
`members18 and dielectric member 26. This is the func-
`tion of flexible transmission members 28. In the illus-
`trated embodiment, transmission members 28 are made
`from compressible, conducting polymeric foam and are
`dimensioned to be under compression when dielectric
`member 34 and substrate 16 are positioned as illustrated
`in FIG. 2. Because transmission members 28 are under
`compression,air is excluded from the interface between
`the transmission members and their respective plate
`members 18 and between the transmission members and
`surface 32 of dielectric member 26.
`A detailed description of control circuit 14 is pro-
`vided in U.S. Pat. No. 4,731,548, issued Mar. 15, 1988 to
`Ronald Ingraham, the disclosure of which is hereby
`incorporated herein by reference. Although a complete
`description of the operation of circuit 14 will not be
`repeated herein, suffice it to say that a voltage divideris
`established between a capacitor 47 and the capacity 42
`of the user touching one input portion 13. Capacitor 47
`extends between one powerline 61 and the base 52 of a
`PNPtransistor 50. Thus, when a useris not touching the
`particular touch input portion 13, capacitor 47 pulls
`base 52 of transistor 50 to a high level which reverse-
`biases the base-emitter junction of the transistor. Thus,
`transistor 50 will not be conducting. Whenan individual
`touches input portion 13, the voltage level on base 52
`drops sufficiently to establish a forward-biased base
`emitter junction for the transistor. This causestransistor
`50 to conduct, which provides an inputsignal on line 57
`to a microcomputer 80. Microcomputer 80 has an out-
`put port 90 connected throughresistor 96 to the gate 65
`of a triac 67. Triac 67 is connected in series with a load
`69 that is proportional to the phase angle provided by
`microcomputer 80 with respect to line 61. The signal
`produced on output port 90 is determined by the pro-
`gram logic of microcomputer 80. Thus, when a user
`touches an input portion, the currentto load 69 is either
`Started, stopped, increased or decreased.
`In order to accommodate close spacing betweenindi-
`vidual switches in touch plate assembly 12, a guard
`band 20 is provided to electrostatically separate plate
`members 18 from each other. All conductor segments
`22 which make up the grid-like arrangement of guard
`band 20 are electrically interconnected and are con-
`nected to emitter 51 of transistor 50 which,in the illus-
`trated embodiment, coincides with power source line
`61. With this guard band arrangement, the increase in
`capacity for one plate member 18 does notresult in a
`corresponding increase in the capacity of adjacent plate
`members 18. There is no requirement for an electrically
`conductive transmission member between guard band
`20 and dielectric member 26. However, one or more
`isolation resistors similar to resistors 44, 46 that isolate
`plate member 18 for ensuring safety, may also be pro-
`vided between guard band20 and emitter 51. Guard
`band 20 may also be extended to the physical space,
`Separating lines 57 extending to microcomputer 80.
`In the illustrated embodiment, substrate 16 is a con-
`ventional printed circuit board with plate members 18
`
`'
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`15
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`20
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`25
`
`30
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`35
`
`40
`
`45
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`50
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`55
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`65
`
`4
`and guard band 20 etched from one conductive surface
`thereof. Dielectric member26is preferably polycarbon-
`ate in order to reduceits susceptibility to breakage. In a
`preferred embodiment, the thickness ofdielectric mem-
`ber 26 is 0.1" but the thickness may extend up to one-
`half and even three-quarter inches. Glass is a suitable
`alternative to polycarbonate. Compressible, conductive
`transmission members 28 may be made from any com-
`pressible open-cell or closed-cell polymeric foam in
`which a high percentage of carbon particles are mixed
`with the binder. Such foams are commercially available
`and are sold by Wescorp under the WESTAT foam
`trademark, Model Series W-2700.
`In the illustrated
`embodiment, indicia layer 30 is screened and dried on
`surface 32 of dielectric member 36 but may optionally
`be applied to outer surface 34. Transmission members
`28 maybe electrically connected with the correspond-
`ing plate members 18 or may contact a conformal coat-
`ing covering the entire surface of substrate 16 facing
`dielectric member 26. The primary requirementis that
`air be eliminated from the interface between transmis-
`sion members28 and substrate 16. It has been found that
`the response of the keyboard system may be “tuned” by
`adjusting the values of capacitor 47 for each input por-
`tion 13 to provide equal sensitivity.
`The present invention overcomesthe difficulties of
`assembling true capacity-responsive switching devices
`in a keyboard assembly. The ability to eliminate an
`air-entrapped interface between the pad members and
`the dielectric member in a keyboard presents planar
`alignment problems because of the multitude of switch
`members. The present invention overcomesthis diffi-
`culty without the necessity of physically attaching ter-
`minals to the back of the dielectric member. Further-
`more, the use of a guard band around the pad members
`and connected to one of the powersources of the con-
`trol circuit, allows a close spacing of the individual
`switch members.
`Changes and modifications to the specifically de-
`scribed embodiments can be carried out without depart-
`ing from the principles of the invention which is in-
`tended to be limited only by the scope of the appended
`claims, as interpreted according to the principles of
`patent law, including the doctrine of equivalents.
`The embodiments of the invention in which an exclu-
`sive property or privilege is claimed are defined as
`follows:
`1. A capacity responsive keyboard system compris-
`ing:
`.
`a substrate having a plurality of electrically conduc-
`tive plate members mounted thereto,. said plate
`members being insulated from each other;
`a substantially rigid dielectric member overlying said
`plate members having a surface facing said plate
`members;
`a plurality of electrically conductive transmission
`members, each ofsaid transmission members being
`a compressible conductive polymer extending be-
`tween said dielectric memberand oneofsaid plate
`members in order to provide an airtight interface
`with said dielectric member and with the associ-
`ated one of said plate members; and
`circuit means connected with said plate members and
`responsive to capacity between said plate members
`and ground for providing a signal indicative of a
`change in capacity between at least one of said
`plate members and ground.
`
`6
`
`6
`
`

`

`5
`2. The keyboard system in claim 1 in which said
`circuit means includes a plurality of voltage responsive
`switch means, each of said switch means having an
`input connected with one of said plate members and
`means responsive to a change in voltage of said input
`for producing a signal.
`3. The keyboard system in claim 2 further including a
`reference voltage and at least one capacitor having a
`first terminal connected to said reference voltage and a
`second terminal connected to one said input such that
`the voltage across said one capacitor produced by said
`reference voltage is changed when the capacity be-
`tween the associated one of said plate members and
`groundis changed.
`4. The keyboard system in claim 3 further including
`an electrically conductive guard member extending
`between at least two of said plate members, said guard
`member being connected to said reference voltage.
`§. The keyboard system in claim 1 further including
`an indicia layer adhered to said surface of said dielectric
`memberfacing said plate members.
`6. The keyboard system in claim 1 wherein said di-
`electric member is made from polycarbonate.
`7. A capacity responsive keyboard system compris-
`ing:
`a planar substrate having a plurality of electrically
`conductive plate members mounted thereto, said
`plate members being insulated from each other;
`a substantially rigid planar dielectric member overly-
`ing said plate members and having a surface facing
`said plate members and spaced a predetermined
`distance therefrom;
`a plurality of electrically conductive transmission
`members between said dielectric member and said
`planar substrate, each ofsaid transmission members
`made from compressible conductive polymer and
`extending between said dielectric member and one
`of said plate members, each of said transmission
`members having a length greater than said prede-
`termined distance such that said transmission mem-
`bers will be compressed between said dielectric
`memberand said plate members; and
`circuit means connected with said plate members and
`responsive to the value of capacity between said
`plate members and ground for producing output
`signals in response to changesin capacity between
`said plate members and ground.
`8. The keyboard system in claim 7 in which said
`circuit meansincludes a plurality of voltage responsive
`switch means, each of said switch means having an
`input connected with one of said plate members and
`means responsive to a change in voltage of said input
`for producing an output signal.
`9. The keyboard system in claim 8 further including a
`reference voltage and at least one capacitor associated
`with each of said switch means and havinga first termi-
`nal connected to said reference voltage and a second
`terminal connected to the input of the corresponding
`said switch means such that the voltage across said one
`capacitor produced by said reference voltage is
`changed whenthe capacity between the associated one
`of said plate members and ground is changed.
`
`25
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`5,087,825
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`6
`10. The keyboard system in claim 9 further including
`an electrically conductive guard member extending
`between each adjacent pair of said plate members, said
`guard members being connectedto said reference volt-
`age.
`11. The keyboard system in claim 7 further including
`an indicia layer adheredto said surface of said dielectric
`member facing said plate members.
`12. The keyboard system in claim 7 wherein said
`dielectric memberis made from polycarbonate.
`13. A capacity responsive keyboard system compris-
`ing:
`a planar substrate having an isolation grid thereon,
`said grid including a plurality of interconnected
`intersecting electrical conductor segments defining
`spaces between said conductor segments;
`a plurality of electrically conductive plate members
`mounted to said planar substrate within said grid
`with one of said plate members in each of said
`spaces between conductor segments;
`a substantially rigid planar dielectric member overly-
`ing said plate members and having a surface facing
`said plate members and spaced a predetermined
`distance therefrom;
`a plurality of electrically plate transmission members
`between said dielectric member and said planar
`substrate, each of said transmission members made
`from compressible conductive polymer and ex-
`tending between said dielectric member and one of
`said plate members, each of said transmission mem-
`bers having a length greater than said predeter-
`mined distance such that said transmission mem-
`bers will be compressed between said dielectric
`memberand said plate members; and
`circuit means connected with said plate members and
`said isolation grid for producing distinct output
`signals in response to changes in capacity between
`each of said plate members and ground,said circuit
`means including a reference voltage, said isolation
`grid being connected with said reference voltage.
`14. The keyboard system in claim 13 in which said
`circuit means includesa plurality of voltage responsive
`switch means, each of said switch means having an
`input connected with one of said plate members and
`means responsive to a change in voltage of said input
`for producing one ofsaid distinct output signal.
`15. The keyboard system in claim 14 further includ-
`ing at least one capacitor associated with each of said
`switch means and having a first terminal connected to
`said reference voltage and a second terminal connected
`to the input of the corresponding said switch means
`such that the voltage across said one capacitor pro-
`duced by said reference voltage is changed when the
`capacity between the associated one of said plate mem-
`bers and ground is changed.
`16. The keyboard system in claim 13 further includ-
`ing an indicia layer adhered to said surface of said di-
`electric member facing said plate members.
`17. The keyboard system in claim 13 wherein said
`dielectric member is made from polycarbonate.
`18. The keyboard system in claim 13 wherein said
`dielectric member is made from glass.
`*
`*
`s
`*
`«
`
`7
`
`

`

`PATENTNO.
`
`:
`
`5,087,825
`
`DATED
`
`: February 11, 1992
`
`INVENTOR(S): Ronald D.
`
`Ingraham
`
`It is certified that error appears in the above-identified patent and that said Letters Patent is hereby
`corrected as shown below:
`
`Column 6, Claim 13, Line 25;
`
`After "electrically" insert
`
`-- conductive --,
`
`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE OF CORRECTION
`
`
`
`Signed and Sealed this
`
`Twenty-sixth Day of October, 1993
`
`ArestWSsnceChma
`
`Altesting Officer
`Commissioner of Patents and Trademarks
`
`
`BRUCE LEHMAN
`
`8
`
`