GB1593414A

GB1593414A - Keyboard switch assemblies

Info

Publication number: GB1593414A
Application number: GB2966877A
Authority: GB
Original assignee: JERMYN HOLDINGS Ltd
Current assignee: JERMYN HOLDINGS Ltd
Priority date: 1978-05-19
Filing date: 1978-05-19
Publication date: 1981-07-15

Description

(54) IMPROVEMENTS IN KEYBOARD SWITCH ASSEMBLIES (71) We, JERMYN HOLDINGS LI MITED, a British Company, of Sevenoaks Kent, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement.

This invention relates to a keyboard switch assembly by which is meant a plurality of electrical switches arranged together in a group, and usually situated on a single plane. Such keyboard switch assemblies are used, for example, in electronic typewriters, calculators, computer terminal equipment and process control. The present invention is particularly, although not exclusively concerned with the last named use of keyboard switch assemblies, where the conditions of use demand unusual requirements for the switch assembly, in particular in hazardous environments where the assemblies may be roughly handled, and may be subjected to spillage.

One form of keyboard switch assembly developed for such hazardous environments comprises a series of conductors which are selectively bridged by a further conductor which may be pressed onto the firstmentioned conductors by the operator. A plurality of such further conductors may be arranged to bridge a series of conductors thus providing a plurality of completely independent or semi-independent switches.

All such switches may be arranged as a single unit in the form of a board, the upper surface of the board being protected against the environment by a thin layer of plastics material, for example nylon or melinex (Registered trade mark). Operation of the individual switches is effected by pressing an appropriate one of said further contacts, through the protective layer of plastics material, until it bridges two of the firstmentioned contacts, thus effecting a connection.

One problem with this type of switch is the absence of tactile feel, which is very important in ensuring that the operator is able to satisfy himself that the switch has been operated. Existing switch designs which provide a degree of tactile feel suffer from the problem of contact bounce. In power switches a small degree of contact bounce can be tolerated and is, indeed, almost inherent in such switches due to the high contact pressure requirements leading to high relative speeds of movement of the fixed and moving contacts. In switches for process control, however, even a small amount of contact bounce cannot be tolerated owing to the very high operating speeds of the circuits being switched. The requirements of no contact bounce and good tactile feel are incompatible and the present invention seeks to overcome this difficulty and to provide a switch having low contact bounce and which possess a degree of tactile feel.

According to the invention there is provided a keyboard switch assembly comprising a plurality of electrical switches mounted on a common circuit board, each of said switches comprising a pair of mutually electrically isolated contacts on said circuit board; a sheet of insulating material overlying and in physical contact with said contacts, said sheet having a plurality of holes therein, each hole lying over a respective pair of contacts; a foil of conductive material overlying and in physical contact with said sheet of insulating material; a plurality of snap-action devices distributed about the surface of the foil in such a way that each device occupies a position substantially in register with a respective hole in the sheet of insulating material, but on the opposite side of the foil therefrom; and a further sheet of insulating material overlying said devices, the arrangement being such that pressure on said further sheet of insulating material in the region of a snap action device causes said foil to electrically bridge the associated pair of contacts.

Each individual switch is operated by pressure applied at an appropriate position on the further sheet of insulating material, such position being indicated to the operator by a pattern printed or otherwise visible on the surface of said further sheet. Finger pressure at the correct position is passed to the appropriate snap-action device which, at a preset pressure, snaps over, thus releasing a quantity of potential energy in the form of a sudden surge of pressure onto the conduc tivc foil beneath. This surge of pressure depresses the foil into the corresponding aperture in the sheet of insulating material and eventually bridges the contacts beneath.

The release of this potential energy is, of course, felt by the operator who is thus aware that the switch has operated correctly. The sudden release of energy also ensures that the foil, when it comes into contact with the contact is travelling fairly rapidly, thus ensuring a very positive contact.

As mentioned above, however, the present invention secks to provide a switch having a degree of tactile feel atid freedom from contact bounce. The rapid movement of the foil as it comes into contact with the contact gives rise to contact bounce which is unacceptable in many of the circumstances in which this switch will be used. In a preferred embodiment of the invention therefore, the switch is constructed to operate such that the pressure required to cause said foil to bridge its respective pair of contacts is less than the pressure required to snap over the snap-action device. In this event, the contacts are bridged by a relatively slowly moving foil, the release of potential energy which supplies the tactile feel not occurring until contact has successfully been made.

Various small snap-action devices are suitable for this application and are known in the art. The devices may be made of any suitable material. for example metal or plastics, since they do not themselves carry currcnt. In a preferred embodiment of the invention, each snap action device comprises a small dished disc of stainless steel, sometimes known as a bursting disc.

In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with refercnce to the accompanying drawing which shows a diagrammatic side section (not to scale) of a section of a keyboard switch according to this invention.

Referring to the drawing. the keyboard switch comprises a printed circuit board 1 of insulating material on which are printed, in the conventional manner, patterns of mutually isolated gold-plated copper conductors. The drawing shows two switches, each comprising a pair 2, 3 of mutually isolated conductors. In practice, however, the arrangement of conductors is more complex than this for efficient switching, although the various arrangements are well known and will not be described further.

Overlying the conductors 2, 3 is a sheet 4 of insulating material, which may be made of any mechanically stable insulating material, for example a suitable electrically insulating plastic film. Polyester film manufactured under the registered trade names of Mylar and Melinex has been found suitable for this application. A typical thickness for sheet 4 is about two thousandths of an inch.

Above each pair 2, 3 of conductors a respective hole 5, 6 is formed in the sheet 4, one such hole being formed at each switch position. Overlying the sheet 4 is an electrical conducting foil 7. The foil 7 is typically of stainless steel, about 1.5 thousandths of an inch thick; however, the foil 7 may also be made of copper or aluminium plated plastic.

Positioned above each hole 5, 6 is a respective dished disc 8, 9. The discs are retained in position by any convenient means - for example simple adhesive tape can be used to adhere each disc to the foil 7.

These discs have the property of storing the potential energy of the finger when pressed, and releasing it suddenly when the disc collapses or snaps over. In the particular embodiment illustrated, the discs 8, 9 are slightly smaller in diameter than the holes 5, 6 since this is important in ensuring that the disc does not snap over before contact between the foil 7 and the contacts is made.

Typical diameters are: disc: 7/16 inches hole: 5/8 inches The two factors which most affect operation of the switch are the ratio of the diameter of the disc to that of its respective hole 5, 6 in the sheet 4, and the ratio of the stiffness of the disc to that of the foil 7. These two ratios should be carefully adjusted so that the pressure required for the foil 7 to make initial contact with contacts 2, 3 is slightly less than the pressure required to snap over the disc. This ensures that bounce-free contact is made.

A packing layer 10 of mylar (Registered Trade Mark) or polychloroprene elastomer, for example, neoprene, covers the discs 8, 9 (this may not be necessary), and finally an outer protective cover in the form of a sheet 11 of insulating material overlays the layer 10. The sheet 11 may be made of any tough chemically resistant material, for example mylar or melinex (both Registered Trade Marks). A typical thickness is ten thousandths of an inch.

The whole structure is maintained in place by a gasket 12 which surrounds the perimeter of the keyboard switch, and to which the sheet 11 is bonded around its edge. The gasket may, for example, be of neoprene.

The switch is operated by finger pressure on the sheet 11. For example pressure represented by the arrow A will operate the left-hand switch. Pressure acting through the perimeter of disc 8 causes the foil 7 to be pressed downwards until it eventually reaches contacts 2 and 3 and forms an electrically conducting bridge between them. Continued pressure on disc 8 will cause the disc to snap over thus producing a tactile feel to the operator signalling that contact has been established.

The above described switch is simple in structure and is thus relatively cheap to manufacture, and is very robust. In addition, the snap-action ensures a significant tactile feel to the switch in operation. The outer protective cover ensures that liquids spilt onto the switch do not affect operation, and the switch can be used for extended periods in extremely hazardous environments.

WHAT WE CLAIM IS: 1. A keyboard switch assembly comprising a plurality of electrical switches mounted on a common circuit board, each of said switches comprising a pair of mutually electrically isolated contacts on said circuit board; a sheet of insulating material overlying and in physical contact with said contacts, said sheet having a plurality of holes therein, each hole lying over a respective pair of contacts; a foil of conductive material overlying and in physical contact with said sheet of insulating material; a plurality of snap-action devices distributed about the surface of the foil in such a way that each device occupies a position substantially in register with a respective hole in the sheet of insulating material, but on the opposite side of the foil therefrom; and a further sheet of insulating material overlying said devices, the arrangement being such that pressure on said further sheet of insulating material in the region of a snapaction device causes said foil to electrically bridge the associated pair of contacts.

2. A keyboard switch assembly as claimed in claim 1 wherein the switch is constructed to operate such that the pressure required to cause said foil to bridge its respective pair of contacts is less than the pressure required to snap over the snapaction device.

3. A keyboard switch assembly as claimed in either one of claims 1 or 2 wherein the positions of the switches are printed or otherwise visible on the surface of said further sheet.

4. A keyboard switch assembly as claimed in any one of claims 1 to 3 wherein each snap action device comprises a small dished disc.

5. A keyboard switch assembly as claimed in claim 4 wherein each said disc is made of plastics material.

6. A keyboard switch assembly as claimed in claim 4 wherein each said disc is made of stainless steel.

7. A keyboard switch assembly as claimed in any one of the preceding claims further including, between said further sheet of insulating material and said snapaction devices, a packing layer of resilient plastics material.

8. A keyboard switch assembly as claimed in any one of the preceding claims wherein said foil is about 1.5 thousandths of an inch thick.

9. A keyboard switch assembly as claimed in any one of the preceding claims wherein said foil is made from stainless steel.

10. A keyboard switch assembly substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. thousandths of an inch. The whole structure is maintained in place by a gasket 12 which surrounds the perimeter of the keyboard switch, and to which the sheet 11 is bonded around its edge. The gasket may, for example, be of neoprene. The switch is operated by finger pressure on the sheet 11. For example pressure represented by the arrow A will operate the left-hand switch. Pressure acting through the perimeter of disc 8 causes the foil 7 to be pressed downwards until it eventually reaches contacts 2 and 3 and forms an electrically conducting bridge between them. Continued pressure on disc 8 will cause the disc to snap over thus producing a tactile feel to the operator signalling that contact has been established. The above described switch is simple in structure and is thus relatively cheap to manufacture, and is very robust. In addition, the snap-action ensures a significant tactile feel to the switch in operation. The outer protective cover ensures that liquids spilt onto the switch do not affect operation, and the switch can be used for extended periods in extremely hazardous environments. WHAT WE CLAIM IS:

1. A keyboard switch assembly comprising a plurality of electrical switches mounted on a common circuit board, each of said switches comprising a pair of mutually electrically isolated contacts on said circuit board; a sheet of insulating material overlying and in physical contact with said contacts, said sheet having a plurality of holes therein, each hole lying over a respective pair of contacts; a foil of conductive material overlying and in physical contact with said sheet of insulating material; a plurality of snap-action devices distributed about the surface of the foil in such a way that each device occupies a position substantially in register with a respective hole in the sheet of insulating material, but on the opposite side of the foil therefrom; and a further sheet of insulating material overlying said devices, the arrangement being such that pressure on said further sheet of insulating material in the region of a snapaction device causes said foil to electrically bridge the associated pair of contacts.