GB2151379A - Key pad - Google Patents

Abstract

A key pad for use as an input for a computer comprises a sheet of flexible material provided with indicia identifying a plurality of pressure areas and, beneath the sheet, a printed circuit board comprising one or more series of resistors connectable to a source of electrical potential, the arrangement being that the application of pressure to any one of the pressure areas of the flexible sheet effects the provision of a distinct input signal for the computer. <IMAGE>

Description

SPECIFICATION Key pad patent The invention relates to apparatus for provid ing a plurality of distinct inputs for a com puter comprising a key pad including a sheet defining a plurality of distinct pressure areas and means whereby the application of pres sure to any of said areas provides a distinct input for the computer.

The sheet may be provided with indicia indentifying the said pressure areas or it may provide a support for a further sheet or sheets provided with alternative indicia.

The key pad may inciude at least one set of resistors in series, means to connect one end of the or each series to a source of electrical potential and means whereby the application I of pressure to any one of the areas of the key pad taps the or one of the series of resistors at a unique position in the series for connecting the same position to an input terminal of the computer.

The key pad may include a printed circuit board under the pressure sheet and each set of resistors may be defined by an elongate uniform strip or a plurality of resistive material applied to a face of the board connected at substantially equal length intervals to conduc tive strips, each conductive strip terminating under a unique one of the pressure areas. The board may also carry at least one common conductor, connectible to the computer input, with a branch terminating below each of a plurality of the pressure areas and spaced from but adjacent to the terminal portion of one of the conductive strips.The underside of the pressure sheet may be provided with a conductive surface in the region of each pres sure area which surface is brought by applica tion of pressure to the pressure sheet into electrical contact with the terminal portion of the associated conductive strip and with the terminal portion of a branch of the common conductor whereby electrical connector is made between the resistor series and the common conductor.

By operation of such apparatus a unique resistance is applied between the source of electric potential and the computer input ter minal according to the particular pressure area depressed, and this resistance can be ar ranged to control the time taken to charge/ discharge a condensor, and the time lapse can be recognised by the computer as a signal attributable uniquely to the particular area depressed.

However, a significant variation of the sys tem has been devised in accordance with which the other end of the or each series of resistors may be connected to a common ground or earth, so that the series of resistors forms a potential divider and each tapping point represents a unique voltage intermediate the source potential and zero.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings of which; Figure 1 is an exploded and part cut-away simplified key pad for connection to a computer.

Figure 2 represents a printed circuit for a more complex key pad, with insulating overlays shown in part.

Figure 3 is a part sectional view of an assembled key pad on line Ill to Ill of Fig. 2.

Figure 4 is a circuit diagram in conventional form, corresponding to Fig. 2; and Figure 5 is a circuit diagram of a keypad in an alternative embodiment.

As shown in Fig. 1 the key pad comprises a printed circuit board 1 defining a base and having on its upper surface 2 a plurality of conductive strips 3 each connected to a terminal 5 in a terminal block 11, and plurality of resistive strips 4.

A flat pressure sheet 6 of flexible but elastically resilient material overlies the board 1, with the underside adhesively bonded in the assembled key pad to the upper face of the board 1 and with the edges of the sheet and of the board in register. The upper surface 7 of the sheet 6 is provided with a series of indicia in the form of lines 8 deliniating a plurality of pressure areas 9. The sheet has an opening 1 1a through which in the assembled pad the terminals 5 nd terminal block can project upwardly from the board 1.

The lower surface 7a of the sheet 6 has a plurality of squares 10 of conductive foil secured thereto, each square being disposed centrally below one of the deliniated pressure areas 9.

Fig. 1 shows, for the sake of simplicity, a pad with a matrix of only 30 pressure areas.

Fig. 2 on the other hand represents a printed circuit for a key pad in an embodiment providing 100 pressure areas.

Of the conductor strips 3 on the circuit board 1, strip 1 2 is a common conductor which has ten rectilinear branches 1 2a, 1 2b, 1 2c etc. each of which extends over substantially the whole width of the board, the branches being equidistantly spaced along the length of the board. Each of the branches is provided, at ten equidistantly spaced positions 14 along its length, with four terminal fingers 1 3 equally spaced and perpendicular to the branch.

Of the conductor strips 3 on the circuit board 1, strips 15, 16, 17 and 18 are each connected to a separate terminal in block 11.

Strip 1 5 is in electrical connection with a plurality of short lengths 1 9 and layer lengths 20 of conductive strip material by the medium of the resistive strips 4.

Each of the strips 4 is of substantially equal length in the embodiment shown, but a plu rality of such strips may be provided by applying a continuous length of resistive strip material over a number of lengths of conductive strip. It will be understood that conductiv ity of the length 1 9 of the conductive strip will not be diminished by the overlying resistive strip.

In Fig. 2 the resistive strips are distinguished from the conductive strips by their being shown as narrower but in practice they may be equally wide or even wider. From what has been said above it will be clear that the resistive strips have a degree of conductivity and it is expected that the conductive strips will have a certain small resistance, but it will be understood that the strips 4 have a very much greater electric resistance per unit length than the strips 3. In the embodiment shown, the conductive strips 3 are of copper and the resistive strips 4 are of resistive ink.

As is well known in the field of printed circuits, the strips 3 and 4 are very much thinner than they are wide.

Each of the short lengths 1 9 of conductive strip 10 directly connected to a set of three fingers 21 of conductive strip material, each set being arranged at a separate one of the positions 14 with the individual fingers disposed between and parallel to the terminal fingers 1 3 of the common conductor 1 2.

In the assembling of the Key Pad and before the pressure sheet 6 is bonded to the board 1, the resistive strips 4 and the conductive strips 3 apart from the fingers 13 and 21 at the positions 14 are at least substantially covered by portions 22 of thin insulating sheet material (some, only, shown in Fig. 2) which are themselves bonded to the surface 2 of the board 1. Although the insulating sheet material is thin, it is thicker than the combined thickness of the fingers 1 3 or 21 and of the foil which the squares 10 are made. Thus, as will be seen from Fig. 3 in which the vertical scale is greatly exaggerated, in the assembled Key Pad and in the absence of pressure on any particular pressure area, the foil square underlying the sheet 6 in that area is slightly spaced from the upper surface of the fingers 13 and 21.

In this condition there is no electrical connection between the fingers 1 3 on the one hand and the fingers 21 on the other, but on applying sufficient downwards pressure to one of the pressure areas the pressure sheet 6 is depressed in that area and only in that area, to the extent that the foil square 10 underlying it is brought in contact with at least one of the fingers 21 in the associated area 14.

Electrical connection is thus made between the common conductor 1 2 and one of the strips 15, 16, 1 7 or 18 and will continue to obtain while the pressure continues to be applied. On releasing the pressure the pressure sheet returns to its original flat shape by reason of its resilience and lifts the foil square so that it is no longer in contact with the fingers.

The foil squares 10 and the fingers 13, 21 thus essentially form a switch and the circuit board 1 can be seen to be electrically equivalent to the circuit shown in Fig. 4, in which like components are referred to by like numerals.

Effectively the circuit provide a twenty five resistors R (1).. . R (25) connected in a first series to conductor 15, and further groups of twenty five resistors R (A).. . R (Y), R (I). .. R (XXV), and R (a). . . R (y) each group being connected in series and one end of the several series being connected respectively to connectors 16, 17, and 18.

Spring-loaded switches S are provided to connect common conductor 1 2 to one or other of the conductors 15, 16, 17, or 18 at tapping points 1 9 between adjacent resistors.

The resistors are each of substantially the same resistance value, and so, depending on which switch S is made, a total resistance xr, ranging between rand 25rips effectively introduced between conductor 1 2 and one or other of conductors 15, 16, 17, or 18.

The terminal block 11 as shown in Fig. 1 is connected in use to the computer via a suitable cable and interface means, not shown, whereby, in a manner known in the art, the value xr of the total resistance can be identified in terms of the time required to charge/ discharge a condensor through the total resistance, that is to say between conductor 1 2 and one of conductors 15, 16, 17, or 18.

Experience has shown that with existing technology the value xr can be measured with sufficient accuracy reliably to distinguish each value in the range rto 25rand the arrangement described thus provides means whereby each of 4 X 25 = 100 distinct signals can be fed into the computer and identified with a unique pressure area on the key pad.

Of course, the invention can readily be adapted to provide a smaller number of distinct pressure areas and by an increase in the number of conductors or, subject to the accuracy of measurement of total resistance, by increasing the number of resistors in each series a larger number of pressure areas could be provided.

The pressure areas may be identified with numerical indicia as shown in Fig. 1 or otherwise. In some applications the key pad will be used as a support for a data sheet bearing other significant indicia, and the location of specific indicia thereon can be identified in terms of the pressure areas on the pressure sheet below.

Whilst the arrangement described above is convenient for certain types of computer it is not suitable for others and after further investigation we have described an alternative system which can be used with a wider range of computers. The alternative system uses similar components to those described above and in particular uses lengths of resistive strip to provide resistors and conductive foil overlying interdigitated "fingers" of printed circuit strips to provide make-and-break switches between conductive strips. Electrically however the resistors are connected to provide a voltage divider as shown in Fig. 5.

Each of four groups of n equal value resistors R connected in series is connected at one end to a source of electrical potential V while the other end of each series is connected to earth. Each series is tapped between successive resistors and the tappings are connectible through press switches P in the case of the respective series to conductors 35, 36, 37, and 38.

At successive tapping points along each of the series of resistors beginning at the earthed end the voltage will be 1 2 3 n-l V-, V-, V V-, V n n n n where n = number of resistors in the series.

By means of an appropriate cable and interface the terminal block 39 is connected to a computer type which can recognise different voltages as distance signals so that in use of the apparatus of this embodiment the computer can distinguish which of the switches has been made by a combination of the voltage supplied and the conductor by which it was supplied.

Claims (8)

1. A key pad for connection as input to a computer comprising a sheet of flexible material defining a plurality of distinct pressure areas, and means whereby the application of pressure to any of said areas provides a distinct input signal for the computer.

2. A key pad a claimed in claim 1 wherein the sheet is provided with indicia identifying said pressure areas.

3. A key pad as claimed in claim 1 or claim 2 wherein the flexible sheet supports a further sheet or sheets provided with indicia for identifying said pressure areas.

4. A key pad as claimed in one of the preceding claims and including at least one set of resistors in series, means to connect one end of the or each series to a source of electrical potential, and means whereby the application of pressure to any one of the areas of the flexible sheet taps the or one of the series of resistors at a unique position in the series for connecting the same position to an input terminal of the computer.

5. A key pad according to claim 4 including a printed circuit board beneath the flexible sheet on which board each set of resistors is defined by an elongate uniform strip or a plurality of strips of resistive material, said strips being connected at substantially equal length intervals to conductive strips, and each conductive strip terminating under a unique one of the pressure areas.

6. A key pad according to claim 5 wherein the circuit board also carries at least one common conducter, connectable to the computer input, with a branch terminating below each of a plurality of the pressure areas and spaced from but adjacent to the terminal portion of one of the conductive strips, and the underside of the flexible sheet is provided with a conductive surface in the region of each pressure area, which surface is brought by application of pressure to the flexible sheet into electrical contact with the terminal portion of the associated conductive strip and with the terminal portion of a branch of the common conducter whereby electrical connection is made between the resistor series and the common conducter.

7. A key pad according to claim 6 wherein the end of the or each series of resistors opposite to that connected to the source of electrical potential is connected to common ground or earth, so that the series of resistors forms a potential divider and each tapping points represents a unique voltage intermediate the source potential and zero.

8. A key pad for connection as input to a computer substantially as described with reference to the drawings.