Browse Prior Art Database

Sense Capacitor for Capacitive Keyboards

IP.com Disclosure Number: IPCOM000052398D
Original Publication Date: 1981-Jun-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 48K

Publishing Venue

IBM

Related People

Harris, RH: AUTHOR [+2]

Abstract

Capacitance keyboards are often exposed to relatively large changes in capacitance resulting from small contaminating particles between the capacitive plate members. As a result, it is desirable to have a key-board sensing capacitor which is able to change its capacitance at relatively large air gap separation distances to overcome the effect of small contaminating particles separating the plates.

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Sense Capacitor for Capacitive Keyboards

Capacitance keyboards are often exposed to relatively large changes in capacitance resulting from small contaminating particles between the capacitive plate members. As a result, it is desirable to have a key-board sensing capacitor which is able to change its capacitance at relatively large air gap separation distances to overcome the effect of small contaminating particles separating the plates.

Fig. 1 illustrates a partial exploded view of a typical capacitive key actuator, such as that shown in U.S. Patent 4,118,611. Fig. 2 shows the assembly. Modifications are included to improve the overall capacitance and the capacitive change ratio as well as to improve the sensitivity of the sensing capacitor, making it sensitive with relatively large air gaps.

The main elements are the key top 1 and a buckling spring 2 which fits over a mounting post 3 with a movable fly plate 4 having pivot locator members 5. The improvements comprise the addition of a metallized plastic film 6 adhered with adhesive to the bottom of fly plate 4. The metallized plastic film 6 is only about one thousandth of an inch thick and does not appreciably affect the operation of the fly plate member 4 when the buckling spring 2 causes it to snap in an up or down fashion about the pivots 5. The conductive metallized plastic 6 may have an insulating layer on the face thereof which is towards the conductive sense and drive pads 7 and 8, respectively. Sense pad 7 and drive pad 8 may be copper metallized pad areas on a circuit board 9 or some other suitable conductive material.

In operation, electrical drive signals are applied to plate 8 and are coupled through the capacitive action between plates 6 and 8 to the output or sense pad
7. Because the m...