Keyboard Switch Assembly With Conductive Plastic Contactor And Actuator Spring Sleeve Biasing Means

Abstract

An encoded switch array having a keyboard comprising a plurality of buttons, each button when depressed adapted to force a conductive contactor against a conductive wiring pattern supported on a board or the like, the contactor preferably being of a conductive plastic normally held away from the wiring pattern by resilent biasing means.

Description

This invention is directed to a new and improved encoded switch array or keyboard structure for converting key or button depressions to a coded electrical output. More particularly this invention is directed to a new and improved encoder switch array in which conductive contactors are used to engage electrical contacts in order to generate a coded electrical output.

In the past most keyboard type encoders have comprised a plurality of keys which when depressed close one or more electrical connectors to generate electrical signals. Various types of mechanical switches have been used for this purpose, most of which have relied upon metal to metal contact for completing the electrical circuits. The generated electrical signals are then usually converted using some type of external logic, (e.g. diode logic) to generate a code representative of the key which was depressed.

Such encoders of the prior art have generally been quite costly to construct and accordingly any equipment using prior art encoders has been quite costly.

There has thus developed a need for less costly encoded switch array which could be constructed on a volume basis at substantially less cost per key. An important feature of the new keyboard for the array is a new and improved switch which incorporates a highly conductive contactor.

The switch of this invention is preferably of the pushbutton type which preferably incorporates a conductive contactor having a hole therethrough in which there is positioned a first resilient biasing means to hold it above electrical contacts positioned on a board and a second resilient biasing adapted to engage the contactor above the first resilient biasing adapted means to cause said contactor to be forced against the contacts.

In its most preferred form the new and improved switch uses a conductive elastomer as the contactor, said conductive elastomer exhibiting less than a 1 ohm per cubic centimeter change in resistance in 10 milliseconds with applied pressure used to press it against electrical contacts on which it closes to complete a circuit. In addition, the conductive elastomer used preferably has a resistivity less than 0.01 ohm centimeters.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view partially broken away showing the encoded switch array according to the invention;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a top view of a circuit board suitable for use in the invention;

FIG. 4 is a bottom view of the encoded switch array with parts broken away to show the bottom of the circuit board shown in FIG. 3;

FIG. 5 is a sectional view taken along line 5--5 of FIG. 4;

FIG. 6 is a perspective view showing one key and the parts cooperating with it as incorporated in the encoded switch array of the invention;

FIG. 7 is a view of the preferred form of the contactor of this invention in engagement with the wiring pattern of the circuit board shown in FIG. 3; and

FIG. 8 is a graph showing the rise or response time of a contactor comprising a conductive plastic incorporating metal particles as the conductive material versus a contactor comprising a conductive plastic incorporating carbon black as the conductive material.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference should now be had to FIGS. 1-6 which shows the preferred construction of the coded switch array of this invention. In FIG. 1, the encoded switch array is shown generally at 10 and includes an outer frame 11 as for example, of plastic. In the confines of the frame there is provided a plurality of keys 12 forming a keyboard. The keys may have numbers, letters or other indicia thereon as per conventional keyboards. Each of the keys 12 of this invention are provided with a key stem 13 and a guide pin 14. The keys 12 may be constructed of plastic, (e.g. nylon, polypropylene), or metal.

At 15 there is shown a circuit board used in the encoder of this invention. The circuit board (e.g. of plastic, e.g. Formica, Bakelite) is provided with an electrical circuit pattern 16, (e.g. of copper plate thereon conventional in the art) for developing encoded output signals indicative of the depression of a particular key.

The circuit pattern shown is adapted to provide output signals from twelve connector pins coupled thereto as shown at 17, each pin providing an indication of the depression of one or more keys. The odd or thirthteenth pin shown at 18 is used as a common to which power may be applied or which may be grounded to complete a circuit. The circuit pattern associated with each key comprises a plurality of spokes 19 on either side of holes 20 through which the stems of the key extend. In addition, holes 21 are provided for the passage of the key guide pins 14. The circuit board additionally is provided with holes 22 to permit pins 23 of the frame 11 to extend therethrough (see FIG. 5).

At 25 in FIGS. 2,4,5 and 6 there is shown a back plate (e.g. of plastic) for holding the board 15 and the other parts of the switch not yet described together. The back plate is provided bores 26 into which pins 14 are adapted to extend. In addition, plate 25 includes bores 27 in which the stems 13 are adapted to slide up and down upon depression and then release of the keys 12. To retain the stems 13 within the bores 27, the stems are provided with heads 13a as shown in FIG. 2.

Further, as shown in FIGS. 2 and 6 the plate 25 is provided with additional bores 29 for holding therein resilent biasing means or springs 30. Each spring 30 is constructed such that the lower portion 30a thereof rests against the side walls of the bore and the top portion 30b thereof extends above the bores 29 through the holes 19 of the board 15.

The spring 30 is most preferably of the Belleville type (in the nature of a drain plunger) to impart the sensation that the key has been fully depressed. With this type of spring upon depression of a key a snap action or toggle is felt by the finger as spring portion 30a tends to try to invert about spring portion 30b.

The spring 30 is preferably made of plastic material such as polyethylene, polypropylene or silicone rubber. It should be understood that other types of springs although less preferable may be used to perform the function of spring 30 as for example, metal coil springs and the like whether or not they provide the preferred snap action.

Positioned on top of each of the spring portion 30b there is provided a contactor 35. In the most preferred form the contactors are in the shape of a cylinder having a central bore 36 through which the stem 13 is adapted to move through as a key 12 is depressed. The contactor 35 is preferably comprised of a plastic material and most preferably a conformable and resilent plastic material which is adapted to conform to the shape of the surface of the wiring pattern contacts on the circuit board 15 to insure good contact. The plastic material is most preferably filled with metal particles such as silver, copper, gold, etc., or any combination thereof which exhibit good conductive properties.

In some applications, the conductive particles may be carbon black, although the use of carbon black particles in switch applicators is much less desirable because of the substantial change in the resistance of such particles upon application of pressure to the particles (e.g. upon depression of the key 12). In addition, in certain cases the contactor may be constructed of solid metal as for example, silver, copper, tin foil, although the use of solid metal contactors in lieu of a resilent and comformable plastic is significantly less desirable because of bounce problems which ensue as well as the lack of consistently good contact.

In its most preferred form the contactor is constructed of plastic materials containing 10 to 80 volume percent of metal particles (e.g. silicone rubber filled with silver flake). Examples of other conductive plastics suitable for use in this invention are further disclosed in U.S. Pats. No. 3,140,342 and No. 3,576,387.

Most preferably the preferred contactor 35 of this invention is a conductive plastic which has a volume resistivity less than 0.01 ohm centimeters so that is is of low resistance. In addition, most preferably, the conductive plastic exhibits less than a 1 ohm per cm.sup.3 change in resistance in 10 microseconds when under pressure applied by key depression. This has been accomplished in this invention by using as heretofore mentioned the preferred conductive plastic comprising a conformable plastic material (e.g. silicone rubber or the like) filled with metal particles, (e.g. silver) in the amount of 10 to 80 volume percent.

It should be understood that in addition to the metal particles used to fill the conductive plastic, inert or nonconductive fillers such as silica, talc, etc. may also be incorporated therein as long as the mechanical and electrical properties of the contactor is not negatively effected in a substantial manner.

As shown in FIGS. 2 and 6 the array also includes a dust cover 39 such as of paper, polyethylene film or the like. The dust cover 39 is provided with holes therein at 39a and 39b for permitting the movement of the stem 13 and the pin 14 therethrough.

Above the dust cover and about each key stem 13, but below the key surface there is provided another resilent biasing means or spring 41. This spring is shaped so that a portion 41a is adapted to engage the cover about contactor 35. Upon depression of a key, the spring 41 is moved downwardly and forces the contactor 35 against the board contact spokes 20. As the key continues downwardly, the spring portion 41b is compressed downwardly. When the key is released, spring portion 41b then returns to its original configuration and urges the key upwardly. The spring 41 is preferably constructed of the same material as the spring 30.

In operation, one or more keys 12 of the array are depressed. The depression of a key causes the pin 14 and stem 13 to move downwardly. As the key moves downwardly, the stem shoulder 13a (FIG. 2) engages the spring portion 30b while at the same time moving spring 41 downwardly. As the contactor 35 continues to move downwardly, and engages the circuit board and its contact as shown in FIG. 7 to make electrical contact, springs 30 and 41 are depressed. Further depression produces a toggle or snap action touch in the finger of the individual depressing the key as the spring 41 trys to invert upon itself. At this point the individual using the device knows that the key may be released because contact has been made.

In FIG. 8, there is shown a graph comparing the response produced by the preferred conductive plastic contactor of this invention, that is one filled with highly conductive metal particles versus a usable conductive plastic contactor containing carbon black particles as the conductive material therein when both are used in series with a source voltage and a resistance in series with the source and the switch.

With the preferred contactor composition of this invention (conductive plastic which is metal filled), it has been found that it is possible to achieve response times for the switch such that upon closure of the switch, substantially full source voltage will appear across the resistance in less than 10 microseconds. It has been found that the fast response time is most preferred to meet the rise time requirements of solid state devices presently used in most applications.

With slower response time switches it has been found that noise or false signals may appear at the input of the semiconductor device (this causes it to turn on or off erroneously) during the extended time of full switch closure. This is particularly true of a switch using carbon black as the conductive material as shown in FIG. 8.

With a conductive plastic containing carbon black, a considerable amount of time elapses between initial depression of the key until full contact closure is made. This is due to the properties of carbon black and the change resulting from the application of pressure thereto. With a conductive carbon black the resistance is initially high and then changes slowly becoming lower as more pressure is applied. Because of this property, contactors using conductive plastic having carbon black particles are much less desirable in high speed switching application. Accordingly, to avoid the disadvantages of conductive plastics filled with carbon black it has been discovered that the preferred contactors of this invention should exhibit less than a 1 ohm Per cm.sup.3 absolute change in resistance with pressure to meet the rise time requirements of presently used circuits. When contactors are made with this property, it has been found that the properties of the switch are such that they are compatible for use with fast rise time semiconductor devices. The preferred contactors may also be of the materials disclosed and described in U.S. Pat. application filed Apr. 7, 1971 in the names of George T. Hodgson,Jr., Vincent Squitieri and William J. Lynn entitled Interconnector for Integrated Circuits and the Like, assigned to the same assignee of this application, this application being incorporated herein by reference hereto.

Claims

We claim:

1. A switch comprising a key, a key stem extending therefrom, a first spring positioned below said key and about said stem, a contactor positioned and movable about said stem, a member having a hole therethrough for the passage of the stem, said member positioned below said contactor and said member having at least two electrical contacts about said hole, support means positioned below said member for slidably supporting the stem therein, and a second spring positioned on the support means and having a portion extending through said hole to maintain said contactor above said contacts.

2. A switch according to claim 1 in which he contactor is a conductive plastic.

3. An array according to claim 2 in which the conductive plastic is filled with carbon black.

4. A switch according to claim 2 in which he conductive plastic comprises a conformable plastic loaded with metal particles.

5. An array according to claim 4 in which the conductive plastic is filled with carbon black.

6. A switch according to claim 4 in which the metal comprises 10 to 80 volume percent of the conductive plastic.

7. A switch according to claim 6 in which the conductive plastic has a volume resistivity less than 0.01 ohm centimeters.

8. A switch according to claim 7 in which the conductive plastic exhibits less than a 1 ohm per cm.sup.3 change in resistance with pressure applied thereto by key depression.

9. An encoded switch array comprising a circuit board having a plurality of holes formed therethrough, electrical contacts supported by and positioned about at least some of said holes of said circuit board, a base plate having a plurality of bores in alignment with the plurality of holes having the electrical contacts positioned about them, a plurality of keys, key stems attached to said keys and extending through said holes having contacts positioned about them, a contactor positioned about each of the stems and adapted to move about said stems, first means for holding each of said contactors above said contacts, second means for pressing each of said contactors against said contacts upon the depression of said keys, said circuit board having a wiring pattern thereon coupled to said electrical contacts, and each stem adapted to slidably move within a different one of said bores, and wherein said first means comprises a plurality of springs which are positioned on said base plate and which have a portion which extends through said holes in said board to hold said contactors above said electrical contacts.

10. An array according to claim 9 in which said second means comprising a plurality of springs each having a portion positioned to press said contactor against said contacts and wherein the springs of each plurality of springs are constructed to force the keys back to their original position after they are released following depression.

11. A switch comprising a circuit board having at least one electrically conductive contact provided on one surface thereof, a conductive plastic contactor for electrically engaging said contact, first plastic insulator means extending between the contact and the conductive plastic contactor for positioning the conductive plastic contactor out of engagement with said contact, depressible key means supported by the board for forcing said contactor to engage said contact, insulator cover means positioned in physical contact with said contactor and spring means positioned in physical contact with said cover means and said key means to provide resistance to key means depression.

12. A switch according to claim 11 in which the conductive plastic contactor is filled with carbon black.

13. A switch according to claim 11 in which the cover is a plastic film.

14. A switch according to claim 11 in which the conductive plastic contactor has a resistivity less than 0.1 ohm centimeters.

15. A switch according to claim 14 in which the conductive plastic exhibits less than a 0.1 ohm per cm.sup.3 change in resistance with pressure applied thereto by key depression.

16. A switch comprising a key, a key stem extending therefrom, a first spring positioned below said key and about said stem, a member having a hole therethrough for the passage of the stem, said member positioned below said contactor and said member having electrical contact means about said hole, support means positioned below said member for slidably supporting the stem therein, and a second spring positioned on the support means and having a portion extending through said hole to maintain said contactor above said contact means.