GB2273203A

GB2273203A - Electrical switches

Info

Publication number: GB2273203A
Application number: GB9222297A
Authority: GB
Inventors: Peter Gordon Hart; Andrew Richmond Davison
Original assignee: TRW Transportation Electronics Ltd
Current assignee: TRW Transportation Electronics Ltd
Priority date: 1992-10-23
Filing date: 1992-10-23
Publication date: 1994-06-08
Anticipated expiration: 2012-10-23
Also published as: GB9222297D0; GB2273203B

Abstract

An electrical switch assembly has two membrane switches (2, 3) operated mutually exclusively by opposite ends of a rocker beam (4) which is biased to an intermediate, non-operating position. The beam (4) has a central fulcrum (6, 8) shaped so that as it tilts towards either operating position, the pivot point rises slightly. The ends (10) of the beam have surfaces (11, 12) which cooperate with the switches (2, 3) and which are so shaped, eg angled, that, with the tilting and raising of the beam, the bearing force on either membrane switch is substantially directly downwards, with a negligible sideways component. A crossbeam co-operating similarly with another pair of membrane switches can use the same fulcrum (6, 8). <IMAGE>

Description

"Improvements relating to Electrical Switches" This invention relates to electrical switches.

For low voltage applications membrane switches are coming into increasing use. Their common form is frusto conical, and they are often mounted base down on a printed circuit board (PCB) spanning two conductors. Inside the top of the frustum there is a carbon disc or pellet, and when the membrane switch is pressed axially, as by an aligned plunger, the conductive disc is brought into contact with the two conductors and forms a bridge. The sloping side of the membrane folds in annular ridges symmetrically around the axis.

However, it is not always appropriate or convenient to operate such a switch by a member which has true axial movement. For example, it may be required to operate it by the end of a rocker arm, a similar switch being alternately operated by the other end of the arm. These ends move in an arc, and therefore there is bound to be a component of sideways movement on the switch as it is actuated. If this is excessive, then the frustum can collapse lopsidedly, and only imperfect contact, or even none at all, is achieved.

It is the aim of this invention to reduce the risk of this type of malfunction.

According to the present invention there is provided a rocker switch assembly comprising a rocker beam, a fulcrum for the beam, means for retaining the beam in a mid-position of stable equilibrium, and membrane switches selectively operable by opposite ends of the beam according to its tilt either side of said mid-position, the fulcrum comprising a socket and a member rigid with the beam received in and spring urged into the socket with freedom only to tilt and to rise and descend within the socket, wherein the tilting of the beam is arranged to generate such rise and descent and the surfaces at the ends of the beam which co-operate with the membrane switches are shaped to exert a substantially exclusively axial force on each switch as the pivot centre of the beam alters.

In one basic form the member received in the socket may be part-cylindrical,; its axis being the pivot axis of the beam, with a bottom flat which abuts the base of the socket in the equilibrium position. When the beam is tilted, the part-cylindrical member is levered up about an edge of the flat, so raising the pivot axis of the beam. In a more complex arrangement the member received in the socket is part-spherical with a bottom flat, and that can be tilted in any direction. Typically, there will then be two beams forming a cross, and four membrane switches at the corners of a square.

Each beam end that engages a membrane switch conveniently has two surfaces meeting at a shallow angle, one surface acting on the membrane switch during a first part of its operation and the other surface so acting during a final part. They can be so angled that any tendency due to their arcuate movement to drag the switch sideways is counteracted by their thrust on top of the switch.

For a better understanding of the invention, one embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic partly sectioned side elevation of a rocker switch assembly in a stable, centred position, Figure 2 is a diagrammatic partly sectioned side elevation of the switch of Figure 1 tilted to one operative position, Figure 3 is a diagrammatic partly sectioned side elevation of another form of rocker switch assembly in an operative position.

Referring to Figures 1 and 2, a horizontal printed circuit board (PCB)1 has two membrane switches 2 and 3 which are to be operated mutually exclusively by a rocker beam 4.

At its centre, the beam is enlarged and has a downwardly projecting boss 5 which terminates in a part-cylindrical portion 6 with its axis horizontal and perpendicular to the length of the beam. This portion 6 has a bottom flat 7 which is in a horizontal plane when the beam is level in the equilibrium position and rests face to face with the top of the PCB1. Laterally, the portion 6 is confined closely by a socket 8.

Another socket 9 is formed centrally of the beam 4 itself, and extends down into the boss 5. Its base is a shallow V shape, symmetrical with respect to the transverse bisecting plane.

At the ends of the beam 4 there are downwardly projecting lugs 10 with undersides that engage the tops of the membrane switches 2 and 3. Each underside is divided into two flat surfaces at a shallow angle with respect to each other, each inner surface 11 being horizontal and engaging the respective switch when the beam 4 is level, and each outer surface 12 becoming level and engaging the respective switch when the beam 4 is tilted to operate that switch. The surfaces 11 and 12 themselves are above the axis of the part-cylinder 6 in the equilibrium position, but when the switch is operated one or the other pair is brought down to that level.

The operating member for this switch assembly is a knob 13 pivotally carried on a superstructure 14 over the PCB 1.

A dumbell shaped stem 15 is fixed co-axially to the underside of the knob 13, its upper part-cylindrical portion providing the pivot for the knob 13 in a socket 16. The pivot axis is parallel to that of the beam 4. The other end of the stem 15 is drilled axially to the mid-length to receive a spring 17 and a plunger 18 with a rounded nose 19.

This engages the V-shaped base of the socket 9, and in the equilibrium position of Figure 1 it rests at the vertex of that base.

For closure of the switch 2, for example, the knob 13 is tilted clockwise, as viewed in the drawings, to the attitude shown in Figure 2. It swings the stem 15 clockwise and the nose 19 of the plunger 18 slides up the left hand slope of the base of the socket 9. The spring 17 is much stiffer than the resilience of the membrane switches, and so the beam 4 is forced to tilt in the anti-clockwise direc tion, urging the left hand lug 10 down on the switch 2 while at the other end the switch 3 is cleared completely.

The geometry of the arrangement is such that the beam 4 tilts about the axis of the part cylinder 6, but as this tilting progresses, that is moved vertically upwards by the engagement of the rim of the flat 7 nearest the switch 2 with the PCB 1. At the start, the inner surface 11 will move in an arc tending to push the membrane switch 2 slightly outwards. But it immediately acquires a slope: as seen from the side it will be inwards and slightly upwards. Therefore, the thrust on the top of the switch will have a small inwards component to counteract the outwards push. At an intermediate stage the only contact with the top of the switch is by the vertex of the underside of the lug 10, and while that descends the axis of the part-cylinder 6 continues to rise so that they are substantially level over this part of the movement. This means that the vertex movement has very little horizontal component in it: it is almost entirely vertically downwards. Finally, the outer surface 12 comes into contact with the top of the switch 2, and at that point it is moving slightly inwards, towards the pivot as well as downwards. But although this might tend to drag the switch off symmetry, its sideways viewed angle, until final contact is made, is outwards and slightly upwards and therefore its thrust on the top of the switch will have a small outwards component to counteract the inward drag. Thus, lopsided collapsing will be avoided.

When the knob 13 is released, the slope of the base of the socket 9 on which the nose 19 rests will give a reaction through the plunger 18 and the spring 17 to restore the knob to the equilibrium position, and the beam 4 follows.

In Figure 3, the structure and operation is similar in many respects, but instead of having a socket 9 in the beam 4, the centre of the modified beam 20 is provided with an upwardly projecting part cylindrical boss 21 which is closely confined within a downwardly open socket 22 of an operating member 23. This boss co-axially contains a spring 24 and plunger 25, whose rounded nose 26 engages an inverted V shaped notch at the head of the socket 22. The operating member moves horizontally, transverse to the pivot axis of the beam. It is thus an inverse arrangement of the previous embodiment.

It will be understood that these switch assemblies can be combined. An operating member could have the facility to tilt and to move laterally. Also, only "two-dimensional" embodiments have been described, but the principles are readily extendable to "three-dimensional" equivalents. The portions 6 and 21 could be part-spheres rather than part cylinders, and the beams 4 and 19 could each be in the form of a cross with arms projecting into and out of the plane of the drawings to operate another pair of membrane switches.

The operating members 13 and 23 would have corresponding further freedom of movement.

Claims

1. A rocker switch assembly comprising a rocker beam, a fulcrum for the beam, means for retaining the beam in a mid-position of stable equilibrium, and membrane switches selectively operable by opposite ends of the beam according to its tilt either side of said mid-position, the fulcrum comprising a socket and a member rigid with the beam received in and spring urged into the socket with freedom to tilt and to rise and descend within the socket, wherein the tilting of the beam is arranged to generate such rise and descent and the surfaces at the ends of the beam which cooperate with the membrane switches are shaped to exert a substantially axial force on each switch as the pivot centre of the beam alters.

2. A rocker switch assembly as claimed in Claim 1, wherein the member received in the socket is part-cylindrical, its axis being the pivot axis of the beam, with a bottom flat which abuts the base of the socket in the equilibrium position.

3. A rocker switch assembly as claimed in Claim 1, wherein the member received in the socket is part-spherical, its centre being the pivot point of the beam, which abuts the base of the socket in the equilibrium position.

4. A rocker switch assembly as claimed in Claim 3, wherein there are two beams forming a cross with a common fulcrum, and four membrane switches at the corners of a square.

5. A rocker switch assembly as claimed in any preceding claim, wherein each beam end which engages a membrane switch has two surfaces meeting at a shallow angle, one surface acting on the membrane switch during a first part of its operation and the other surface so acting during a final part of such operation.

6. A rocker switch assembly as claimed in Claim 5, wherein said two surfaces are so angled that any tendency due to their arcuate movement to drag the switch sideways is counteracted by their thrust on top of the switch.

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