GB2133625A - Electrical switch - Google Patents

Abstract

A tactile switch has first and second (typically upper and lower) membranes 10, 12 bearing conductive tracks 16, 18 and if required are separated by a spacer 14, apertured in a contact closure region (typically below a keyboard) where the tracks 16, 18 are formed with contacts 20, 22. The membrane assembly 10, 12, 14 seats on a moulded deck which has a depression 28 aligned with the switch, thus providing for overtravel after predetermined displacement necessary to close the contacts has been effected either by direct manual depression in the membrane or via a button or key. In a modification, instead of providing a depression in the deck or in a layer overlying the deck, depressions in the form of raised bubbles eg 38, 40 (Figure 3, not shown) are formed in the first and second membranes, the bubble in the lower membrane providing the required overtravel for the key. Ridges 60 (Figures 5 and 6, not shown) in one or both membranes may be used to house and protect the tracks to prevent "crossing" tracks from making contact. <IMAGE>

Description

SPECIFICATION Improvements in Tactile Switches Background to the invention This invention relates to tactile switches and particularly to keyboards incorporating such switches.

Known tactile switches comprise upper and lower membranes bearing conductive tracks and separated by a spacing layer apertured under the keys, where the conductive tracks incorporate contacts. A flat keyboard deck moulding underlies the lower mem brane. The contacts are closed when a key is depressed to deform the upper membrane into the corres ponding aperture in the spacing layer. When contact is made, the key is at the bottom of its travel.

In another form of a switch using conductive ink contacts, a thinner spacing layer can be employed in conjunction with an upward bubble in the contact bearing area of the upper membrane. Again, the membrane lie on a deck moulding, and contact clos ure is effected at the bottom travel limit of the key.

Although it is known that overtravel for a tactile switch is to be preferred, the only known method of affording this advantage is to interpose a layer of soft rubber or thin foam between the deck moulding and the lower membrane.

Object of the invention It is an object of this invention to provide an improved tactile switch having overtravel.

The invention According to one aspect of the invention, there is provided a tactile switch comprising first and second membranes carrying contacts for defining a switch, the membranes being spaced apart, the switch being closable by a small, predetermined displacement of the first membrane towards the second membrane, where there is provided means defining a depression in the region of the contacts which are to be pressed together whereby said switch is afforded limited over-travel beyond the predetermined displacement necessary to close the switch.

A tactile keyboard switch according to the invention thus typically comprises first and second (typically upper and lower) membranes carrying contacts for defining a switch under a key, the membranes being spaced apart at least in the region under the key so that the switch is closable by a small, predetermined, downward displacement of the key, and means defining a depression in such relation to the region ofthe membranes under the key that said key is afforded limited overtravel beyond the predetermined displacement necessary to close the switch.

Preferably, the first and second (or upper and lower) membranes are separated by a spacing layer apertured in the region under the key.

The invention is applicable to switches in which contact closure is affected by direct tactile contact and pressure on one of the membranes or by indirect tactile pressure or through a key or button.

In one embodiment, the depression defining means comprises a depression formed in a base layer at the region of the latter beneath the key. In this embodiment, the base layer may be constituted by a keyboard deck moulding, preferably with depres sions of spherical form, or alternatively, the base layer may be constituted by a duplicate ofthe spacing layer apertured in the contact closure region, under a key when provided.

In another embodiment, the depression defining means comprises raised, contact-carrying bubbles and/or ridges formed in both the first and second (or upper and lower) membranes. Preferably, the bubble or ridge formed in the second (lower) membrane is shallower than the bubble formed in the first (upper) membrane. In the latter instance, the bubble or ridge in the second (lower) membrane may also be of small ler diameter. This can make it possible to dispense with the spacing layer, conductive ink contacts being confined to the facing surfaces of said bubbles and/or ridges so that said ink contacts are normally spaced apart even when the first (upper) membarne lies directly against the second (lower) membrane, as when a spacing member is dispensed with.

Description of embodiments Embodiments of the invention are now described by way of example with reference to the accompany ing drawings, in which: Figure 1 shows a first embodiment of switch in axial cross section; Figure 2 shows a modification; Figure3 shows a second embodiment in axial cross section; and Figure 4 shows a modification; Figure 5 is a similar view to Figure 4 where ridges are formed to protect interconnecting conductors, and Figure 6 is a plan view of Figure 5.

Figure 1 shows the region of the switch forming components of a keyboard beneath a key (not shown). An upper membrane 10 is spaced from a lower membrane 12 by an apertured spacer 14. The facing surfaces of the membranes 10, 12 are formed with printed or otherwise formed conductive tracks 16, 18 which have contact areas 20, 22 on the axis of the switch. The spacer 14 has an aperture 24 centred on this axis, whereby pressing of the key effects deformation of the upper membrane 10 through the aperture to close the contacts 20, 22.

The lower membrane 12 lies on the keyboard deck moulding 26, which, instead of being wholly flat, has a spherical depression 28 centred on the key axis.

This depression 28 permits a limited overtravel ofthe key after it has moved through the small predetermined distance necessary to close the contacts. The depression in the deck moulding 26, conveniently made of Krypton (Trade Mark), may be about 0.20 mm in maximum depth to afford a convenient overtravel distance.

While a spherical depression is slightly advantageous to reduce strain on the upper membrane due to increased deformation thereof, a straight sided depression achieves the same major advantage in providing overtravel. Thus, in the modification of Figure 2, instead of providing depressions in the deck moulding, a duplicate apertured spacing layer 30 is interposed between the deck moulding and the lower membrane, with its aperture 32 in alignment with the aperture 24 in the spacer 14.

Figure 3 shows an alternative construction oftactile keyboard switch wherein the outer surfaces of upper and lower membranes 34, 36 bear conductive circuits, and the inner surfaces have contacts 38, 40 formed of conductive ink only in the facing regions thereof beneath a key (not shown). As illustrated, both the upper and lower membranes are formed, in a manner in itself known, with raised bubbles 42,44 at these regions. Thus, limited overtravel is provided by denting of the lower bubble 44 after the contacts 38, 40 have been closed due to denting of the upper bubble 42 by a predetermined key displacement.

Preferably,the lower bubble44 is of smaller diame- ter and smaller depth than the upper bubble 42. This makes possible the modification shown in Figure 4, in which the spacing layer, referenced 46 in Figure 3, is dispensed with. To this end the conductive ink contacts 48, 50 are confined strictly to the opposed, facing surfaces of the bubbles 52, 54 and, the lower bubble being smallerthan the upper bubble, shorting between the upper and lower circuits is avoided even though the membranes 56, 58 are in direct engagement around the periphery of the upper, larger bubble 52.

If conductors 62, 64 and 66, 68 are required to contacts 48, 50 and a number of such contact sets are required in a single membrane set then the conductors 62,64 leading to the contact 48 on one membrane can be prevented from touching the conductors 66,68 on the other membrane by forming ridges in the surface of one or both membranes to join the bubbles and locating the conductors appropriately as shown in Figures 5 and 6.

The above-described arrangements have the advantage of providing overtravel at minimal cost.

Only the embodiment of Figure 2 requires an additional component, and this is merely a duplicate of a component already present, whilst the embodiment of Figure 4 requires a reduced number of components.

It will be appreciated that the above-described arrangements may be modified in various ways within the scope of the invention as defined in the appended claims.

Claims (14)

1. A tactile switch comprising first and second membrane carrying contacts for defining a switch, the membranes being spaced apart, the switch being closable by a small, predetermined, displacement oi the first membrane towards the second membrane, wherein there is provided means defining a depression in the region of the contacts which are to be pressed together whereby said switch is afforded limited over-travel beyond the predetermined displacement necessary to close the switch.

2. A switch according to claim 1, wherein the membranes are separated by a spacing layer apertured in the said contact closure region.

3. A switch according to claim 1 or claim 2, wherein the depression defining means comprises a depression formed in a base layer at the region ofthe latter behind the key.

4. A switch as claimed in any of the preceding claims in which the first membrane is acted on by a key or push-button which when depressed causes the switch to be closed.

5. A switch as claimed in claim 4 when forming part of a keyboard made up of a plurality of such switches.

6. A switch as claimed in claim 5 wherein the keys of the keyboard are arranged above the two membranes so that downward pressure on the key results in the contacts making contact.

7. A switch according to claim 5 or 6, wherein said base layer is constituted by a keyboard deck moulding.

8. A switch according to any of claims 3 to 7, wherein said depression is of spherical form.

9. A switch according to claim 3 when appendant to claim 2, wherein said base layer is constituted by a duplicate spacing layer apertured in the said contact closure region.

10. A switch according to any of claims 1 to 9, wherein the depression defining means comprises raised, contact-carrying bubbles and/or ridges formed in the membranes.

11. A switch according to claim 10, wherein a ridge formed in the second membrane is shallower than the ridge formed in the first membrane, and the conductive contacts and conductors thereto are confined to the underside of the ridge in the first membrane so that the contacts and the conductors are separated from crossing conductors on the second membrane even in the absence of an intermediate apertured sheet of insulating material.

12. A switch according to claim 10, wherein the bubble or ridge formed in the second membrane is of smaller radius than the bubble or ridge formed in the upper membrane, conductive ink contacts being confined to the facing surfaces of said bubbles or ridges and to non-overlying regions of the membranes so that the contacts and conductors thereto are normally spaced apart even when the two membranes are otherwise in contact.

13. A keyboard incorporating a plurality oftactile keyboard switches as claimed in any of claims 1 to 9.

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