GB2158291A

GB2158291A - Pressure sensitive electrical switch device

Info

Publication number: GB2158291A
Application number: GB08510615A
Authority: GB
Inventors: Malcolm Batchelor
Original assignee: Nottingham County Council
Current assignee: Nottingham County Council
Priority date: 1984-05-02
Filing date: 1985-04-26
Publication date: 1985-11-06
Also published as: GB8510615D0

Abstract

A membrane switch comprises an electrically insulating base 2 having at least one region comprising a plurality of spaced electrically conductive elements 3 alternately connected to opposed terminals and an electrically insulating layer 4 having electrically conductive particles distributed therethrough laid over and in contact with the elements 3 for bridging at least two adjacent elements 3 when pressure is applied between the layer 4 and conductive elements 3. The switch includes a waterproof layer 5 and an upper overlay 6 having symbols marked thereon. The switch is designed such that no significant "dead" or non- sensitive areas exist, thereby enabling the switch to be used especially by small children or mentally or physically handicapped persons. <IMAGE>

Description

SPECIFICATION Pressure sensitive electrical device This invention relates to a pressure sensitive electrical device and is especially applicable to a touch sensitive keyboard, for example for use with a microcomputer.

Conventional keyboards for computers, especially microcomputers, contain a large number of small discrete keys which require a manual dexterity to operate far greater than that exhibited by, for example, very young children, or those with physical or mental disability. Further, it is difficult to provide adequate waterproofing for conventional keyboards to protect them from, for example, water spillage, dribbling or vomiting. While key pads with larger individual sensitive areas are known, a problem which is experienced with such key pads is that significant "dead" or non-sensitive areas exist between the sensitive areas, and this can give rise to operating difficulties, especially for small children or the mentally or physically handicapped.

An object of the invention is to provide a pressure sensitive electrical device which is simple in construction and which may readily be used as a key pad for use with microcomputers.

Accordingly, the invention provides a pressure sensitive electrical device, comprising an electrically insulating base having thereon at least one region comprising a plurality of spaced electrically conductive elements alternately connected to opposed terminals of the device, and a layer of electrically conductive material laid over and in contact with the elements, such that on pressure being applied between the electrically conductive material and the plurality of spaced electrically conductive elements the electrical contact resistance between at least two adjacent elements and the electrically conductive material is reduced to a level allowing detection of the current flow.

Preferably, the material comprises an electrically insulating matrix having electrically conductive particles distributed therethrough.

In a preferred embodiment of the invention, the base has a plurality of regions, for example sixteen regions arranged in a four by four grid, each having a plurality of spaced electrically conductive elements, the elements of each region being separately connected to a respective pair of terminals or to a respective terminal and to a common ground terminal.

A water impermeable layer may extend over the layer of resiliently compressible material, and an overlay may be positioned on top of the water impermeable layer, the overlay having marked thereon a plurality of areas each having a different picture, symbol, or word, and each area corresponding to one of the regions of the base. The device may include within the base thereof an electrical circuit arranged to encode the signals produced by pressure on the device into a form which can be recognised by a computer.

Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawings in which Figure 1 shows an exploded view of one portion of a device in accordance with the invention; Figure 2 shows in plan view a first printed circuit pattern for use in the present invention; Figure 3 shows an equivalent electrical circuit explaining the operation of the electrical switching of the device; Figure 4 shows a decoding circuit for the printed circuit pattern of Fig. 2; and Figures 5 to 13 show various different printed circuit patterns suitable for the present invention.

The device comprises a support 1 which may be formed of a rigid plastics material and may be in the form of a frame into which the other components are located. An electrically insulating base 2 is located on the support 1.

The base 2 is in the form of a printed circuit board having a plurality of separate regions, each consisting in turn of a plurality of closely spaced parallel conductive tracks 3, the conductive tracks or elements 3 being alternately connected to one of two opposed common terminals.

Positioned over the base 2 and tracks 3, and in partial contact therewith, is a sheet of electrically conductive material 4. A particularly suitable material is that sold under the Trade Mark "VERMAHIDE" by the British company Vermason Ltd. The material typically has a thickness of 1.5 mm.

A flexible waterproof sheet 5 is positioned over the material 4 and is sealed at the edges of the support 1. Plastics films such as polyvinylchloride and polyethylene are suitable materials.

The waterproof layer 5 may be marked with areas corresponding to the separate regions of electrically conductive elements 3 on the base 2 and the support 1 may be arranged to receive in a fixed position thereon any of a plurality of separate overlays 6 which are formed of a flexible sheet material, such as paper, or plastics, having symbols, pictures, words or numbers marked thereon.

In the case of a key pad arranged to operate a microcomputer, the functions of the keys may readily be changed by changing the overlay, with a corresponding change to the computer programme. The key pad is therefore especially suitable for educational use, particularly for young children or for mentally or physically handicapped children and adults.

The overall dimension of the keyboard so formed is typically 320 X 320 mm and there fore the area of each key or sensitive area is 80 x 80 mm assuming a 4 X 4 matrix arrangement. One example of a printed circuit pattern suitable for a 4 X 4 array is shown in Fig. 2. In Fig. 2 there are 1 6 separately identifiable sensitive areas 10 to 25. Each area comprises two sets of fingers spaced alternatively within each area. Each set of fingers is identified by a particular shading e.g. horizontal or vertical and each line of a particular shading is joined together by for example through hole plating and suitable conductive patterns (not shown) on the reverse side of the printed circuit board.

With reference to Fig. 3 it can be seen that each area 10 to 25 forms a switch which is identified by a combination of 2 out of the 8 sets of fingers. Within each area e.g. 10 it may be seen that pressure of the material 4 onto any area will cause conduction to occur between one or more of the lines in the two sets. It is virtually impossible to press the sheet material 4 at any point of the area 10 without achieving conduction. The conduction of current identifies which two out of the 8 sets are "active" and may be used as shown in Fig. 4 to identify the particular area.

With reference now to Fig. 4 the printed circuit board 3 is shown with eight output lines 26 to 33. These lines are connected for decoding to an integrated circuit decoder 34 which is for example an SL490B or similar decoder. The decoder converts the 2 out of eight logic inputs to for example one of sixteen pulse position modulation "words" which may be transmitted via infrared diodes 35 to a remote location or passed via a wired link (not shown) to operate a computer or other device. Preferably an LED 36 is provided to give an indication of the functioning of the decoder.

With reference to Figs 5 to 14 alternative pattern arrangements are shown which are suitable for each individual area 10 to 25 of Figs. 2 and 5. In Figs. 2 and 5 a 4 x 4 array is shown but an 8 x 4 or other size of array may be used providing appropriate decoding circuitry is used.

The conductive tracks 3 forming the pattern can be copper-plated or can be made from gold or nickel plating on a suitable substrate.

The tracks may be tinned to give better environmental protection.

The conductive material 4 may be of any thickness suitable for the desired use of the touch sensitive device. The touch sensitive device may comprise a conductive membrane including a foil pattern on a flexible sheet of plastics, which may be placed on the floor or on a suitable base. The sensitive areas may be made extremely large such that the device may sense the position of a person's feet.

Alternatively, the foil pattern and associated layers may be mounted on a wall. In this position the device may comprise a single pad, the pad being receptive to being touched in any position over a large area.

Claims

1. A pressure sensitive electrical device comprising an electrically insulating base having thereon at least one region comprising a plurality of spaced electrically conductive elements alternately connected to opposed terminals of the device, and a layer of electrically conductive material laid over and in contact with the elements such that on pressure being applied between the electrically conductive material and the plurality of spaced electrically conductive elements and electrical contact resistance between at least two adjacent elements and the electrically conductive material is reduced to a level allowing detection of current flow.

2. A pressure sensitive electrical device as claimed in claim 1, in which the electrically conductive material comprises an electrically insulating matrix having electrically conductive particles distributed therethrough.

3. A pressure sensitive electrical device as claimed in claim 1 or claim 2, in which the insulating base is divided into a plurality of regions, each region having a plurality of spaced electrically conductive elements, the elements of each region being separately connected to a respective pair of terminals.

4. A pressure sensitive electrical device as claimed in any one of claims 1 to 3, including a water impermeable layer extending over the layer of electrically conductive material.

5. A pressure sensitive electrical device as claimed in claim 4, including an overlay positioned on top of the water impermeable layer, the overlay having marked thereon one or more pictures, symbols or words corresponding to the regions of the base.

6. A pressure sensitive electrical device as claimed in any preceding claim, in which the device includes an electrical circuit arrangement to encode the output signals to a form suitable for response by a computer.

7. A pressure sensitive electrical device as claimed in any one of the preceding claims, in which the insulating base comprises a plastics sheet and a plurality of spaced electrically conductive elements are formed by a foil pattern on the plastics sheet and in which the plastics sheet is suitable for positioning on a floor such that the device may be walked on and may thereby sense the position of feet.

8. A pressure sensitive electrical device constructed substantially as herein described with reference to the accompanying drawings.