U.S. patent number 3,710,209 [Application Number 05/171,640] was granted by the patent office on 1973-01-09 for variable capacitor.
This patent grant is currently assigned to Colorado Instruments, Inc.. Invention is credited to James R. Webb, Richard C. Webb.
United States Patent |
3,710,209 |
Webb , et al. |
January 9, 1973 |
VARIABLE CAPACITOR
Abstract
An electrostatic impulse key suitable for use in electronic
keyboards or as a separate item is described. An individual key
comprises a single or plural metallic target area which may be
formed on one surface of a printed circuit board. A dome spring is
mounted above the metallic target and a foam cylinder is mounted
between the dome spring and an actuator element. The actuator
element is operated by a key mechanically connected thereto. In
addition, a coil spring located about the foam cylinder between the
printed circuit board and the actuator is provided for loading the
actuator and key to a predetermined level prior to actuation
(depression) of the key. When the key is depressed, the actuator
presses the foam cylinder against the upper surface of the dome
spring to cause the dome spring to change position. This position
change changes the separation distance between the dome spring and
the metallic target in a rapid or "snap action" manner. Because the
separation distance is changed, the capacitance between the dome
spring and the metallic target increases to cause a rapid decrease
in a voltage connected between the dome spring and the target. This
voltage decrease is in the form of a signal pulse and is suitable
for use in electronic keyboard subsystems.
Inventors: |
Webb; James R. (Boulder,
CO), Webb; Richard C. (Boulder, CO) |
Assignee: |
Colorado Instruments, Inc.
(Bloomfield, CO)
|
Family
ID: |
22624568 |
Appl.
No.: |
05/171,640 |
Filed: |
August 13, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
13091 |
Feb 20, 1970 |
3653038 |
Mar 28, 1972 |
|
|
Current U.S.
Class: |
200/600 |
Current CPC
Class: |
H01H
13/7006 (20130101); G06F 3/02 (20130101); H01H
2239/006 (20130101) |
Current International
Class: |
H01H
13/70 (20060101); G06F 3/02 (20060101); H01g
005/16 () |
Field of
Search: |
;317/246,249R,261
;340/365C ;200/DIG.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; E. A.
Parent Case Text
This is a continuation-in-part of commonly assigned U.S. Pat.
application, Ser. No. 13,091, filed on Feb. 20, 1970, which issued
on Mar. 28, 1972 as U.S. Pat. No. 3,653,038 and entitled
"Capacitive Electric Signal Device and Keyboard Using Said Device."
In accordance with a notice in Volume 859, page 346 of the Official
Gazette dated Feb. 11, 1969, the subject matter of application,
Ser. No. 13,091 is incorporated herein by reference .
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An electrostatic impulse key comprising:
a target board including an electrically conductive target and an
electrically conductive ground plane surrounding said target and
separated therefrom;
a curved spring mounted on the other side of said electrically
conductive ground plane from said target in a manner such that the
distance between said curved spring and said target can be abruptly
decreased when pressure is applied to the upper surface of said
curved spring; and,
key means for applying pressure to the upper surface of said curved
spring to abruptly decrease the distance between said curved spring
and said target when force is applied to said key means.
2. An electrostatic impulse key as claimed in claim 1 wherein said
target board comprises:
a single-sided printed circuit board having said target formed in
the conductive surface thereof;
an insulating layer covering the surface of said single-sided
printed circuit board in which said target is formed, including
covering said target; and,
a printed circuit board structure having two separated conductive
layers, said printed circuit board structure being aligned over
said single-sided printed circuit board and including an aperture
aligned over said target, said curved spring being mounted on the
conductive surface of said printed circuit board structure most
remote from said target and the other conductive surface of said
printed circuit board structure forming said electrically
conductive ground plane surrounding said target.
3. An electrostatic impulse key as claimed in claim 2 wherein said
insulating layer is a doped glass epoxy film.
4. An electrostatic impulse key as claimed in claim 2 wherein said
insulating layer is an imperfect insulating film.
5. An electrostatic impulse key as claimed in claim 2 wherein the
conductive surface of said printed circuit board structure that
supports said curved spring is highly wear resistant.
6. An electrostatic impulse key as claimed in claim 5 wherein said
wear resistant material is nickel.
7. An electrostatic impulse key as claimed in claim 2 wherein said
printed circuit board structure is a double-sided printed circuit
board.
8. An electrostatic impulse key as claimed in claim 2 wherein said
printed circuit board comprises two single-sided printed circuit
boards arrayed in parallel on said insulating layer with their
conductive surfaces remote from said insulating layer.
9. An electrostatic impulse key as claimed in claim 1 wherein said
key means comprises:
a depressable key;
an actuator attached to said key; and,
a cylinder of foam mounted between said actuator and the upper
surface of said curved spring for transferring force applied to
said depressable key to said curved spring.
10. An electrostatic impulse key as claimed in claim 9 including a
housing mounted about said cylinder of foam and said actuator, said
housing including a cylindrical opening aligned with said cylinder
of foam, said cylindrical opening having a plurality of
semi-circular recesses longitudinally formed in the outer surface
thereof; said actuator including a plurality of semi-cylindrical
legs which fit into the semi-cylindrical recesses in said housing
so as to maintain said actuator in a predetermined alignment.
11. An electrostatic impulse key as claimed in claim 10 including a
coil spring, said coil spring encompassing said cylinder of foam
yet being out of contact therewith, said coil spring being mounted
between said target board and said actuator so that a predetermined
amount of force must be applied by said depressable key to said
actuator prior to said curved spring changing position.
12. An electrostatic impulse key as claimed in claim 11 wherein
said target board comprises:
a printed circuit board having said target formed in the conductive
surface thereof;
an insulating layer covering the surface of said printed circuit
board in which said target is formed, including covering said
target; and,
a printed circuit board structure having two separated conductive
layers, said printed circuit board structure being aligned over
said printed circuit board and including an aperture aligned over
said target, said curved spring being mounted on the conductive
surface of said printed circuit board structure most remote from
said target and the other conductive surface of said printed
circuit board structure forming said electrically conductive ground
plane surrounding said target.
13. An electrostatic impulse key as claimed in claim 1 wherein said
curved spring is dome-shaped.
14. An electrostatic impulse key as claimed in claim 13 wherein the
edges of said dome-shaped spring are clipped to form an equilateral
triangle supported at its corners.
15. A target board suitable for use in an electrostatic impulse key
wherein a key applies pressure to a dome spring to vary the
distance and, thereby, the capacitance between the dome spring and
an electrically conductive target, said target board
comprising:
a target printed circuit board having a target formed of
electrically conductive material;
an insulating layer covering the surface of said target printed
circuit board in which said target is formed, including covering
said target; and,
a printed circuit board structure having two separated conductive
layers, said printed circuit board structure being aligned over
said target printed circuit board and including an aperture aligned
over said target, one of said conductive layers of said printed
circuit board structure located to support said dome spring and the
other conductive layer of said printed circuit board structure
forming an electrically conductive ground plane that surrounds said
target.
Description
BACKGROUND OF THE INVENTION
The referenced application as well as this invention relate
generally to impulse keys of an electrostatic or capacitive nature
that are suitable for use in electronic keyboards or as separate
switching elements. Reference is made to for a discussion of
electronic keyboards and the various disadvantages of prior art
impulse keys used therein.
While the electrostatic (capacitive) electric signal devices or
impulse keys described in the referenced patent application have
been found to be satisfactory in solving many of the disadvantages
of prior art devices that perform the same function, there are
certain areas still subject to improvement. For example, while they
provide the desired output pulse under normal operating conditions,
it has been found for at least one embodiment that resistive
leakage at high humidity and temperature raise the signal electrode
potential (i.e., the target potential) to a level higher than
desirable.
Other structures generally made in accordance with the invention
described in the referenced patent application, or slight
modifications thereof, have been found to have other disadvantages.
For example, in one structure surface leakage on the top side of
the insulating film above the target brought the bias voltage onto
the surface of the target thereby destroying the signal output.
While various approaches have been proposed to solve the foregoing
problems, they have not been entirely satisfactory in all
environments. And, those that have been somewhat acceptable have
been more complicated and, therefor, more expensive than
desirable.
Therefore, it is an object of this invention to provide a new and
improved electrostatic impulse key.
It is another object of this invention to provide a new and
improved electrostatic impulse key suitable for use with electronic
keyboards that operates over a wide range of humidity and
temperature conditions without deterioration of the signal
generated by the impulse key.
It is a still further object of this invention to provide new and
improved electrostatic impulse keys wherein the bias voltage
leakage path between the actuated element and the target is
circumvented.
SUMMARY OF THE INVENTION
In accordance with principles of this invention, the basic key
structure comprises a board or plate, which may be of a printed
circuit board, on which one or more targets is formed. Located
above the targets is a dome spring with preferably, three edges
clipped so that the remainder essentially forms an equilateral
triangle when viewed from above. Located above the dome depressable
is a foam "spring" or pressure pad. Located above the foam spring
is an actuator which is depressable by the action of a key member
attached thereto. When the key is pressed, the actuator moves
against the foam. The foam transmits the force of the actuator to
the top of the dome spring. When a sufficient force is applied the
dome spring rapidly changes position. The dome spring rapid change
in position rapidly changes the capacitance between the target and
the dome spring. If the dome spring is connected to a source of DC
power and the target or targets are connected through resistors to
ground, a voltage pulse is generated across the resistors each time
the key is depressed because of the capacitance change.
In accordance with further principles of this invention, a coil
spring encompasses the foam and the dome spring between the target
board and the actuator to preload the key so that a predetermined
amount of pressure must be applied to the key prior to the foam
causing the dome spring to rapidly change position.
In accordance with other principles of this invention, a printed
circuit (target) board has the target or targets formed thereon and
the remaining portion of the conductive surface of the board is
removed. An insulating layer is then disposed on the target surface
of the printed circuit board so as to cover the entire surface
including the target area. Thereafter, a double-sided circuit board
having an aperture located around the target area is located above
the insulating layer. The top conducting layer of the double-sided
printed circuit board supports the dome spring in an electrically
contacting arrangement. The top conducting layer is connected to a
predetermined voltage source so as to form a bias voltage plane.
The lower conducting layer of the double-sided printed circuit
board is connected to ground and forms a suitable ground plane. The
targets are connected through suitable resistors to ground. This
structural arrangement alleviates bias voltage-target leakage
problems of prior art devices.
In the preferred form, the insulating layer is formed of a suitably
doped glass epoxy film. In an alternate form, the insulating layer
is formed of an imperfect insulating film which has been screened,
dipped or sprayed to coat the surface.
In accordance with alternate principles of this invention, the
target area comprises a first printed circuit board with a target
region formed on its upper surface. The entire upper surface of the
first printed circuit (target) board is covered by an imperfect
insulating film. Thereafter, two single-sided printed circuit
boards having apertures of approximately the size of the target
region are mounted above the screened insulating film in parallel
with their conducting surfaces being on the top of each board. The
upper surface of the top of the two single-sided printed circuit
boards supports the dome spring and is connected to a source of
voltage. The conducting surface of the lower of the two
single-sided printed circuit boards is connected to ground and the
target region is connected through resistor(s) to ground.
It will be appreciated by those skilled in the art from the
foregoing brief description that the invention provides an
electrostatic impulse key suitable for use in an electronic key
board or as an individual signal generating element. Because a
spring is used to preload the actuator key, a positive action force
must be applied prior to a signal being generated. However, when
the positive action force is applied a fast pulse is generated.
Moreover, an audiable "click" can be heard. In addition, because a
ground plane that essentially surrounds the dome spring and the
target between these two items is included, the leakage problems of
prior art devices are alleviated without the use of a complex
system. Moreover, because the invention uses well known and readily
available printed circuit boards, it is inexpensive to manufacture
and construct.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing objects and many of the attendant advantages of this
invention will become more readily appreciated as the same becomes
better understood by reference to the following detailed
description when taken in conjunction with the accompanying
drawings, wherein:
FIG. 1 is an expanded view of a preferred embodiment of an
electrostatic impulse key formed in accordance with the
invention;
FIG. 2 is a cross-sectional view of an electrostatic switch of the
type illustrated in FIG. 1;
FIG. 3 is a top view of an alternate form of a target suitable for
use in the embodiment of the invention illustrated in FIG. 1;
FIG. 4 is a cross-sectional view illustrating in detail a printed
circuit board sandwich arrangement suitable for use in the
embodiment of the invention illustrated in FIG. 1;
FIG. 5 is an alternative cross-sectional view of a printed circuit
board arrangement suitable for use in the embodiment of the
invention illustrated in FIG. 1;
FIG. 6 is a further alternative cross-sectional view of a printed
circuit board sandwich arrangement suitable for use in the
embodiment of the invention illustrated in FIG. 1; and,
FIG. 7 is a top view of the dome spring portion of the invention
and is used to illustrate the cross section of the spring dome
shown in FIGS. 3-6 .
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the overall impulse key structure of the
invention is illustrated in FIGS. 1 and 2 and comprises: a target
board 11; a dome spring 13; a cylinder of foam 15; a coil spring
17; a housing 19; an actuator 21; a lid 23; and a key 25.
The target board 11 is actually a sandwich of the type illustrated
in FIGS. 4, 5 and 6, hereinafter described. The dome spring 13 lies
above the target board and is a thin, dome-shaped metal spring,
essentially the segment of a sphere about the size of a dime. Three
edges of the sphere segment are clipped from the circle so that the
remainder essentially forms an equilateral triangle when viewed
from the top as shown in FIG. 7. A sufficient portion of the points
of the triangle remain to provide contact with certain regions of
the target board as hereinafter described. In a preferred
embodiment the dome spring arches to a height of about 0.025 inches
above the upper surface of the target board 11, when at rest.
Preferably the cylinder of foam 15 is formed of polyurethane and
rests gently upon the top of the arched dome spring as illustrated
in FIGS. 1 and 2. The coil spring 17 encompasses the cylinder of
foam but is out of contact therewith. The coil spring also
surrounds the dome spring and is out of physical contact therewith.
In other words, the coil spring surrounds both the cylinder of foam
and the dome spring and rests on the upper surface of the target
board 11 as seen in FIG. 2.
The housing 19 is illustrated in FIGS. 1 and 2 as generally a cube,
however, it may be of any suitable shape. The housing has four
downwardly projecting legs 27 (only three of which are viewable in
FIG. 1) located at the corners of the cube. The legs 27 pass
through four aligned apertures 29 formed in the target board 11.
Hence, when the housing 19 is arrayed atop the target board 11, the
housing is located in a predetermined, fixed alignment above the
target board. The housing includes a cylindrical aperture 31 in
which the coil spring 17, the cylinder of foam 15 and the dome
spring 13 reside. Located around the periphery of the cylindrical
aperture 31 in vertical alignment are four semi-cylindrical
recesses 33.
The actuator 21 is generally cylindrical in shape and includes four
semi-cylindrical legs 35 (only three of which can be seen in FIG.
1) vertically arrayed along the outer surface thereof. The
semi-cylindrical legs 35 fit into the semi-cylindrical recesses 33
formed in the housing 19. The actuator also includes an upwardly
projecting cylinder 37 that includes an upwardly projecting
X-shaped element 39. The lid 23 fits about the upwardly projecting
cylinder 37 of the actuator 29 atop the housing 19, and includes a
ring shaped portion 41 that surrounds the upwardly projecting
cylinder. The remaining portion of the lid 23 is generally flat and
includes four downwardly projecting legs 43 (only two of which are
viewable in FIG. 1) that fit into four apertures 45 formed in the
upper surface of the housing 19 about the cylindrical opening 31.
The key 25 has a suitable aperture 38 (FIG. 2) formed in its bottom
that tightly fits over the X-shaped element 39 formed atop the
actuator 21 and is large enough to slide over the ring shaped
portion 41 of the lid 23.
In operation, when the key 25 is depressed it forces the actuator
21 downwardly against the coil spring 17 and the cylinder of foam
15. When a sufficient amount of pressure is applied, the dome
spring 13 rapidly collapses to change the separation distance
between the dome spring and a target on the target board. This
separation distance change changes the capacitance between these
elements in a very rapid manner. In one actual embodiment the
resulting capacitance increases to its maximum value in
approximately 100 microseconds. Upon release of finger pressure on
the key 25, the dome spring quickly recovers to its arched position
over the target and the capacitance between the dome spring and the
target rapidly returns its low value.
The coil spring does not impinge on the dome spring. Rather, as
indicated above, the coil spring lies between the target board 11
outside of the target area and the actuator 21 and is merely
provided to pre-bias the key so that a predetermined amount of
force must be applied to the key prior to actuation of the dome
spring, 25 grams of force, for example. The four semi-cylindrical
legs 35 of the actuator which, as illustrated in FIG. 1, extend
below the lower surface of the actuator maintain the coil spring in
its desired alignment position.
As illustrated in FIG. 1, the invention may include three separated
target rings 47, 48 and 49 concentrically arrayed on the target
board 11. Any one of these rings will provide an output signal of
the desired nature. Alternatively, as illustrated in FIG. 3, a
solid ring segmented into three separated sections 51, 53, and 55
can be utilized. In either case, the target is surrounded by a
further ring 57 on which the impinging portion of the spring dome
resides. In addition, the target is not limited to three regions,
rather the target can be a single target region, two target
regions, or more than three target regions as desired.
It will be appreciated by those skilled in the art that one of the
major problems with a structure of the type described above is
leakage between the dome spring and ground and/or leakage between
the targets and ground, depending upon which side of the
"capacitor" is connected to the bias voltage source. The invention
overcomes this problem by using one or the other of the target
structures illustrated in FIGS. 4, 5 and 6 and hereinafter
described. While the preferred target structure is illustrated in
FIG. 4, the target structures illustrated in 5 and 6 also overcome
the leakage problems of prior art electrostatic impulse keys of a
similar nature.
The target structure illustrated in FIG. 4 comprises a printed
circuit (target) board 61 having formed thereon a target 63 which
may be a single target region or a plurality of target regions of
the type heretofor described. However, for purposes of discussion
FIG. 4 illustrates a target 63 formed of three target regions which
may be rings of the type illustrated in FIG. 1. By any of several
well known techniques, the target 63 is "layed out" on the upper
conducting surface of the target printed circuit-board 61;
thereafter, the remaining portion of the conductive surface is
removed by any suitable process, such as by chemical etching, for
example. Electrical connection to the three target rings is made by
means of "plated thru" (conductive) holes which permit connection
to the target rings to be made from the bottom side of the printed
circuit board.
After the target printed circuit board has a target formed in its
upper surface, a suitable thin and slightly imperfect insulating
layer 65 such as, a doped glass epoxy film, or other screened,
dipped or sprayed insulating material is formed over the entire
upper surface of the target printed board 61, including the target
63. Following this formation, a double-sided printed circuit board
67 having an aperture 69 formed therein is arrayed atop the
insulating layer 65. The aperture 69 is aligned directly over the
target and has a cross-sectional area that is slightly larger than
the area covered by the target 63. The upper conductive surface of
the double-sided printed circuit board 67 is etched slightly
rearwardly from the edges of the aperture 69. The dome spring 13
lies atop the upper conductive surface 71 of the double-sided
printed circuit board 67 and is in electrical contact therewith. In
addition, the upper conductive surface 71 is connected to a
suitable DC bias voltage source +V and forms a voltage plane. The
lower conductive surface 73 of the double-sided printed circuit
board 67 is connected to ground and forms a ground plane. While
FIG. 4 illustrates that the entire upper conductive surface 71 of
the double-sided printed circuit board 67 is retained, only a
portion thereof, such as a ring 57 as shown in FIGS. 1 and 3 need
be retained, as desired.
In general, as somewhat previously described, when the dome spring
13 changes position, the capacitance between the dome spring and
the target 63 changes. This capacitance change causes a voltage
change across resistors R1, R2 and R3 connected between the target
regions and ground. The voltage decreases are in the form of pulses
which flow along three output lines 81, 83 and 85 connected
respectively between R1, R2 and R3, and their respective target
regions.
As a signal source, it will be appreciated, that the invention is
in essence a "snap variable capacitor" and resembles a capacitor
microphone, except that the signal voltage is much larger because
of the comparatively extreme changing capacitance. For example, in
one actual embodiment of the invention using a bias voltage of
about 150 volts and a suitable target load resistance, a signal
pulse of 15 to 20 volts was generated. The pulse duration was about
one-half milli-second. While some ringing was found to occur, this
was highly damped by the cylinder of foam 15.
FIG. 5 illustrates an alternate embodiment-of a target board formed
in accordance with the invention which is somewhat similar to the
target board illustrated in FIG. 4 but differs therefrom. For ease
of illustration and description, only a single target region is
illustrated in FIG. 5, however, a plurality of targets regions
could be used in this embodiment, if desired.
The target board illustrated in FIG. 5 comprises a single-sided
target printed circuit board 91 having a target 93 formed thereon.
The target may be formed in any well known manner. However, in
general, the target 93 is "layed out" on the conductive surface of
the target printed circuit board 91. Thereafter, the remaining
portion of the conductive surface is etched or removed in some
other manner by any suitable process.
After the target 93 is formed, the entire upper surface of the
printed circuit board 91 is covered with a suitable insulator
layer. In the case of the embodiment of the invention illustrated
in FIG. 5, an imperfect insulating material 95 is screened, dipped
or sprayed to coat the upper surface of the target printed circuit
board 91 with a suitable film.
After the insulating film 96 has been applied to the upper surface
of the target printed circuit board 91, a double-sided printed
circuit board 97 having an aperture 99 formed therein is arrayed
over the insulating film 95 so that the aperture 99 is aligned over
the target 93. Preferably, the cross-sectional area of the aperture
99 is slightly larger than the area covered by the target 93.
The upper conductive surface 101 of the double-sided printed
circuit board 97 is etched slightly rearwardly from around the
periphery of the aperture 99. Alternatively, if desired, only a
ring around the aperture (set slightly back therefrom) could be
left on the upper conductive surface. In any event, the remaining
portion of the upper conductive surface 101 of the double-sided
printed circuit board 97 supports the dome spring 13 and is
connected to a voltage source +V and forms a voltage plane. The
lower conductive surface 103 of the double-sided printed circuit
board 97 is connected to ground and forms a ground plane. In
addition, the target region 93 is connected through a resistor R4
to ground. The output voltage is taken across R4 and passes along
an output line 105.
FIG. 6 is a further alternative embodiment of a target board formed
in accordance with the invention and comprises a single-sided
target printed circuit board 111 having a target 113 formed on its
upper conductive surface in the manner generally described above.
The upper surface of the target printed circuit board 111 is
covered with an insulating film 115 such as an imperfect insulating
film, for example. Thereafter, second and third single-sided
printed circuit boards 116 and 117 are arrayed in parallel above
the lossy screened insulating film 115 with their conducting
surfaces being upwardly. The second and third printed single-sided
circuit boards 116 and 117 each include an aperture 118a and 118b
which align over the target region 113. The apertures 118a and 118b
have a cross-sectional area that is slightly larger than the area
of the target 113. In addition, the upper surface of the third
single-sided printed circuit board 117 has its conductive area
etched slightly rearwardly from its aperture 118a. The upper
conductive surface 119 of the third single-sided printed circuit
board supports the dome spring 13 and is connected to a voltage
source designated +V to form a voltage plane. The conductive
surface of the second single-sided printed circuit board 116 is
connected to ground and forms a ground plane. In addition, the
target 113 is connected through a resistor R5 to ground. The
voltage across R5 is sensed on an output line 120.
It will be appreciated from viewing FIGS. 4, 5 and 6 that the
invention can take on various structural forms. The forms are all
related because each provides a ground plane that is physically
located between the target area and the spring dome. The ground
plane prevents leakage from destroying the signal generating
capabilities of the invention. In addition, an insulating layer
which is preferably a doped glass epoxy film but may be some other
imperfect insulating film is located atop the target area, between
the target area and the ground plane.
While the invention has been described as using conventional
printed circuit boards which are normally copper clad on their
conductive surfaces, preferably, the portion of the conductive
surface on which the spring dome impinges is formed of a harder
material, such as nickel, for example, that is more wear resistant.
That is, as the spring dome is moved up and down as the key 25 is
actuated wearing occurs between the legs of the dome spring 13 and
the conductive surface on which it resides. Hence, it is desirable
that this conductive surface be highly wear resistant.
Consequently, it is preferred that a good electrically conductive
material that is also highly wear resistant, such as nickel, be
used in this region. This material may be plated or vacuum
deposited over a conventional copper layer or applied in any other
conventional manner, as desired.
It will be appreciated from the foregoing description that the
invention provides a new and improved electrostatic impulse key
that is suitable for widespread use. Because signal destroying
leakage between the dome spring and the target area is prevented,
the invention is suitable for widespread use, even in environments
where temperature and humidity are high.
While the preferred embodiments of the invention have been herein
described, it will be appreciated by those skilled in the art and
others that various changes can be made therein without departing
from the spirit and scope of the invention. Hence, the invention
can be practiced otherwise than as specifically described
herein.
* * * * *