United States Patent
`
`[191
`
`[11] Patent Number:
`
`5,087,825
`
`1118111th
`[45] Date of Patent:
`Feb. 11, 1992
`
`
`lllII1111111111111111111111111
`U5005087825A
`
`[75]
`
`[54] CAPACITY RESPONSIVE KEYBOARD
`,
`.
`,
`Inventor: Ronald D. Ingraham, Qumcy1'Mlch
`.
`.
`‘
`.
`.
`[73] Ass‘gnee‘ Sign?“ C°"P°"t‘°“’Rced CW1
`”3 -
`21 A 1N ., 480 93
`1
`_pp
`0
`’2
`[
`[22] Filed:
`Feb. 15, 1990
`
`Int. C1.s ................... H0111 47/00, HOIH 35/00
`[51]
`{52] US. Cl, ............................. 307/132 EA; 307/116;
`,
`307/132 R; 200/511; 361/280
`[58] Field of Search ................... 307/116,125, 132R,
`307/132 13,132 EA, 132 M, 139;200/5 41,511,
`512, 6“), DIG. 1‘ 361/280' 178/18‘ 341/22 33
`’
`’
`’
`’
`References Cited
`
`[56]
`
`llSPATENTDOCUMENTS
`2/1957
`2,783,308
`Rug .................................. 379/167
`6/1962
`3,040,178
`.. 250/213 A
`Lyman et a1.
`
`8/1965
`..
`Atkins et a1.
`361/179
`3,200,304
`
`8/1965
`..
`Atkins et a1.
`361/179
`3,200,305
`
`8/1965
`..
`Atkins et a].
`361/179
`3,200,306
`
`..
`Atkins et a].
`331/111
`5/1966
`3,254,313
`
`9/1966
`..
`Atkins et a].
`361/181
`3,275,897
`
`Adelson ...................
`307/631
`12/1970
`3,549,909
`
`2/1972
`3,641,410
`Vogelsberg ..............
`388/830
`
`3/1972
`3,651,391
`Vogelsberg
`318/466
`5/1972
`Bellis .........
`315/208
`
`3,666,988
`3/1974
`Hurst .......... 178/ 18
`3,798,370
`
`11/1974
`Foster .......
`. 341/33
`3,846,791
`
`8/1975
`Barium et a1.
`.
`.. 315/34
`3,899,713
`
`Hurst ct al. ............... 178/18
`10/1975
`3,911,215
`
`Alexander ............
`340/527
`6/1976
`3,965,465
`
`Gott et a].
`323/326
`10/1976
`3,984,757
`
`4/1977
`4,016,453
`Moenning .
`315/312
`
`H012 ..........
`307/116
`6/1977
`4,031,408
`Jordan ..........
`200/5 A
`11/1977
`4,056,699
`
`1/1978
`4,071,689
`Talmage et a1.
`.
`..... 178/ 18
`
`5/1978
`Serrano .
`307/116
`4,090,092
`Stone .............. 315/74
`7/1978
`4,101,805
`
`Grimes et a}.
`..... 165/14
`7/1978
`4,101,886
`
`Petrizio ........
`307/116
`10/1978
`4,1 19,864
`Lewis ........
`200/600
`10/1978
`4,123,631
`Waldron ..............
`307/308
`1/1979
`4,136,291
`3/1979
`4,145,748
`Eichelberger et a].
`307/125
`5/1979
`4,152,629
`Raupp ..................
`315/362
`6/1979
`Senk .....................
`340/565
`4,159,473
`7/1979
`4,161,766
`Castleberry et a].
`............... 361/280
`
`..
`
`(List continued on next page.)
`Assistant Examiner—Jeffrey A. Gaffin
`Attorney, Agent, or Firm—Price, Heneveld, Cooper,
`DeWitt & Litton
`
`[57]
`
`ABSTRACT
`
`A capacity responsive keyboard which eliminates the
`necessity for metal films on the touch surface includes
`an insulated substrate having a plurality of electrically
`conductive plate members mounted thereon and a di-
`electric member positioned over the plate members. A
`piece of compressible, electrically-conductive foam is
`compressed between each plate member and the dielec-
`tric member to eliminate air from the electrical path
`between the dielectric member and the plate members.
`A grid of conductor segments separates the plate mem-
`bers defining a guard band to reduce interference be-
`tween the switches. The guard band is connected with
`one power supply line of a switching circuit which
`produces outputs in response to capacity changes
`caused by a user touching the dielectric member.
`
`18 Claims, 2 Drawing Sheets
`
`APPLE 1025
`
` 1
`
`4,174,517 11/1979 Mandel ........................... 340/310A
`
`7/1980 Wem ................ 3o7/116
`4,210,822
`..... 315/362
`4,211,959
`7/1980 Dcavenport
`
`
`7/1980 Conner ...........
`.307/116
`4,213,061
`4,220,815
`9/1980 Gibson etal...
`178/18
`
`9/1980 Grimes eta]. ..
`. 340/500
`4,223,301
`
`
`4,237,38612/1980 Instance ........... 307/116
`4,237,421 12/1980 Waldron .......... 307/116
`
`..... 323/349
`4,246,533
`1/1981 Chiang
`
`4,257,117
`3/1981 Besson ....................
`..307/116
`4/1981 Wem ..........................
`.. 307/638
`4,264,831
`
`9/1931 wcm ,,,,,,,,,,,,,,,, 307/116
`4,239,972
`4,289,980 9/1981 McLaughlin ..........
`.307/308
`
`4,290,052
`9/1981 Eichelbergeretal.
`341/33
`
`4,293,987 10/1981 Gottbrecht .............
`....29/25.42
`
`.. 219/1055 B
`4,304,976 12/1981 Gottbrecht .....
`......
`219/10493
`4,308,443 12/1981 Tucker ct a1.
`
`....... 318/55
`4,323,829 4/1982 Whitney eta].
`
`.. 307/116
`4,360,737 11/1982 Leopold ..........
`43u3m ywm Ngafl”
`sun“
`
` 4,380,040 4/1983 Posset ......
`
`4,394,643 7/ 1983 Williams .
`4,400,758 8/ 1983 Frame .....
`
`................:"361/288
`
`APPLE 1025
`
`1
`
`

`

`
`PageZ
`
`5,087,825
`
`U.S. PATENT DOCUMENTS
`
`........................... 341/33
`:11.
`4,413,25211/1983 Tyler Ct
`..200/5A
`4,439,647
`3/1984 Calandrello eta].
`
`4,476,463 10/1984 Ng a a1,, ,,,,,,,,,,,,,
`340/712
`4,493,377
`1/1985 Giimher et a1.
`173/170
`
`1/1985 Smith ................
`4,495,485
`341/33
`7/1985 Hilsum c181. ....................... 340/635
`4,529,968
`4.535.254
`8/1985 Khatri ................................... 307/38
`.. 341/33
`4,550,310 10/1985 Yamaguchi ct a1.
`.
`
`4,561,002 12/1985 Chui .....................
`.. 341/26
`...... 341/33
`4,567,470
`1/1986 Yoshikawa eta]
`
`....323/245
`4,584,519 4/1986 Gruodis ............
`4,614,937 9/9186 Poujois ................ 341/33
`
`...................... 341/26
`4,651,133 3/1987 Gancsan ct a1.
`
`3/1988 Ingraham ............................ 307/116
`4,731,548
`3/1988 Grabneretal.
`.. 361/280
`4,731,694
`
`.. 200/517
`4,736,190 4/1988 Fiorella .......
`4/1988 Brown ................................. 340/712
`4,740,781
`4,743,895
`5/1988 Alexander ........................... 340/712
`4758.735
`7/1988 Ingraham
`.- 307/116
`4,831,279
`5/1989 Ingrahamn
`~307/116
`4,855,550
`8/1989 Schultz, Jr..
`.. 200/600
`4,894,493
`1/1990 Smith c1a1..
`.200/5A
`4,901,074 2/1990 Sinn et a1.
`341/22
`4,920,343 4/1990 Schwartz ........
`341/33
`
`4,924,222
`5/1990 Antikidis et a1.
`.200/600
`5,012,124 4/1991 Holloway ............................ 307/116
`,,
`Prime/y Examiner—A. D. Pellinen
`
`
`
`2
`
`

`

`US. Patent
`
`Feb. 11, 1992
`
`Sheet 1 of2
`
`5,087,825
`
`— n““ -— ss“~— .“‘-— .“‘~—
`Wfllmmaymm
`
`
`2616
`
`
`28
`
`FIG.2
`
`IG.
`
`1 F
`
`3
`
`

`

`US. Patent
`
`Feb. 11, 1992
`
`Sheet 2 of 2
`
`5,087,825
`
`.5 on 115
`
`VAC
`
`62
`
`64
`
`63
`
`NEUTRAL
`
`1;:
`
`ll;:4r--1
`
`§§\_-
`
`4
`
`

`

`1
`
`CAPACITY RESPONSIVE KEYBOARD
`
`BACKGROUND OF THE INVENTION
`
`5,087,825
`
`10
`
`15
`
`20
`
`25
`
`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 of a plurality of such capacity
`responsive switches.
`Touch-responsive switches in the form of “capacitive
`glass" are common in the appliance field. This special
`glass has conductive elements fired on each side in a
`manner that forms a pair of series—connected capacitors
`having input and output terminals on the glass surface
`facing away from the user. A common plate 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 nodnput
`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 US. Pat.
`No. 4,308,443, issued to Tucker et al.
`Such “capacitive-glass" switch systems are not truly
`capacity responsive but require physical contact by the
`user with the electrically-conductive common plate of 30
`the series capacitors. This requires a metal film, such as
`tin oxide, be deposited on the outer surface of the glass.
`Such metal film 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 US. Pat. Nos. 4,731,548 and 4,758,735 I dis-
`close touch control switch circuits which respond to a
`change in 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.
`SUMMARY OF THE INVENTION
`
`35
`
`45
`
`The present 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 member and individ-
`ual conductive members. The transmission members are
`sized to provide airtight interfaces with the dielectric
`member and 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 overcome by 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 band is 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 touch plate
`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 may include 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 adjacent pairs 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 and is positioned to overlie one of the plate members
`18. An indicia layer 30 may be adhered to surface 32 of
`dielectric member 26 which faces substrate 16. The
`purpose of indicia layer 30 is to provide an indication of
`the function of each input portion 13.
`
`5
`
`.
`
`5
`
`

`

`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 member 26,
`it has been found to be necessary to exclude substan-
`tially all of the air from the electrical path between plate
`members 18 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 US. 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 divider is
`established between a capacitor 47 and the capacity 42
`of the user touching one input portion 13. Capacitor 47
`extends between one power line 61 and the base 52 of a
`PNP transistor 50. Thus, when a user is 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. When an 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 causes transistor
`50 to conduct, which provides an input signal on line 57
`to a microcomputer 80. Microcomputer 80 has an out-
`put port 90 connected through resistor 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 current to load 69 is either
`started, stopped, increased or decreased.
`In order to accommodate close spacing between indi-
`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 not result 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 band 20 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
`
`4
`and guard band 20 etched from one conductive surface
`thereof. Dielectric member 26 is preferably polycarbon-
`ate in order to reduce its susceptibility to breakage. In a
`preferred embodiment, the thickness of dielectric 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 may be 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 requirement is that
`air be eliminated from the interface between transmis-
`sion members 28 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 overcomes the 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 overcomes this 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 power sources of the con-
`trol circuit, allows a close spacing of the individual
`switch members.
`
`'
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`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:
`
`45
`
`50
`
`1. A capacity responsive keyboard system compris-
`ing:
`'
`
`55
`
`65
`
`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 of said transmission members being
`a compressible conductive polymer extending be-
`tween said dielectric member and one of said 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
`ground is 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.
`5. The keyboard system in claim 1 further including
`an indicia layer adhered to said surface of said dielectric
`member facing 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-
`mg:
`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 of said 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
`member and 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 changes in capacity between
`said plate members and ground.
`8. The keyboard system in claim 7 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 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 having a 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 when the capacity between the associated one
`of said plate members and ground is changed.
`
`5,087,825
`
`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 connected to said reference volt—
`age.
`11. The keyboard system in claim 7 further including
`an indicia layer adhered to said surface of said dielectric
`member facing said plate members.
`12. The keyboard system in claim 7 wherein said
`dielectric member is 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
`member and 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 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 one of said 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.
`‘
`t
`t
`t
`t
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`55
`
`65
`
`7
`
`

`

`
`
`UNITED STATES PATENT AND TRADEMARK OFFICE
`
`CERTIFICATE OF CORRECTION
`
`PATENT N0.
`
`:
`
`5,087,825
`
`DATED
`
`: February 11, 1992
`
`INVENTUFHS): 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 -—.
`
`Signed and Sealed this
`
`Twenty-sixth Day of October, 1993
`
`Am:
`
`6mg W
`
`BRUCE LEI‘LMAN
`
`
`
`Cummissioner of Patents and Trademarks
`Attesfing Ofi‘ICEI‘
`
`
`8
`
`