Variable capacitance push-button switch

In a capacitive-type switch having two capacitive levels, a relatively thin blade of a desired dielectric material is interposed between two capacitive plates by downward actuation of a push-button switch so that the plates become separated by the blade. When the push button is released, the blade is removed from between the two capacitive plates and a high capacitance level is once again achieved.

FIELD OF THE INVENTION 
The present invention relates to a push-button switch of a type operable to 
change its capacitance value upon actuation of a push button. 
BACKGROUND OF THE INVENTION 
In a keyboard or other data input device, variable capacitance push-button 
switches (hereinbelow termed "capacitance type switches") have come into 
wide use. In such switches, actuation of the push-button changes the 
capacitance of the switch. In a keyboard, for example, the changed 
capacitance of a depressed switch is detected and its row and column 
determined, causing to be generated a binary signal uniquely associated 
with the depression of that particular switch. Thus, the difference 
between the two capacitive levels of the switch needs to be made 
sufficiently great, and the capacitance level at each of the two switch 
positions must remain uniform throughout the life of the push-button 
switch and under a variety of pressures applied to the push-button switch 
by an operator's finger. Typically, capacitance type switches achieve two 
levels of capacitance by, upon actuation of the push-button, effectively 
separating or bringing together two conductive plates separated by a 
dielectric in order to change the distance between the plates and, 
consequently, the capacitance between the plates. 
Capacitance is calculated using the equation: 
EQU C=AK.epsilon..sub.0 /t, 
where 
C is the capacitance 
.epsilon..sub.0 is the permittivity of empty space (8.85.times.10.sup.-2 
coul.sup.2 /newton.sup.2 -m.sup.2) 
K is the dielectric constant 
A is the plate area, and 
t is the dielectric thickness. 
As seen, by decreasing the dielectric thickness t between the plates, the 
capacitance is increased. 
FIG. 1 is a sectional view of the essential portions of a known capacitance 
type switch, and principally illustrates the electrical construction of 
the switch. As shown in FIG. 1, a push rod 31 carries a movable electrode 
36 having a dielectric film 35 thereover. The electrode 36 is movable 
relative to a pair of stationary electrodes 33 and 34 disposed on an 
insulating substrate 32. One of the stationary electrodes, such as 
electrode 33, serves as an input and the other electrode serves as the 
output of the switch. In FIG. 1, let d.sub.a and d.sub.f denote the 
thicknesses of the air gap and the dielectric film 35, respectively. When 
the push-button is pressed and dielectric film 35 approaches and contacts 
stationary electrodes 33 and 34, the air gap d.sub.a is reduced and the 
capacitance is thereby increased. Capacitance of the switch is determined 
by the series capacitance of the capacitor formed by stationary contact 33 
and movable electrode 36 and the capacitor formed by stationary electrode 
34 and movable electrode 36. Two patents which utilize this general means 
of providing two capacitance levels by actuation of a push-button switch 
are U.S. Pat. Nos. 3,965,399 to Walker, Jr. et al. and 4,423,464 to Tamura 
et al. These prior art patents concern themselves with increasing the 
reliability of the switch by obtaining a consistent dielectric thickness 
between the capacitive plates in both switch positions so that the two 
capacitance levels remain uniform throughout the life of the switch. As 
seen by inspection of FIG. 1, the dielectric thickness of, for example, 
dielectric 35 in FIG. 1 as well as the mechanical movement of the switch, 
controlling the air gap, must be precisely controlled to obtain uniform 
capacitances in both switch positions. These types of switches are 
therefore manufactured with high precision, resulting in a concomitant 
increased cost of manufacture. Further, the switch must be depressed to 
its full extent to achieve the full high capacitance level, adding to the 
complexity of the manufacture of the switch since the dielectric material 
must not be damaged by contact with the stationary electrodes. 
SUMMARY OF THE INVENTION 
My invention is a more reliable, yet relatively inexpensive, capacitive 
type push-button switch which comprises two resilient capacitive plates 
each comprising a dielectric substrate with a conductive layer thereon 
which are, in their normal position, resiliently pressed together with a 
force determined by their resiliency. A relatively thin blade of a desired 
dielectric material is interposed between the two capacitive plates by 
downward actuation of the switch so that the plates are separated by the 
blade. The penetration of the blade between the capacitive plates is 
determined by a stop in the structure of the switch which prevents the 
push-button to be pressed beyond a certain point. With the above-described 
switch design, the tolerances in the manufacture of the switch can be 
relaxed from those required for prior art capacitive type switches since 
the penetration of the blade between the capacitive plates may vary 
slightly without significantly changing the capacitance between the plates 
when the push-button switch is in its down position. This is because once 
the blade is even partially inserted between the plates, the plates have 
already been spaced apart in accordance with the thickness of the blade 
and any further penetration of the blade merely changes the proportion of 
blade dielectric to air dielectric between the plates. Further, the area 
of the plates can be increased if necessary without increasing the surface 
area of the switch since the plates are perpendicular to the surface of 
the switch, unlike prior art capacitive plates wherein the maximum area of 
the stationary plates may be limited by, for example, keyboard space. 
Further, the thickness of the blade, determining the distance between the 
plates, can easily be varied to form switches with a desired low 
capacitance level. Still further, the depth of the blade interposed 
between the capacitive plates can also be varied to increase the 
proportion of blade dielectric to air dielectric between the capacitive 
plates, hence, enabling an even greater ability to select a desired low 
level capacitance.

DETAILED DESCRIPTION 
FIG. 2 shows the preferred embodiment of my inventive capacitive type 
switch 40 where key cap 42 is pressed downwardly, resisted by a spring 
(not shown), to interpose end blade portion 46, located at the end of 
plunger 48, between capacitive plates 50 and 52. Knob or stop 54, jutting 
out from plunger guide wall 56, coacts with plunger cavity 58 to limit the 
up and down movement of plunger 48. Opposite guide wall 56 is guide wall 
60, which, in combination with guide wall 56, aligns plunger 48 and end 
blade portion 46 for insertion of blade portion 46 between capacitive 
plates 50 and 52. Top and bottom support members 62 and 63, respectively, 
provide the mechanical support for the structure. 
In the embodiment of FIG. 2, spring 64 is a tactile spring clip which 
provides a satisfying abrupt change in resistance in the pressure 
resisting the operator's finger when key cap 42 is pressed, giving 
indication that the key has indeed been pressed. Spring 64 may be 
beneficially omitted if the change in mechanical resistance when blade 
portion 46 interposes between capacitive plates 50 and 52 is to provide 
the desired tactile signal. In such an embodiment, with spring 64 omitted, 
a first resistive force is sensed by the operator in pressing key cap 42 
until blade portion 46 first contacts capacitive plates 50 and 52. At this 
point, the increased resistance in interposing blade portion 46 between 
capacitive plates 50 and 52 is sensed by the operator as the operator 
presses key cap 42 down further. After blade portion 46 has spread 
capacitive plates 50 and 52 apart, the resistance is decreased until 
further downward travel is mechanically stopped by stop 54. Hence, the 
changes in mechanical resistance provide the desired tactile signal to the 
operator. Thus, the tactile signal inherently occurs at the precise moment 
that the capacitive switch begins to change its capacitance. The change in 
mechanical resistance, and hence the magnitude of the tactile signal, as 
blade portion 46 interposes between capacitive plates 50 and 52 can be 
adjusted by making the edge of blade portion 46 either more V-shaped, 
decreasing the change in mechanical resistance, or more blunt, increasing 
the change in mechanical resistance. 
A spring for returning a pressed and released push-button back to its 
normal position will be described later with reference to FIG. 3. 
The thickness and dielectric constant of blade portion 46, along with the 
area of the capacitive plates and the amount of insertion of end blade 
portion 46 between plates 50 and 52, determine the capacitance of the 
switch in its pressed or low capacitance position. 
Capacitive plates 50 and 52 are, in one embodiment, comprised of a 
Mylar.RTM. (or similar dielectric) substrate with a thin electrically 
conductive film deposited thereon. This film may be copper, deposited on 
the substrate by sputtering. In one embodiment, capacitive plates 50 and 
52 also have a thin film of dielectric deposited on the conductive film so 
that as they resiliently contact each other, capacitive plates 50 and 52 
are separated by the thickness of the dielectric film. Since the 
mechanical pressure of capacitive plate 50 against capacitive plate 52 is 
not affected by the downward pressure on the key cap but on the resiliency 
of the capacitive plates 50 and 52, there is no possibility of destroying 
the dielectric films on capacitive plates 50 and 52 due to an excessive 
amount of pressure on key cap 42--a problem inherent in the prior art of 
FIG. 1. The thickness of the dielectric film on capacitor plates 50 and 52 
can be made as thin as desired to provide a desired high capacitance in 
this normally off push-button position. If desired, the deposition of the 
dielectric film may be omitted and the dielectric substrate may provide 
the desired separation of the electrically conductive films. 
In a practical embodiment of capacitive switch 40, the Mylar.RTM. substrate 
is between 2 to 4 mils thick; the conductive film is between 5-10 microns 
thick; the dielectric film is approximately 1 mil thick; the dimension of 
each of capacitive plates 50 and 52 is approximately 0.2 in. by 0.5 in.; 
and blade portion 46 is approximately 0.03 in. thick and interposes 
between capacitive plates 50 and 52 a maximum of between 0.08-0.2 in. with 
a total travel of approximately 0.140 in. 
In the preferred embodiment, capacitive plates 50 and 52 are held in 
position by sandwiching the ends of capacitive plates 50 and 52 between 
top support member 62 and bottom support member 63. 
In an example of an application of capacitive switch 40, capacitive plates 
50 and 52 are coupled in series with other capacitive plates, so as to be 
equivalent to a capacitor in series with a plurality of other capacitors, 
wherein the total series capacitance is coupled to a capacitance detection 
means. The series capacitances are arranged to form rows and columns so 
that, given an adequate sensitivity of the capacitance detection means, 
each switch when pressed would produce a characteristic change in the 
total series capacitance of both a particular row and column, identifying 
which capacitive switch has been actuated. 
FIG. 3 shows a side view of capacitive switch 40. In FIG. 3, capacitive 
plate 50 is shown along with end blade portion 46, center blade portion 
65, and end blade portion 66. As seen from FIG. 3, the three blade 
portions, all commonly fixed to plunger 48, are positioned to interpose 
between capacitive plates 50 and 52 (capacitive plate 52 not shown). The 
purpose of the three blade portions is to provide a centered location for 
return spring 68 through which center blade portion 65 travels. Return 
spring 68 remains positioned between fixed supports 70 and 72 and plunger 
48 so that return spring 68 urges blade portions 46, 65, and 66 out of 
between capacitive plates 50 and 52 and into the high capacitance 
position. Stops 54 and 74 are clearly shown which interact with cavities 
58 and 78 in end blade portions 46 and 66, respectively. 
FIG. 4 shows a front view of capacitive switch 40 in its depressed position 
where end blade portion 46 is shown interposed between capacitive plates 
50 and 52. As seen, stop 54 prevents plunger 48 from further downward 
movement. The capacitance between capacitive plates 50 and 52 is 
determined by the capacitance between capacitive plates 50 and 52 across 
the blade dielectric material in parallel with the capacitance between 
plates 50 and 52 across the air gap. As seen, capacitance in this 
depressed position can be modified by either elongation of the blade 
between the capacitive plates 50 and 52, assuming the dielectric constant 
of end blade portion 46 differs from that of air, or varying the thickness 
of end blade portion 46. By both adjusting the thickness of the blade and 
the depth of the blade between the plates, a wide range of low level 
capacitances are easily obtainable. 
The preferred manufacture of capacitive plates 50 and 52 in their 
arrangement shown in FIGS. 2-4 is shown in FIGS. 5a-5e. FIG. 5a shows 
capacitive plates 50 and 52 with capacitive plate 50 comprising dielectric 
substrate 80 and conductive film 81 deposited thereon, and capacitive 
plate 52 comprising dielectric substrate 82 and conductive film 83 
deposited thereon. In another embodiment, conductive film 81 may be 
deposited on the top of dielectric substrate 80 and a separate dielectric 
film may be deposited on the bottom of dielectric substrate 80 so that 
when capacitive plates 50 and 52 are in their resiliently opposing 
positions, they will be separated by the dielectric film. In this way, the 
separation of capacitive plates 50 and 52 may be made as small as desired 
without affecting the thickness and resiliency of the substrate. As the 
thickness of dielectric substrate 80 is reduced, a higher capacitance in 
the normally off position is achieved, enabling a larger difference 
between capacitances in the on and off switch positions. Deposited on the 
underside of capacitive plate 52 is a ground potential conductive layer 84 
to decrease noise. However, grounded layer 84 is not required for 
acceptable operation of the capacitive switch and may be eliminated. 
The structure of FIG. 5a is manufactured as two overlapping sheets using 
well known processes whereby dielectric substrate 80 of capacitive plate 
50 has first deposited on it conductive film 81, and dielectric substrate 
82 of capacitive plate 52 has deposited on one side conductive film 83 and 
deposited on its other side grounded layer 84. By using a thinner 
dielectric substrate 80, the capacitance of the switch in its high 
capacitance position will be made higher. Since dielectric substrates 80 
and 82 must be thick enough to provide a desired amount of resiliency and 
mechanical strength, if a desired dielectric thickness between conductive 
films 81 and 83 is less than about 2 mils, conductive film 81 would have 
to be deposited on the top of dielectric substrate 80 and a thin 
dielectric film must then be deposited over the conductive film. 
Next, as shown in FIG. 5a, cuts are made, or breakable perforations are 
made, in the flat sheets at points 86 and 88, and the two sheets are then 
placed flat together and fixed in position on, for example, a keyboard by, 
as shown in FIG. 2, top support 62 and bottom support 63. 
As shown in FIG. 5b, a separating tool 90 may be used to press down on a 
center position between points 86 and 88 to separate a portion of 
capacitive plate 50 from the upper remaining sheet and concurrently 
separate a portion of capacitive plate 52 from the lower remaining sheet. 
In FIG. 5c, separating tool 90 is shown continuing its downward movement 
until capacitive plates 50 and 52 are completely broken from their sheets 
and are facing each other perpendicular to the sheet, as shown in FIG. 5d. 
In FIG. 5d, capacitive plates 50 and 52 are resiliently pressed together 
by the resiliency of the capacitive plates 50 and 52. 
Thus, a novel and improved capacitive type switch has been taught along 
with a preferred method of manufacture. My inventive switch is relatively 
simple to manufacture and overcomes many of the problems associated with 
prior art type capacitive switches. The concepts used in this invention 
may be modified in an obvious manner by one of ordinary skill in the art 
for various purposes while keeping with the spirit and scope of the 
invention, that being the insertion of a dielectric blade actuated by a 
push button to separate two capacitive plates which are separated by a 
dielectric when the blade is removed.