Capacitance controlled keyswitch

A tri-state capacitance controlled keyswitch having a deformable concave electrode moveable by means of an actuator toward and away from a pair of capacitor plates underlying a dielectric layer in order to change the capacitive coupling between the capacitor plates. The electrode is normally remote from the dielectric layer in a weak capacitance open circuit position, a first higher capacitance closed circuit position exists when the electrode is undeformed and is in contact with the dielectric layer, and a second highest capacitance closed circuit position exists when the electrode is deformed and is in contact with the dielectric layer.

This invention relates to a capacitance controlled keyswitch in which a 
movable electrode is moved toward and away from a pair of spaced coplanar 
capacitor plates, sandwiched between a substrate and a dielectric layer, 
in order to vary the capacitive coupling between the plates. 
Keyswitches as commonly used in keyboards of electronic text input devices 
have only two states, ON and OFF. Of these, the capacitive coupled 
keyswitch is well known as represented by that disclosed in U.S. Pat. No. 
3,965,399 and U.S. Pat. No. 3,659,163. Also, well known are snap-action 
capacitive keyswitches as represented by that disclosed in U.S. Pat. No. 
3,643,041 and U.S. Pat. No. 3,751,612. 
There are instances, however, when it is desirable to have a third state 
which becomes activated if the force applied to the keyswitch is 
substantially beyond that needed to switch it to its ON state. For 
example, this third state could control the simultaneous display of an 
alphabetic symbol with a related accent mark such as the German "umlaut", 
the French "cedilla" or the Spanish "tilde". In the known prior art text 
input devices the accented letter may be displayed by depressing a 
keybutton dedicated soley to that function or by depressing the selected 
keybutton, backspacing, and then depressing the accent mark, usually in 
conjunction with the shift key. Alternatively, a third keyswitch state 
could control the automatic repetition of a selected character. 
The present invention comprises a capacitance controlled keyswitch which 
undergoes a first abrupt transition from a relatively low capacitance 
state to a higher capacitance state under normal keystroke pressure, and 
which undergoes an additional abrupt and pronounced capacitance increase 
under still greater application of pressure. Thus, in normal keystroke 
operation the keyswitch is moved from the low capacitance OFF position to 
the higher capacitance ON.sub.1 position. In order to address the 
extraordinary ON.sub.2 position a substantially higher keystroke force 
required. The clear benefit derived over the known multi-keyswitch array 
is the elimination of plural keystrokes for achieving some beneficial 
functions. The discrete signal levels of capacitance for each keyswitch 
may be detected with suitable circuitry and applied through machine logic 
to perform the desired functions.

With particular reference to the drawings there is illustrated in FIG. 1 a 
keyboard 1 of an electronic text input device bearing an array of 
keybuttons 2 each having marked thereon a legend representive of the 
output character to be displayed upon actuation of its associated 
keyswitch. Three keybuttons are shown bearing legends which make them 
particularly suitable for use with this invention, namely accent marks 
associated with particular letters. 
In FIGS. 2-4 the novel tri-state keyswitch of this invention is illustrated 
in each of its three states. It comprises a pair of conductive capacitor 
plates 3 and 4 (as seen in plan view in FIG. 5) disposed upon a rigid 
insulating substrate 5 and underlying a dielectric layer 6. The dielectric 
layer 6 is preferably stepped, having a peripheral annulus of thicker 
dielectric material forming a seat 7 around the outer edge of a thinner 
interior portion 8 (as seen in plan view in FIG. 6). 
The moveable portion of the keyswitch of this invention comprises a 
deformable concave electrode 9 made of a conductive material connected 
near its center by keyplunger 10 to keybutton 2. Suitable resilient means 
urges the entire moveable portion of the keyswitch upwardly so that 
electrode 9 and capacitor plates 3 and 4 are normally separated. In the 
accompanying drawings, the resilient member is seen to be a spring 11 
disposed between the keybutton 2 and the face of the keyboard 1. Output 
leads 12 and 13 attached to capacitor plates 3 and 4, respectively, extend 
through the insulating substrate 5 to provide an output signal 
representative of the capacitance between the plates. 
In the alternative embodiment illustrated in FIG. 7 like parts are 
designated by like numerals with a prime (') added. The rectangular 
dielectric layer 6' includes a stepped annular portion 7' and a thinner 
interior portion 8'. The conforming movable concave electrode which will 
also be rectangular is not shown. 
To better enable those skilled in the art to practice this invention the 
following specific example, reciting the mode of fabrication and materials 
which may be used, is provided. A known circuit board comprising the 
insulating substrate 5 is copper clad and then laminated with a dry film 
resist such as a 0.001 inch thick layer of Riston (a trademark of E. I. 
Dupont and Nemours) photopolymer. The pattern of the capacitor plates 3 
and 4 is then exposed and etched in the copper layer. The photopolymer 
film is left over the copper capacitor plates to serve as a dielectric. 
Then, the circuit board is again covered with another layer of Riston 
photopolymer of approximately 0.005 inch and is again exposed and 
developed to form the annular stepped portion 7 of the dielectric. The 
width of the annular ring is kept small to allow the moveable deformable 
electrode 9 to be readily depressed by "snap" action so that the center of 
the electrode is brought into contact with the thin central section 8 of 
the dielectric layer. Electrode 9 is fabricated from a thin sheet of 
spring material, such as a 0.005 inch thick foil of beryllium copper, 
which has been formed into a slightly concave geometry whose dish depth is 
0.005 inch. Thus, the "throw" of deformable electrode 9 from concave to 
convex configuration will be approximately 0.01 inch. 
As is well known, the capacitance of a parallel plate capacitor is directly 
proportional to the dielectric constant and is inversely proportional to 
the separation of the plates. In the configuration of this invention, the 
capacitive effect is equivalent to two parallel plate capacitors connected 
in series, each of the plates 3 and 4 coupled with the electrode 9. When 
the keyswitch is in the OFF position, as shown in FIG. 2, the capacitive 
coupling is very weak yielding a very low reading across leads 12 and 13. 
In operation, the operator will apply a force to each selected keybutton 2 
as on a standard keyboard. A normal force of about three grams will move 
the keybutton 2 downwardly as graphically illustrated by the legend "Zone 
A" in FIG. 8. This force will compress spring 11 to move concave electrode 
9 to the point at which its peripheral lip becomes seated on the annular 
stepped seat 7 of the dielectric layer, as seen in FIG. 3. When the 
keyswitch has arrived at this On.sub.1 position the capacitive coupling 
becomes abruptly higher than at the rest position. The text input device 
would be programmed to display the single selected keyboard character at 
ON.sub.1. 
Alternatively, in accordance with the novel feature of this invention, the 
operator may wish to display something other than the single selected 
character such as, for example, the "o", "n", or "c" shown in FIG. 1 or a 
repetition of the selected keyboard character. This may be accomplished by 
a single keystroke of substantially greater than normal force, such as six 
to ten grams. Upon striking the keybutton 2 with greater force, the 
concave electrode 9 will initially move downwardly until it comes to rest 
against the annular stepped seat 7 (through Zone A), then the force 
strongly urges the electrode and dielectric together with no displacement 
(through Zone B) until the deformation threshold of the electrode is 
reached and passed (through Zone C) thus "snapping" the concave electrode 
into a convex configuration as seen in FIG. 4. When the keyswitch has 
arrived at this ON.sub.2 position the capacitive coupling abruptly 
achieves its highest value. 
A preferred detection circuit for the keyswitch is shown in FIG. 9. The 
detection circuit includes an a-c signal source 14 desirably in the RF 
range (e.g. 3 MH.sub.z) which is connected to the plate 3 of the tri-state 
keyswitch. The other plate 4 of the keyswitch is coupled through an 
amplifier 15 and rectifier 16 to a two-stage analog comparator 17 
comprised of a pair of Schmidt triggers 18 and 19. A voltage divider in 
the form of resistors R1, R2 and R3 and voltage source +V establish the 
threshold levels for the Schmidt triggers 18 and 19. More specifically, 
the input to the Schmidt trigger 18 is connected to the voltage source +V 
through the resistor R1 and to ground through resistors R2 and R3. The 
output of the rectifier 16 is applied directly to the input of Schmidt 
trigger 18. On the other hand, the input to the Schmidt trigger 19 is 
connected to the voltage source +V through the resistors R1 and R2 and to 
ground through the resistor R3. The output of the rectifier 16 is applied 
through resistor R2 to the input of Schmidt trigger 19. 
The output of Schmidt trigger 18 is applied to one input of an AND-gate 20 
and the output of Schmidt trigger 19 is applied as an inverted input to 
AND-gate 20. The two outputs 21 and 22 of the detector circuit 17 include 
the output of AND-gate 20 and the output of Schmidt trigger 19. 
In operation, the output of the tri-state keyswitch as amplified and 
rectified through 15 and 16, respectively, is applied to the two-stage 
analog comparator 17. In the OFF condition illustrated in FIG. 2 the input 
signal to Schmidt trigger 18 is below its threshold level yielding a low 
(binary 0)output which is applied to one input of AND-gate 20. The input 
signal to Schmidt trigger 19 is also below its threshold level yielding a 
low (binary 0) output which is inverted and applied as a high (binary 1) 
signal to the other input of AND-gate 20 resulting in a first low output 
signal from the comparator 17 at 21. The low output signal from Schmidt 
trigger 19 also directly provides a second low output signal from the 
comparator 17 at 22. When both outputs 21 and 22 are low, the normal low 
capacitance OFF condition exists. 
In the ON.sub.1 condition as seen in FIG. 3 the input signal to Schmidt 
trigger 18 is higher than its threshold resulting in a high output signal 
to AND-gate 20 and the input signal to Schmidt trigger 19 is lower than 
its threshold resulting in a low output signal which is inverted and 
applied to the AND-gate 20 resulting in a first high output signal from 
the comparator 17 at 21. The low output signal from Schmidt trigger 19 
also directly provides a second low output signal from the comparator 17 
at 22. When output 21 is high and output 22 is low, the normal higher 
capacitance ON.sub.1 condition is indicated. 
In the ON.sub.2 condition as seen in FIG. 3 the input signal to Schmidt 
trigger 18 is higher than its threshold resulting in a high output signal 
to AND-date 20 and the input signal to Schmidt trigger 19 is also higher 
than its threshold resulting in a high output signal which is inverted and 
applied to the AND-gate 20 resulting in a first low output from the 
comparator 17 at 21. The high output signal from Schmidt trigger 19 also 
directly provides a second high output signal from the comparator 17 at 
22. When output 21 in low and output 22 is high the highest capacitance 
ON.sub.2 condition is indicated. 
It is understood that the present disclosure has been made only by way of 
example and that numerous changes in details of construction and the 
combination and arrangement of parts may be resorted to without departing 
from the true spirit and the scope of the invention as hereinafter claimed 
.