GB2046524A - Improvements relating to electric switches - Google Patents

Abstract

An electric switch, primarily intended as part of a keyboard switch, comprises a rigid apertured top plate 1 and a tray-like apertured retention plate 6 placed against the back of the top plate to define a cavity therebetween. The operating button 9 operates within the cavity and extends through the apertures in the plates 1, 6, inwards and outwards movement being limited by a flange 11. A sheet 16 of resilient material is sandwiched between the retention plate 6 and a rigid backing plate 17 to thereby effectively seal the cavity. Each button 9 operates a respective electric switch 13 through the resilient sheet 16. The construction provides a very tough switch useable in arduous environments and where vandalism is a problem. <IMAGE>

Description

SPECIFICATION Improvements relating to electric switches This invention relates to electric switches particularly keyboard switches of the type used for operating an array of electrical switches.

Such keyboard switches are used extensively as the human/machine interface in a variety of electronic systems in industry. It has, however, proved difficult to provide a construction which resists the attention of vandals and, with the increasing use of these keyboards in potentially vulnerable circumstances, for example public telephone boxes, the need for a vandal resistant keyboard has become acute.

This invention, therefore, is directed essentially to the actuating mechanism for such a keyboard switch - in other words, the mechanism which transfers the mechanical force applied by the user's finger to the delicate electrical switches.

According to the invention there is provided an electric switch comprising a rigid top plate having a through aperture, an operating button of the same size and shape as the aperture so as to be a sliding fit therein, means for biassing said button in an outwards direction, said button having a flange limiting such outwards movement, a rigid retention plate situated parallel to and beneath said top plate, means for defining, between the retention plate and the top plate, a cavity defining the range of movement of the flange of said button, said retention plate further being formed with a through aperture in registry with the aperture in the top plate, a sheet of resilient material stretched across the aperture in the retention plate, means for sealing said resilient material around the edge of the aperture, and electric switch means positioned behind the aperture in the retention plate in such a way as to be operated by said button through said resilient material.

Preferably means are provided for completely sealing the cavity defined between the two plates so that liquids cannot escape into the interior of the equipment being controlled by the switch. Preferably also, the flange extends right around the perimeter of the operating button thus, in conjunction with the spring means, providing an effective metal-to-metal seal against the ingress of liquid. In order to be fully effective, the smallest clearance possible should exist between the operating button and the top plate. This, besides restricting entry to the sealed interior will also provide a small cross section for film contamination, such as paint or chewing gum, which will tend to shear and allow the operating button to move freely.

Direct blows on the operating button with a heavy instrument will be restrained by the retention plate.

In an embodiment of the invention, a rigid backing plate is situated behind the retention plate to act as an additional protection against such action. Preferably, such backing plate acts also to compress said resilient material around the underside of the retention plate, thus doubling as said sealing means.

Although specified above in relation to a single switch, it will be clear that the invention could be readily applied to a keyboard switch comprising an array of independently switched electrical contacts operated by respective operating buttons. The top plate would be formed with a plurality of throughapertures, one for each button, and a single common retention plate (and backing plate, if fitted) used for the whole array. Preferably the retention plate is such as to define a single cavity beneath the top plate in which the flanges of all the operating buttons move.

In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with reference to the accompanying drawings in which: Figure lisa side elevation of a single switch unit forming part of a complete keyboard switch unit forming part of a complete keyboard switch constructed in accordance with the present invention; and Figures 2,3 and 4 are perspective views showing various parts of the keyboard switch shown partially in Figure 1.

Referring to the drawings, the keyboard switch comprises a top plate 1 made of rigid material such as stainless steel. As will be clear from Figure 2, the plate 1 is formed with a plurality of square throughapertures 2 and is provided, at each corner, with an upstanding bolt 3. Behind the plate 1 is laid a backingsheet 4 of resilient material such as polyprene. The shape of the sheet4 is shown in Figure 3 from which it will be seen that the sheet is in the form of a frame with a large central aperture. Small apertures 5 are formed at each corner so that the sheet may be accurately located on the bolts 3.

The resilient sheet 4 forms a sandwich between the undersurface of the top plate 1 and the edge of a retention cover 6. The retention cover 6, which is shown in detail in Figure 4, is made of rigid material folded into the form of a very shallow tray. It is the upturned edges 7 (see Figure 1 ) of the tray which abut against the sheet 4 right around the perimeter of the tray. As will be described later, the retention sheet is pressed towards the top plate 1 to thereby slightly compress the sheet 4 and form a sealing joint therebetween.

The retention cover 6 is formed with a plurality of circular apertures 8 in registry with the square aper- tures 2 in the top plate 1. Each pair of in-registry apertures 2 and 8 forms the guiding surfaces for a respective operating button 9. Only one operating button 9 is shown (in Figure 1), but it is to be understood that a similar button will be provided in each pair of apertures 2 and 8 - i.e. twelve buttons in all.

Each button 9 comprises three portions: an outer portion 10 of a square cross section which is sized to be a close sliding fit in its respective aperture 2; a central flange portion 11 of greater lateral width than the remainder; and an inner portion 12 of circular cross section which is a sliding fit in the respective aperture 8 in retention cover 6.

The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy. In use, the operating button is biassed outwardly to the position shown in Figure 1, by means of a spring, to be referred to again later. The central flange portion 11 limits outward movement in such a way that the button protrudes only slightly - for example 0.01 inches - above the outer surface of top plate 1.

When it is required to operate the switch, the operating button 9 may be pressed inwards by the finger against the bias of the spring, inwards movement being limited by the flange portion 11 abutting the retention cover 6. The tray-like nature of the cover 6 thus defines a cavity in which the respective flange portions 11 of the operating buttons may move.

Behind each operating button 9 is a respective electrical switch 13. The switches 13 are collectively mounted on a printed circuit board 14 supported on the bolts 3. Each switch 13 is of the push-on, release-off type and has an operating member 15 extending upwardly therefrom into mechanical contact with the inner portion 12 of the respective operating button 9. The spring force inherent in such switches is therefore applied to bias the operating button 9 into its normal position, as shown in Figure 1, without the need for additional spring force. The various parts of the assembly are arranged such that, in the outermost position of the button 9 the switch is only just past its point of operation. Thus only the slightest inwards movement of each button is necessary to operate its respective switch and to provide positive tactile feelto the operator.

In order to completely seal the assembly of switches, together with the remainder of the electronic circuitry, against ingress of liquids, a further sheet 16 of resilient sealing material such as pplychoropropene is positioned behind the retention cover 6. The sheet 16 is not formed with apertures, so that each aperture 8 is covered by the sheet. The whole assembly, including resilient sheets 4 and 16 together with the retention cover 6 are firmly held by being pressed towards top plate 1 by means of a rigid backing plate 17 which is secured onto bolts 3 by means of nuts 18. The nuts 18 (four in all) are screwed down to cause backing plate 17 to firmly hold the remaining parts of the assembly in place.In addition the sandwiching action of the backing plate 17 on the rear face of the retention cover 6 causes the resilient sheet 16 to be compressed and thus form a sealing joint around each aperture 8. The backing sheet 17 has a shape similar to that of backing sheet 4 (see Figure 3), the size of the central aperture being such as to clear all the apertures 8 in retention cover 6. The operating member 15 of each switch thus acts against the inner portion 12 of the operating button 9 via the intermediary of the resilient sheet 16. The sheet 16 is able to take up the necessary movement whilst still retaining its sealing properties.

The close fit between the operating buttons and the top plate 1 is designed to restrict the entry of implements such as screwdrivers which would be used as a lever. In addition, the close fit results in a restricted entry to the sealed interior and also provides a small cross section for film contamination such as paint or chewing gum that will tend to shear and allow free movement of the operating button.

The limited amount the operating button stands above the surface is designed to discourage abuse by pliers or clamps. The total travel of the switch button is limited by the relative length of the flange portion 11 of the operating button 9, and the depth of the cavity defined between the top plate 1 and the retention cover 6. The travel is limited so that in combination with the force of the spring bias of switch 13, ingress of liquids or rosol projected contaminants is limited.

Direct blows on the operating buttons with a hammer or similar heavy implement will be restrained, to a certain extent at least, by the retention cover 6 and backing plate 17.

By carefully selecting the manufacturing tolerances, each switch 13 can be made to operate at a predetermined distance from its mounting on printed circuit board 14. Once determined, this distance can be used to mount a sealed operating panel, as described above, fitted with mechanical restraints that will allow the correct mechanical and electrical functions to be performed even when the assembly is subjected to severe mechanical and chemical abuse.

Although shown in the form of a solid body, it is likely that the operating button 9 will, for manufacturing convenience, be made up of at least two parts; one comprising the square shaped outer portion 10, and one the circular-section central and inner portions 11 and 12. It is preferred that the flange portion 11 extends right around the perimeter of the square portion 10 of the operating button in order to provide a continuous metal-to-metal seal between the flange and the inner surface of the top plate 1.

Claims (10)

1. An electric switch comprising a rigid top plate having a through aperture, an operating button of the same size and shape as the aperture so as to be a sliding fit therein, means for biassing said button in an outwards direction, said button having a flange limiting such outwards movement, a rigid retention plate situated parallel to and beneath said top plate, means for defining, between the retention plate and the top plate, a cavity defining the range of movement of the flange of said button, said retention plate further being formed with a through aperture in registry with the aperture in the top plate, a sheet of resilient material stretched across the aperture in the retention plate, means for sealing said resilient mat erial around the edge of the aperture, and electric switch means positioned behind the aperture in the retention plate in such a way as to be operated by said button through said resilient material.

2. An electric switch as claimed in claim 1 further comprising means for sealing the perimeter of the cavity defined between the retention plate and the top plate.

3. An electric switch as claimed in either one of claims 1 or 2 wherein said retention plate takes the form of a shallow tray, whose bent-over edges face the top plate to thereby form said cavity defining means.

4. An electric switch as claimed in claim 2 and 3 wherein said means for sealing the perimeter of the cavity comprises a sheet of resilient material sandwiched between the bent over edges of the retention plate and the top plate.

5. An electric switch as claimed in any one of claims 1 to 4 wherein the flange extends right around the perimeter of the operating button to thus provide a seal with the underside of the top plate when the button is biased there against.

8. An electric switch as claimed in any one of the preceding claims further comprising a rigid backing plate which is situated behind, and fixed with respect to said retention plate in order to strengthen the latter.

7. An electric switch as claimed in claim 6 wherein said rigid backing plate is positioned so as to sandwich, between itself and the retention plate, said sheet of resilient material, to thereby constitute said means for sealing the resilient material around the edge of the aperture.

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

9. A keyboard switch comprising a plurality of electric switches as claimed in any one of the preceding claims.

10. A keyboard switch as claimed in claim 9 wherein the structure is such as to define a single cavity for all switches.