GB2088637A - Electric Switches - Google Patents

Abstract

A switch comprises a pair of flexible conductors 3, 4 arranged on opposite surfaces of a sheet 1 of compressible electrically insulating material which is provided with one or more intermediate conductor elements 2 which are clinched through the material 1 between its opposite surfaces in such a way that the exposed parts of the or each element 2 lie below the adjacent general surface of the sheet 1. Squeezing of the flexible conductors 3, 4 towards one another causes electrical contact to be made between the flexible conductors via one or more of the intermediate conductor elements 2. <IMAGE>

Description

SPECIFICATION Improvements relating to electric switches This invention relates to switches, particularly strip switches.

According to the invention there is provided a switch comprising a pair of flexible conductors arranged in sandwich fashion on opposite surfaces of a sheet of compressible electrical insulating material in such a way as to be normally electrically isolated from one another, said insulating material being provided with one or more intermediate conductor elements which are clinched through the material between said opposite surfaces in such a way that the exposed parts of the or each element lie below the adjacent general surface of the sheet, the arrangement being such that the squeezing of said flexible conductors towards one another causes electrical contact to be made between the flexible conductors via one or more of said intermediate conductor elements.

The switch of this invention can be fabricated in the form of a mat-for example for use in burglar alarm and machinery safety applications or in the form of a strip. The sensitivity of the switch is dictated by the degree of compressibility of the sheet material. In a preferred embodiment plastics or rubber foam material is used, but solid plastics or rubber could be used where lower sensitivity is required. The thickness of the sheet material is largely a matter of conveni ence-a a thickness of about 3mm has been used successfully. Switches for high voltage will required a sheet of greater thickness.

The intermediate conductor elements may take many forms, for example staples, eyelets or rivets and may be applied by suitable automatic machines-for example automatic staplers. Alternatively, the intermediate elements may take the form of wire clinched through the material by means of a wire stitching machine. The wire may be continuous or preferably, cut into discrete lengths.

The use of wire has the advantage that a greater choice of materials is available--for example tinned copper wire could be used.

The type of flexible conductor used will depend upon the nature of the switch. For mat-type switches, the flexible conductor may take the form of metal foil or mesh. For strip switches, the flexible conductor may be strip metal foil or mesh, but preferably is braided wire. In some circumstances stranded wire could be used.

In order that the invention may be better understood,two embodiments thereof will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a perspective view of a length of strip switch constructed in accordance with the invention; Figure 2 is a sectional side view along the length of the switch of Fig. 1; and Figure 3 is an exploded perspective view of a mat-type switch constructed in accordance with the invention.

In Fig. 1, the switch is shown partly disassembled to show the construction. The switch comprises a strip 1 of electrically insulating foam rubber or plastics material in which are clinched a plurality of lengths 2 of metal, each length being formed in like manner to a staple. Each length 2 of metal forms an intermediate conductor element between two lengths 3, 4 of braid which protrude to form the external connections 5, 6 of the switch.

Two layers of self-adhesive insulating tape 7, 8 complete the assembly, although further protection may be provided in the form of a plastics "dip" coating (not shown), or similar, if desired. Alternatively the assembly may be slipped into a sleeve of plastics or rubber material which may be shaped for a particular purpose-for example as a floor switch.

During construction, as each length 2 of metal is inserted through the foam strip, a certain amount of gripping takes place which acts to compress the material in the region of each element, as shown most clearly in Fig.

2. The exposed conductive surfaces of each element thus lie slightly below the level of the surrounding material. In the embodiment illustrated, the elements are all shown aligned with one another along a straight line, and the lengths 3, 4 of braid overlie the line of the elements on respective opposite sides of the strip 1.

The switch may be operated at any point along its length by squeezing the two lengths of braid together. This can be achieved by squeezing between the fingers such as shown by the arrows A in Fig. 2 or, more likely, between the finger and a solid object. Clearly the switch could also be operated by pressure from other parts of the body, or by pressure from other objects. Connection between the two braids is effected via the intermediary of one of the elements 2, or possibly, via two adjacent elements 2 in the event that the squeezing pressure is applied between elements. In connection with the latter, it will be appreciated that the spacing between adjacent elements is fairly critical if the switch is to operate reliably at any point along its length.

Clearly, account must be taken of the physical size of the expected switch actuator-for example a human finger or foot since, if the identity of the actuator is known there is little advantage to be gained by arranging the elements at a considerably lower separation than the expected length of contact by the actuator.

The switch described above effectively eliminates one serious disadvantage which existing strip switches using braided or similar material possess. In the course of time, or due to local damage to the switch, it is possible for one of the wires comprising the braid to break, leaving a whisker of wire which can permanently short the switch, thus rendering it useless. In strip switches it is well nigh impossible to locate the position of such a break, and the switch usually has to be scrapped. In the switch described herein, the failure of the braid in this way will not result in failure until such time as both pieces of braid become damaged at the same point along the length of the switch an unlikely occurrence.If only one of the braids is damaged, the switch will continue to operate perfectly normally since it requires an electrical connection between both braids and a single element before permanent through contact is made.

Although the embodiment illustrated shows all the elements 2 in line with one another, it is possible to arrange each element across the strip, at 90 or some intermediate angle. In accordance with the particular angle used, greater flexibility in different directions can be obtained. Of course, it is possible that different elements on the same switch may be oriented differently, but this is unlikely to lend itself to efficent continuous production.

Fig. 3 shows a mat-type switch using the principles of the invention. The switch comprises a rectangular sheet 9 of electrically insulating foam rubber or plastics material in which are clinched a plurality of regularly spaced eyelets 1 0. The eyelets 10 perform the same electrical function as the staple-like lengths 2 of the above-described embodiment.

The flexible conductors take the form of respective sheets 11, 1 2 of flexible conductive material such as metal foil, mesh or metallised plastics or paper material to which respective external switch connections 13, 14 are attached. The sheets 11, 1 2 form a sandwich with the sheet 9 of foam material, the whole being encased in a suitable protective covering (not shown).

Switches constructed as shown in Fig. 3 may be used in machine safety and burglar alarm applications where the expected actuator is a human foot. The spacing of the eyelets 10 can thus be suitably adjusted for this purpose.

Although it is obviously necessary that the three sheets 9, 11 and 1 2 be kept in registry with each other, it is not anticipated that the facing surfaces of the sheets will require to be adhered together although this is, of course, a possibility and might enable the basic rectangular switch to be readily cut to shape with scissors in accordance with actual requirements without the danger of the switch falling to pieces.

The switches described above may be made quickly and cheaply from readily available materials and using an automatic stapler or wire threaded machine on a continuous production line basis. It has been found that, after initial assembly, the operation of the switch benefits from being rolled, for example by laying the switch on a smooth hard surface, and rolling it with a hand roller.

Claims (8)

1. A switch comprising a pair of flexible conductors arranged in sandwich fashion on opposite surfaces of a sheet of compressible electrical insulating material in such a way as to be normally electrically isolated from one another, said insulating material being provided with one or more intermediate conductor elements which are clinched through thematerial between said opposite surfaces in such a way that the exposed parts of the or each element lie below the adjacent general surface of the sheet, the arrangement being such that the squeezing of said flexible conductors towards one another causes electrical contact to be made between the flexible conductors via one or more of said intermediate conductor elements.

2. A switch as claimed in claim 1 wherein the sheet takes the form of an elongate strip and said flexible conductors are elongate to match, and wherein said intermediate conductor element or elements are arranged in a row along a central portion of the strip.

3. A switch as claimed in claim 2, wherein each flexible conductor takes the form of a braided wire.

4. A switch as claimed in any one of claims 1, 2 or 3, wherein the intermediate conductor elements take the form of staples.

5. A switch as claimed in any one of claims 1, 2 or 3, wherein the intermediate conductor elements take the form of wire which is clinched through the insulating material.

6. A switch as claimed in claim 5, wherein the wire is continuous, being stitched through the insulating material to retain it in place and so as to define a conducting surface alter nately on one surface of the insulating material and the other.

7. The switch as claimed in claim 5, wherein the wire is cut into discrete lengths to provide a plurality of individual intermediate conductor elements distributed about the surface of the sheet of insulating material.

8. A switch substantially as hereinbefore described with reference to the accompanying drawings.