GB2293046A

GB2293046A - Switch

Info

Publication number: GB2293046A
Application number: GB9417865A
Authority: GB
Inventors: Raymond Edwin Connolly
Original assignee: Permasign Ltd
Current assignee: Permasign Ltd
Priority date: 1994-09-06
Filing date: 1994-09-06
Publication date: 1996-03-13
Also published as: GB9417865D0

Abstract

A switch comprises first and second layers 1, 2 each comprising electrically conductive material 6, 5, the switch having an operative region 8 in which the first layer 1 is spaced apart from the second layer 2 by a spacing member 7 located between the first and second layers and is deformable towards the second layer to make electrical contact with the second layer. The spacing member 7 is free in the operative region 8 to move relative to at least one of the first and second layers. The switch may be positioned in a burglar alarm bell box to detect an attempt to gain access into the box. <IMAGE>

Description

SWITCH The present invention relates to a switch, for example a membrane switch.

In one conventional form of membrane switch two electrically conductive layers are separated by an insulating adhesive or varnish layer which bonds the electrically conductive layers together. The adhesive layer has apertures in it through which one of the electrically conductive layers can be deformed to meet the other and make electrical contact across the switch.

Such a switch has the disadvantage that because the adhesive layer is bonded to both of the electrically conductive layers the switch cannot operate successfully if it is bent or folded; the electrically conductive layers tend to buckle and make contact before the switch is operated.

According to a first aspect of the present invention there is provided a switch comprising first and second layers each comprising electrically conductive material, the switch having an operative region in which the first layer is spaced apart from the second layer by a spacing member located between the first and second layers and is deformable towards the second layer to make electrical contact with the second layer; the spacing member being free in the operative region to move relative to at least one of the first and second layers. Since the spacing member is free in the operative region to move relative to at least one of the first and second layers the tendency of the layers to buckle is avoided and the switch can work correctly even if it is bent or folded.

Preferably, in the operative region of the switch the spacing member is free to move relative to both the first and second layers. The spacing member is preferably a sheet of material having apertures through it, through which one or both of the first or second layers can be deformed to meet the other and make electrical contact across the switch. The spacing member is suitably a mesh sheet.

The spacing member may suitably be fixed between the first and second layers by the spacing member and the first and second layers being joined together near, or preferably around, the periphery of the operative region; most preferably around the perimeter of the switch.

The first and second layers are suitably movable relative to each other in the operative region. Such movement preferably includes lateral movement generally in the plane of the layers.

In security devices to which unauthorised access is to be inhibited, for example bell boxes of burglar alarms, it is useful to be able to detect intrusion, for example by a drill, into the device. A switch according to the present invention may be used for this, preferably with the switch adjacent the inner surface of the outer wall of the device so that if an object intrudes into the device it will cause contact to be made across the switch. This contact can be used to trigger an alarm.

More generally, according to a second aspect of the present invention there is provided an enclosure comprising a membrane switch for detecting an attempt to gain access to the interior of the enclosure.

Preferably, the membrane switch at least partially encloses the interior of the enclosure. The enclosure suitably includes a wall to prevent or inhibit access to the interior of the enclosure. In this case the wall is preferably adjacent the membrane switch, preferably with the membrane switch adjacent the inner surface of the wall. Especially where the wall is not flat a flexible switch, for example a switch according to preferred features of the first aspect of the present invention, may suitably be used.

The present invention will now be described by way of example with reference to the accompanying Figures, in which: Figure 1 shows a schematic plan view of a switch; Figure 2 shows a partial schematic exploded view of the switch of Figure 1; Figure 3 shows a cross section on line I-I in Figure 1; Figure 4 shows a partial plan view of another switch; and Figure 5 shows a schematic cross-section of a bell box containing a switch.

The switch shown in Figures 1 to 3 comprises upper 1 and lower 2 layers each comprising a membrane 3,4 of flexible plastics material and a conductive layer 5,6 on the mutually facing surfaces of the membranes. Between the upper and lower layers is a spacing layer 7 of electrically insulating plastic mesh material. In an operative region 8 of the switch each of the upper and lower layers can be deformed to meet the other through apertures 9 in the mesh so that the conductive layers 5,6 make electrical contact. Electrical terminals 10,11 are provided to allow the switch to be connected to other equipment and to allow electrical contact between the conductive layers to be sensed. The upper and lower layers and the spacing layer are bonded together only at the periphery of the operative region, around the edge of the switch.In the operative region the upper and lower layers and the spacing layer are all free to move relative to each other, although such movement is limited because the layers are joined together outside the operative region. The upper and lower layers and the spacing layer are flexible and the switch can be bent or folded. As it is bent the layers slide relative to each other in the operative region so that the switch can still operate correctly.

The membranes are sheets of thin, plastics film, for example polyester. These sheets are flexible but are sufficiently stiff that they will not deform unduly when the switch is bent and will remain generally parallel to each other. The conductive layers are layers of conductive ink which have been screen printed on to the membranes.

The spacing layer is a sheet of plastics mesh formed of generally linear elements 12 which are welded together at their intersections, for example nylon. The thickness and pitch of the mesh is chosen to give the switch the correct sensitivity. For example, the thickness of the mesh may be around 0.5 mm and the pitch around 5 to 7 mm.

The mesh provides a grid defining regularly spaced apertures.

At the perimeter of the switch an adhesive 13 is applied so as to hold the inner and outer layers together and seal the interior of the switch against the atmosphere.

The spacing layer is sandwiched between the upper and lower layers to prevent electrical contact between the conductive layers 5,6 at the edge.

The first aspect of the present invention allows relatively large membrane switches to be made. For example, switches according to the first aspect of the invention can be made whose operative regions have areas of greater than 2500 mm2. Membrane switches of this size have hitherto not been feasible. As shown in Figure 4 (in which like parts are numbered as for Figures 1 to 3) strips 14 of highly conductive material (for example silver-loaded ink) mayo3'beincluded on the conductive layer to reduce the maximum resistance between the terminals 10,11 when electrical contact is made between the conductive layers.

The switch 15 of Figure 4 is suitable for fixing in place adjacent the outer wall of a burglar alarm's bell box, to detect intrusion into the bell box by, for example, a finger, a drill or expanding foam intended to disable the bell. This use of the switch is illustrated in Figure 5.

The central region 16 of the switch is located against the interior surface of the front outer wall 17 of the bell box. Wings 18 of the switch are folded relative to the central region and located against the interior surfaces of the side walls 19 of the bell box. The bell box is attached to a surface 20. Both the central region and the wings include on them the operative region of the switch. The switch is preferably fixed to the bell box: for example, the wings of the switch may be fixed to the outer wall with adhesive. If an object enters the box it will press the operative region of the switch and thus the intrusion can be detected.

In the membrane switch of Figure 4 the terminals 10,11 are on the end of conductive tracks on flexible tails 21 which can be connected to the alarm system inside the bell box. The switch 15 is flexible and can therefore conform to the shape of the outer wall of the bell box even if the wall (for example at 17) is not flat. The switch can constitute part of the enclosure of the bell box, at least partially enclosing the interior of the bell box.

Claims

1. A switch comprising first and second layers each comprising electrically conductive material, the switch having an operative region in which the first layer is spaced apart from the second layer by a spacing member located between the first and second layers and is deformable towards the second layer to make electrical contact with the second layer; the spacing member being free in the operative region to move relative to at least one of the first and second layers.

2. A switch as claimed in claim 1, wherein there is at least one aperture through the spacing member in the operative region, by which the first member can make electrical contact with the second member.

3. A switch as claimed in claim 2, wherein there is more than one such aperture through the spacing member in the operative region.

4. A switch as claimed in claim 2 or 3, wherein the spacing member comprises a plurality of spacing elements spaced apart so as to define the or each aperture.

5. A switch as claimed in claim 4, wherein the spacing elements are connected together in the operative region.

6. A switch as claimed in claim 4 or 5, wherein the spacing elements define a plurality of regularly spaced apertures in the operative region.

7. A switch as claimed in any of claim 4 to 6, wherein the spacing elements are generally linear.

8. A switch as claimed in any of claims 4 to 7, wherein the spacing elements provide a grid defining apertures throughout the operative region.

9. A switch as claimed in any preceding claim, wherein the spacing member comprises a mesh sheet.

10. A switch as claimed in any preceding claim, wherein the spacing member is free to move relative to both the first and second layers in the operative region 11. A switch as claimed in any preceding claim, wherein the first and second layers lie generally parallel in the operative region.

12. A switch as claimed in any preceding claim, wherein in the operative region the first layer is deformable towards the second layer to make physical contact with second layer.

13. A switch as claimed in any preceding claim, wherein in the operative region the second layer is deformable to make electrical and/or physical contact with the first layer.

14. A switch as claimed in any preceding claim, wherein the first and second layers are free from mutual adhesion in the operative region.

15. A switch as claimed in any preceding claim, the operative region of the switch being flexible.

16. A switch as claimed in any preceding claim, wherein the first and second layers each comprise an electrically conductive layer on their mutually facing surfaces.

17. A switch as claimed in any preceding claim, wherein the first and second layers and the spacing member are joined near the periphery of the operative region.

18. A switch as claimed in any preceding claim, wherein the switch is a membrane switch.

19. A switch as claimed in any preceding claim, wherein the area of the operative region is greater than 2500 mm2.

20. A switch substantially as herein described with reference to the accompanying drawings.

21. A security device including a switch as claimed in any preceding claim.

21. A security device as claimed in claim 21, wherein at least part of the device is at least partially enclosed by a switch according to any of claims 1 to 20.

24. An enclosure comprising a membrane switch for detecting an attempt to gain access to the interior of the enclosure.

25. An enclosure as claimed in claim 24, wherein the membrane switch at least partially encloses the interior of the enclosure.

26. An enclosure as claimed in claim 24 or 25, comprising a wall to inhibit access to the interior of the enclosure, the wall being adjacent the membrane switch.

27. An enclosure substantially as herein described with reference to the accompanying drawings.