GB2086804A - Keyblock assembly - Google Patents

A keyblock assembly for a telephone has a support frame (1) on which rests a printed circuit (10)' covered successively by a dimple pad (14) and a keyguide (17). Pins on the underside of the keyguide (17), each of which extends through holes (15, 11, 4) in the dimple pad (14), the printed circuit (10) and the frame (1), have catches which engage holes (5) in the frame (1) and so hold the assembly together. <IMAGE>

SPECIFICATION Keyblock assembly This invention relates to the construction of electrical push-button operated switches, especially but not exclusively for use in telephone instruments.

A recent development in the field of such manually operable switching is the use of a socalled rubber (or rubber-like material) dimple pad, which is a sheet of rubber (or rubber-like material) with a dimple per push-button position. Each such dimple is a dome on the sheet surface which has on its concave surface a region of conductive material, e.g. metal or conductive plastics. The dimple pad is located over a sheet of an electrically insulating material having electrically conductive regions on it. When a push-button is depressed, its dimple's dome is collapsed so that its conductive region contacts the conductive region on the lower sheet to complete one or more connections. On release of the push-button it restores to rest, assisted by the resilience of the dimple.In the case of a telephone keypad we use a 4 x 3 dimple pad, with a corresponding matrix of conductive tracks on the lower sheet. Other forms of dimple pad exists, e.g. one which is allmetal.

An object of the invention is to provide a pushbutton assembly for a telephone set using such a so-called dimple pad, which is cheap and easy to assemble.

According to the present invention there is provided a push-button assembly, which includes a support frame having a plurality of holes each adapted to receive the end of a locating pin or the like, a printed circuit carrying electrically conductive tracks between which connections have to be made in response to push-button operations, which printed circuit is mounted flat on a surface of the support frame, a contact pad of a resilient material overlying the printed circuit and having a number of domes each with its concave surface facing the printed circuit, at least a portion of each said dome being electrically conductive such that when a dome is collpased its engagement with the printed circuit causes the establishment of one of more electrical connections, a key-guide formed of a flat plate-like portion with a hole for each push-button and its one of the domes, which keyguide has depending portions each carrying one or more of said locating pins or the like, and a number of push-buttons each fitting into a said hole in the keyguide, wherein the assembly is assembled with the printed circuit on the support frame, the contact pad over the printed circuit and the keyguide over the contact pad with each of its push-buttons overlying a said dome, wherein the locating pins or the like are received in the said holes in the support frame to hold the assembly together, and wherein when a push-button is depressed it acts on the dome beneath it to collapse that dome to cause it to make the electrical connection or connections appropriate to that push-button, the resilience of the dome resetting the depressed push-button when the latter is released.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is an exploded (i.e.

disassembled) view of a push-button assembly embodying the invention. Fig. 2 is a perspective view of a modification of the keyguide of Fig. 1, Fig. 3 is a cross-section of one key module assembled, Fig. 4 is a partial cross section assembled) of another modification of the arrangement of Fig. 1, and Fig. 5 is a detail of the support frame of any one of the devices of the preceding figures.

The keypad is shown in the exploded view of Fig. 1, in which it will be seen that everything is fitted together by catches and holes. The lowermost unit of the assembly is a support frame of roughly U-cross-section with two ridges 2, 3 on its upper face which define a flat surface having four locating holes 4. There are also four square holes 5 one at each corner of a rectangular portion of the flat surface.

Each of the longitudinally-extending sides of the frame 1 has a catch 6 for fitting the assembly into a telephone. This catch is shown in the scrap view of Fig. 5, and is formed by two slots. It has an outwardly-extending V-section portion 7, which when the assembly is in a telephone is received in a recess therein. Near the lower end of each of these sides is a notch 8, Fig. 1, which fits over a locating portion in the telephone.

The next unit of the assembly is a flexible (or rigid) printed circuit 10, which carries circuit tracks between which connections are to be made by push-button operations. This may (if rigid) carry circuit components, and has four locating holes 11, corresponding located with respect to the holes 4 in the frame 1.

Overlying the flexible printed circuit 10, we have a contact pad 14, which is a dimple pad of the type referred to in the opening paragraph. In this case the pad is intended for a keyset with only the basic ten digit buttons, hence it has ten dimples. Where other buttons are provided, e.g. as in the very popular 4 x 3 push-button set, the dimple pad has a suitable number of dimples. This pad 14 also has four holes 15, correspondingly located with respect to the holes 11 and 4 already referred to.

Next we come to the keyguide 17, which has ten rectangular holes such as 1 8 each bordered by a low wall such as 1 9 and each aligned with one of the dimples or domes on the pad 14. Each of these holes can receive a push-button of the type shown at 18, which is a rectangular block with a flange on it which fits under the hole such as 1 9.

Each such flange has a corner cut off, as shown, to facilitate its correct location in its hole.

The keyguide 1 7 also has four pins (not shown) moulded into the underside, each of which extends through one each of the holes 15, 11 and 4 when the keypad is assembled. There are also, on the upper surface of the guide 17, two blind holes 20, which co-operate with pins on the telephone cover to locate the keypad. The keyguide 1 7 also has four flexible plastics catches 21, each of which fits into one of the holes 5 when the keypad is assembled. When the assembly is completed and the catches 21 fitted into the holes 5, a suitable adhesive, e.g. epoxy, may be applied to secure each catch in its hole. In some cases the flexible catches may be replaced by simple location pins which are heat stoked to the frame, or secured thereto by adhesives.

The upper edges as champered to facilitate the fitting of the instrument cover when the keypad is assembled in the instrument body. The keyguide unit is approximately located in the instrument by the recesses 8, the final relationship of the push buttons in the holes being achieved by locating pins.

Fig. 2 shows how additional push-buttons can be added. To do this, the keyguide of Fig. 1 is modified by the addition of slotted projections such as 25. Each additional key module, as shown at 26, has tags which can fit into the slots 25. As shown, if one key is added it is added at point A, while if two are added they are added at B, B.

Alternatively, the added modules can be fitted at C, C, if one wishes to use the popular 4 x 3 layout.

Whichever variant is used to add a key module or two, the contact pad and printed circuit are suitably modified.

Fig. 3 is a partial cross-section of one form of push-button and its associated elements when the keypad is assembled. This includes contact carrier 30, which corresponds to the printed circuit 10 on the flat surface of the frame 1 , or to the printed circuit of Fig. 4. This carrier 30 carries fixed contacts 31. The dome or dimple is shown at 32 and is made of a silicone rubber material. The portion of the keyguide frame for this portion of the keyguide frame for this push-button is shown at 33, and the push-button at 34. In this case the keyguide frame is uniquely that of the add-on module, the contact carrier 30 being specific to this module.

When the push-button is depressed it causes the dome or dimple 32 to collapse, which forces a metal cap 35 carried on its inside to be driven into contact with the fixed contacts 31. On release of the push-button 34, the dome 32 re-asserts itself by virtue of its inherent resilience to return the push-button to its non-operated condition. The metal cap 35 is used to give a low contact resistance, which is desirable for some applications, e.g. the recall position. Another form of add-on module which can be used could employ a leafspring carrying a contact which is depressed by the inner surface of the dome or dimple so as to touch a fixed contact carried by the carrier 30.

In the arrangement shown in Fig. 4, we see the support frame 40, which is similar to the frame 1 of Fig.1. 1. Mounted on this is a stiff printed circuit board 41, by contrast with Fig. 1, where the printed circuit is flexible. Overlying this is the contact pad 42, and the keyguide 43, with its push-buttons such as 44.

Considering the keyguide again, it would be possible to so arrange the position and number of the locating pins moulded on the underside of the keyguide that the components cannot be assembled incorrectly orientated with relation to one another. Thus one could make sure that the printed circuit cannot rate with the pins if offered upside down or end to end reversed.

An alternative form of contact pad which could be used is made throughout of conductive material, in which case it is necessary to provide an insulating mask between that pad and the printed circuit. Such a mask would have 10 or 12 holes to allow the underside of the depressed dome to touch the printed circuit at the contact area only. Such a mask can be formed by a thin sheet of insulating material, or by an insulating varnish painted over the printed circuit at areas other than those for the button positions.

Instead of using loose buttons it is possible to mould all the buttons in one unit, they being connected to one another by moulding material or sprue. Hence they can be handled and stored as one component until they have to be assembled.

At this point the connecting sprues can be cut off by special tooling, although the sprues enable the buttons to be held and maintained in the correct position ready to be fitted to the keyguide.