GB1584345A

GB1584345A - Keyboards

Info

Publication number: GB1584345A
Application number: GB542876A
Authority: GB
Original assignee: Alphameric Keyboards Ltd
Current assignee: Alphameric Keyboards Ltd
Priority date: 1977-05-11
Filing date: 1977-05-11
Publication date: 1981-02-11

Description

(54) IMPROVEMENTS IN OR RELATING TO KEYBOARDS (71) We, ALPHAMERIC KEYBOARDS LIMITED, a British Company, of Darven House, Armfield Close, West Molesey, Surrey KT8 OSH, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to capacitive keyboards, to keyboard clusters for forming keyboards, and to apparatus for use with printed circuit boards to form keyboard clusters.

The term "keyboard cluster" as used herein means a single separate key station or group of associated key stations. In the latter case the key stations are aligned in a single row or in array of two or more twos. A keyboard is one cluster or a plurality of clusters arranged for installation in an electronic apparatus at one position, and preferably in the latter case having a common printed circuit board. As an example, a complete keyboard can comprise a main cluster of 60 keys, two auxiliary clusters each of 12 keys, and an auxiliary cluster of 1 key.

As used herein the term "key" means the movable part of a key station of a cluster, excluding the leaf spring means actuated by the key and the key-top bearing an indicium. Although keyboards will normally be manufactured and sold to customers complete with a set of key-tops as required, the situation may arise where the customer already has or will obtain his own key-tops and therefore requires the keyboard to be supplied without key-tops.

In known capacitive keyboards it is necessary either to provide mechanical "overtravel" in order to position the make-point at approximately half travel, or alternatively to use very high drive voltages andl or very sensitive detection so that the moving plate is sensed at approximately half travel.

According to one aspect of this invention there is provided a keyboard having a plurality of key stations and comprising a printed circuit board having a plurality of capacitor plates formed thereon, each key station including a key and means arranged to support the key for operative movement and wherein conductive metal spring means are provided having a plurality of separately movable portions of an integral spring strip, the separately movable portions bearing resiliently against the lower ends of respective keys for separately urging the various keys upwardly, said separately movable portions constituting electrically connected but separately movable key-operated capacitor plates, co-operating respectively with fixed capacitor plates on the printed circuit board.

In the simplest form, each movable portion comprises a spring finger constituting a leaf spring. As explained later, however, more complex arrangements are possible.

In known capacitive keyboards a movable capacitor plate of Area A is mounted at the bottom of the plunger portion of a key and is operative to bridge capacitively two co-planar fixed plates each approximate of area A/2, so as effectively to form two series-connected capacitors of plate area A/2. By using a leaf spring, each movable capacitor plate, in the present invention, can have an area of approximately A, and the fixed capacitor plate can also have an area of approximately A, and therefore compared with the known arrangement using two half-area fixed capacitor plates there is a four-fold increase in capacitance.

This in turn moves the make-point closer to the mid-travel point and also improves the translation of electrical hysteresis into mechanical hysteresis. Further, if the movable capacitor plate is in the form of a leaf spring it is effectively hinged along one edge and driven by the key at the other edge. This gives an effective 2:1 mechanical leverage which moves the make-point even higher and further doubles the mechanical hysteresis.

However, the movable spring portion may alternatively be a spring finger generally concertina-folded having a plurality of finger portions including a bottom finger portion remote from the key. This provides even higher leverage ratios, giving further improvements in make-point positioning and hysteresis. The leverage ratio can be changed by altering the shape of the various parts of the leaf spring means.

At least one concertina spring finger may have at least one of its finger portions other than an associated bottom finger portion at least partly of reduced width.

At least one concertina spring finger may have its finger portions approximately of equal length.

Preferably at least one concertina spring finger has a finger portion other than the associated bottom finger formed with a downwardly directed transversely extending rib disposed so as to exert, in use a force on the associated bottom finger at about its middle either directly, or indirectly via an intermediate finger portion. Conveniently, the or at least one rib is formed by the rolled over free end of the associated top finger portion.

A concertina spring finger can have its bottom finger portion bowed downwardly.

This provides a gradual "roll-down" of the movable capacitor plate over the fixed capacitor. In one alternative form the bottom finger portion can be straight; and in another alternative form, it can be bowed upwardly, in which case the change of capacitance with key movement will not be so gradual as compared with the downwardly bowed bottom finger portion. Furthermore, the shape of the spring fingers will determine the tactile feedback for an operator.

If two or more fingers are employed for each key, the spring return force acting on the plunger may readily be arranged to change during the travel of the plunger.

Each spring finger may be bowed upwardly such that when not depressed the key contacts a spring finger at about the middle of the finger; where there are two or more staggered fingers, the key can subsequently come into contact with the other finger (or fingers) at its (or their respective) middle(s) during the downward travel of the key.

Each movable spring portion may be a spring finger formed so as to provide, in use, a snap action. Conveniently the spring finger has a region which is domed, and when the key is depressed the domed region is forced to change abruptly to its opposite curvature with an accompanying audible click and tactile feedback.

Alternatively, the spring means may be formed of magnetic material and there may be magnetic means fixed to the support means and arranged to attract part of the or movable spring portion such as to provide, in use, a snap action.

The support means for each key may include a wall carrying or having mounted on it at its bottom a further fixed capacitor plate on the printed circuit board and with the spring means such as to provide a capacitance of a first value between said fixed capacitor plates when the key is unoperated and to provide a reduced value of said capacitance when the key is depressed.

Such a further fixed capacitor plate would be used for providing coupling of an antiphase drive signal.

In one arrangement said fixed capacitor plates are on the underside of the circuit board and are thus separated from the associated movable plates (the spring fingers) by the material of the board. In another arrangement the fixed capacitor plates are on the upper surface of the printed circuit board in which case to prevent resistive contact between the fixed and movable plates either a layer of dielectric material can be provided over the board or at least over the fixed capacitor plates, or the leaf spring means can be non-conductive on its under-surface.

As an alternative to applying an antiphase signal to a further fixed capacitor plate, as mentioned above, there is preferably provided on the board one or more conductive regions adjacent each fixed capacitor plate for providing capacitance to circut ground. Such a conductive region can be provided on the same side of the board as the fixed plate, or on the opposite side of the board, depending on the areas of the conductive regions, the layout and the desired capacitance to ground. If required, there can be such a region on both sides of the board.

The drive signal can be coupled directly to the spring means either by means of a lug at the root of the spring means, the lug being secured in a hole in the printed circuit board and soldered to an appropriate conductor strip, or by means of a conductive region on the top of the board positioned under the root of the spring means and in contact with it. The drive signal can be capacitively coupled to the spring means by means of a suitable fixed capacitor plate either formed on the board or formed on or attached to the support means.

In arrangements where the fixed capacitor plates are on the underside of the printed circut board, the necessity for a further dielectric layer between the capacitor plates is avoided and there is thus obtained significant economy with added simplicity to the construction compared with arrangements where the fixed plates are on the upper surface of the board.

In a keyboard cluster comprising a plurality of key stations, a common printed circuit board is preferably employed. Pre ferably the various respective conductive regions on the printed circuit board, as and when present, are interconnected by conductive strips, in a known manner. In other words, these regions are the regions providing the capacitance to circuit ground; the regions for receiving the anti-phase drive signal; and the regions providing capacitive coupling of the drive signal to the spring means (alternatively, those regions providing direct coupling of the drive signal).

Preferably, the respective spring means common to a plurality of keys in a line are together integrally formed with an elongate element. The integral structure thus formed is referred to herein as a single spring device.

In capacitance keyboards it is the usual practice to arrange the keys electrically in a matrix, the rows (or columns) of the matrix being connected to common drive circuits and the columns (or rows) being connected to common input sensing circuits. Typically there might be four or eight rows driven by a drive having a corresponding number of phases, which drive applies pulses in sequence to the row circuits under the control of a clock. A sep zrate sensing circuit may be provided for each column or a single sensing circuit may be arranged to sense the various columns in sequence also under the control of the clock so that the timing of any sensed output identifies the particular key which has been operated. Such an arrangement may conveniently be used with a keyboard of the present invention.

Some stray capacitance to ground is inevitable but, in preferred arrangements of keyboards, this capacitance to ground can be carefully designed into the printed circuit board and thus can be closely controlled. As mentioned conductive regions forming grounded screens may be provided on the printed circuit board adjacent to the fixed capacitor plates on the board. In certain layout arrangements, the grounded screens can extend around, or substantially around, the fixed capacitor plates. Thus the capacitances to ground are determined by the arrangement and areas of the conductive regions on the printed circuit board. Since there is no resistive con tact between the movable and fixed capaci tor plates cross-coupling of the row drive outputs when more than one key per column is depressed is avoided.

The capacitance between a movable spring portion and the fixed capacitor plate on the under surface of the printed circuit board has a maximum value limited by the layout on and the thickness and material of the board, this limited value providing significant benefits as a result of controlling the effective maximum coupling of the variable key capacitance. Firstly this markedly improves the translation of the sensing circuit's electrical hysteresis into mechanical hysteresis which is highly desirable in keyboards. Secondly the maximum input signal is limited thereby eliminating possible overloading of sensing amplifiers and resultant voltage swings causing cross-coupling. It is known for both voltage sensing and current sensing (virtual earth) amplifiers to suffer from these problems.Thirdly, the limiting effect described above allows the use of more drive and/or more sensitive detection which in turn further improves the mechanical hysteresis and effectively enables keys to be sensed at a further distance away from the printed circuit board than has heretofore been possible without the abovementioned overloading or cross-coupling recurring.

The respective support means of a cluster having a plurality of keys may together be in the form of an integral structure, preferably a single moulding formed of rigid plastics material, the structure having a top from which depend elongate walls spaced apart for defining respective rows (or columns) of the cluster. Alternatively, the respective support means may together be constituted by a plurality of separate elongate modules, each having an upright wall and a top and being arranged to interlock in side to side relation, and a corresponding upright wall member arranged to interlock with a module such as to provide, in use, an end wall for the interlocked set of modules. Each of the abovementioned elongate elements may be upright and lie against, or in a recess in, a respective wall.

In the abovementioned integral structure, respective transverse walls may be provided between the keys, the transverse walls being spaced apart along the elongate walls and the keys being disposed in the respective gaps between adjacent transverse walls. Alternatively the modules might have a plurality of transverse walls arranged to guide the keys respectively disposed in the gaps between the transverse walls, but con veniently the keys are made to be closely adjacent so that, when the keyboard clus ter is assembled, each key is retained be tween its own module wall and the adjacent module wall, or end wall as the case may be, so as to be slidable without rocking.

Preferably, the or each key is provided with guide surfaces for cooperation with complementary surfaces on the elongate walls (and/or the transverse walls, if present), to limit the area of contact and hence the friction. If there are guide surfaces only on the elongate walls, the keys will preferably be dimensioned so as to be spaced from adjacent keys, or adjacent transverse walls, if present. Similarly, if there are guide surfaces only on transverse walls, the keys will preferably be dimen sioned so as to be spaced from the elon gate walls.

The abovementioned integral structure and the modular construction will be gener ically referred to as a housing.

In the case where the housing is a single moulding, the top will have respective aper tures for accommodating the respective stem portions of the keys. In the alterna tive case the tops of the modules may simi larly have apertures or may have recesses which become closed to constitute apertures when adjacent modules are interlocked, or when the wall member interlocks with the end module of housing. It will be appre ciated that where the modules have such recesses, the stem portions of the keys can be provided with a corresponding rib whereby the keys are captive while the housing is in its interlocked condition.

The modules may be interlocked by the resilience of the plastics material or by hav ing a recess or recesses to fit over projec tions on adjacent modules for example in the manner well known with toy building bricks. The upright wall member may be in the form of a simple wall, or, if desired, it may be constituted by the upright wall of a further module without its associated keys.

This construction is particularly con venient in that it enables the key assemblies to be manufactured by moulding with the parting line of the mould lying in an up right plane with respect to the direction of movement of the key in the keyboard. This ) facilitates the formation of recesses in the sides of plunger portions of the keys as might be desired for example for lock down cams or detent grooves as will be described in further detail later.

A A single spring device, as referred to above, eliminates the necessity for several moving parts per row which is the com mon practice in capacitive keyboards.

Moreover there is no need to use foam material pads or additional additional springs to provide overtravel or intimate contact, or to soften the collision between the striker, i.e. the bottom portion of the plunger, and the printed circuit board.

Each single spring device, which may con veniently be formed of beryllium copper, serves the triple purpose of constituting a capacitor plate for each of its associated key stations, providing the return spring force for each key and providing the electrical interconnection between the movable capa citor plates of its associated keys. Particu larly if a roll-down type of spring device is used in which the fingers gradually flatten on to the printed circuit board, there is no direct collision with the printed circuit board.

It may be desirable to provide a single spring device which is longer than the row of keys in a cluster. Furthermore two or more clusters can be arranged so as to utilize respective common single spring devices. Also, if a cluster is very large (in the region of 60 keys) instead of a single housing, there may be two or more smaller housings which together support the keys of the cluster, and in this case the single spring devices can extend along the full length of the cluster and be common to the respective sub-groups of the smaller housings.

Preferred embodiments of the invention will now be described by way of ex ample with reference to the accompanying drawings in which: Figure 1 is a section through a number of elongate modules one of the modules having a key in position Figure 2 illustrates a single spring device shown in Figure 1 and the co-operating capacitor plates on one form of printed circuit board; Figure 3 is a section through part of the printed circuit board shown in Figure 2 to show a detail of construction; Figure 4 is a section through part of a module and printed circuit board to show a detail of construction; Figure 5 is a scrap diagram illustrating a key plunger and lock down wire; Figures 6 to 13 and 15 show various al ternative forms of spring fingers; Figure 14 shows an alternative form of applying an antiphase drive signal;; Figure 16 shows an alternative form of the spring device shown in Figure 2; and Figure 17 shows a low profile spring and button arrangement suitable for use in a pocket calculator.

Referring to Figure 1 there are shown three modules 10, 11, 12 each having an upright wall 13 with a top portion 14 extending substantially horizontally and being formed of rigid plastics material. The top portion has a plurality of recesses 15, one of which is shown in section. These recesses serve to accommodate, as a sliding fit, a stem portion 16 of a plunger 17 which slides between the upright wall of a module and the upright wall of an adjacent module.

The modules will typically be provided in three lengths of housing, respectively, three, four, and five keys which are arranged in line normal to the plane of the paper in the drawing of Figure 1. Stem portions 16 each carry a key-top 18 which may be of conventional construction carrying indicia on its upper surface. The key-top conveniently is resiliently held on the stem portion 16 by engagement with grooves 19 (Figure 5) into which projections on the inside of the key-top extend; the plunger and key-top being formed of slightly resilient plastics material so as to permit the key-top being forced on to the top of the stem.

The lower portion 20 of each plunger is of generally oblong form in horizontal plan and has, in this example, a bottom portion 21 of generally square form in plan, which portion fits with a sliding fit between the upright walls of adjacent modules. Around the stem portion 16 is an elastomeric gaiter 24 to provide a liquid-tight seal around the stem portion for sealing the top of the keyboard. This also provides an elastic bump stop limiting the downward movement of the plunger. The gaiter 24 can be omitted, if desired.

Surrounding the apertures 15, the vertical walls are shaped with a radiused topmost portion 25 to allow a degree of plunger misalignment without binding. Further, immediately below the radiused portion 25, the wall is tapered as shown at 26 to cooperate with a similar tapered portion 27 on the plunger such that, when in the released position shown in Figure 1, wobble of the plunger is reduced to a minimum.

The aforementioned single spring devices which constitute a return spring and capacitive electrode are shown in three different forms in Figure 1. At 30 is shown a spring device having a straight spring finger 31. There is a separate spring finger of this form for each key in a module but these spring fingers are formed integrally with an upright portion 32, the spring fingers extending through a slot in the upright wall of the module to the upright portion 32 lying in a recess or rebate in that wall on the opposite side of the wall to the associated key. Alternatively, the upright portion 32 may be disposed in a narroy slot 65 shown in dashed lines in Figure 1. At 33 is shown a spring device of roll-down form in which the spring finger gradually flattens as the plunger is forced downwardly.At 34 there is shown a spring device with two differently positioned fingers so that, as the plunger is depressed, first one finger is engaged and then another one. The free ends of the spring fingers have turned over end to avoid burrs engaging the undersurface of the bottom portion 21. The free ends can be turned over upwardly as shown at 66 in Figure 1, or downwardly as shown at 67.

The plungers are placed in the apertures 15 and the modules are assembled together and interlocked by interlocking means shown at 35. After the springs have been put in position, the upright walls of the module, which at their lower ends are formed as peg portions 36, are assembled on a printed circuit board 37 the peg portions protruding through holes 38 in the board.

As is more clearly seen in Figure 2, the printed circuit board has a series of pad portions 40 which co-operate with the spring fingers to form capacitors. As shown in Figure 3 the portions 40 are covered by a dielectric sheet 41 over the upper surface of the board 37 to prevent direct contact between the spring fingers and the plates on the printed circuit board. The sheet 41 need not be over all the board but can be over only the portions 40. In this particular arrangement the spring devices of the modules are connected by respective leads 42 to a row drive unit (not shown) which provides drive signals in sequence for the various rows of matrix formed by the keys.

Alternatively, the spring devices can be provided with one or more downwardly ex tending lugs 68 which protrude through holes 69 in the board and are soldered to a conductive portion on the upper or the lower surface to receive the drive signals.

Thus, the lugs 68 not only provide electrical connection to the spring device but act to position it accurately and to hold it firmly relative to the board. The pads 40 are con nected together in columns that is to say the pads of separate modules are connected together to column inputs 44.

In Figure 4 the spring finger 60 is of rolldown form similar to but shorter than the spring finger of device 33 in Figure 2. The printed circuit board has a series of pad portions 61 on its underside which co-oper ate with the spring fingers 60 to form capacitors, and this construction does not require a dielectric sheet as does the printed circuit board shown in Figure 3 where corresponding pad portions 40 are provided on the upper surface of the board and the dielectric sheet 41 is necessary to prevent direct contact between the fingers and the pad portions.

The pad portions 61 have respective end portions 61a which are not overlapped by the spring fingers 60 when in their rolleddown condition. Provided on the upper surface of the board above the end portions 61 a are respective conductive regions 62, each arranged so as to be spaced from the end of the associated spring finger 60 in its down position. Further conductive regions 63 and 64 are provided on the upper and lower surfaces of the board, respectively, close to the end portion 61 a and the region 62.

In use, the pad portions of a column are connected together and to the input of a respective sensing circuit (not shown) which typically comprises an MOS device. There is a capacitance between the sensing circuit input and the circuit ground, and the value of this capacitance can be controlled by the provision of one or both of the conductive regions 63 and 64 which are connected to circuit ground. The spring fingers 60 are all connected to a respective output of the row drive unit in the manner shown in Figure 2, and the conductive regions 62 are connected to an anti-phase output of the row drive unit in order to reduce or cancel the effects of stray coupling between the spring fingers 60 and the respective associated pad portions 61.

In Figure 4, the grounded region 63 predominantly determines the capacitance to ground of the anti-phase input (region 62), and the grounded region 64 predominantly determines the capacitance to ground of the sensing circuit input (pad portion 61).

Depending on the relative magnitudes of the various capacitances, it may be possible to obtain satisfactory operation with only one of the grounded regins.

It will be appreciated that the various capacitances can readily be accrately controlled in magnitude since they are constituted by conductive regions on the circuit board formed by high precision printing techniques.

It wil be seen that in the Figure 4 arrangement the spring fingers contact only the upper surface of the board and do not contact any conductive regions. By this means the effect of any wear is kept to a minimum.

Figure 5 shows in further detail the shaping of a plunger. This plunger has a recess 50 for a lamp if required, for illuminating the key-top. The lamp would be energized via leads extending downwardly through the plunger, passing through a hole (not shown) in the spring fingers at about the middle of the fingers, and making connection to conductive regions (not shown) on the board in the middle of, but isolated from, the fixed capacitor plate.

Conveniently the capacitor plate can be on the bottom of the board and the conductor strips for the lamp can be on the top of the board. Also shown in Figure 5 is a heart shaped recess 51 forming a lock down cam for co-operating with a lock down wire 52 secured to the wall of a keyboard module.

It will be particularly noted that the plunger may readily be formed as a moulding of plastics material with the mould opening in the direction indicated by the arrows 55. This readily permits the formation of all the necessary recesses such as the lock-down cam and the lamp housing, on the plunger without any movable cores in the mould.

Whereas in the above described embodiments the modules are separate and are connected together in the completed keyboard cluster by the interlocking means 35, it will be understood that if desired the modules can be integrally formed to provide a single moulding for the keyboard cluster. If it is not desired to form a large keyboard cluster from a single integrally formed housing, or a plurality of modules of the required length, then smaller housings, or modules of shorter length, can be used from which the large cluster can be formed. In this case a single spring device common to all the keys in a row of the cluster can be used. The spring device may be manufactured in strip form and cut into appropriate lengths as desired.

In Figure 3 the fixed capacitor plate 40 is shown on the upper surface of the board and is covered with a dielectric sheet 41.

Alternatively, the sheet 41 can be omitted and the undersurface of the spring fingers rendered non-conductive. This can be achieved by coating the spring fingers with a dielectric layer, or by forming a nonconductive surface region, for example an oxide or nitride layer, or by forming the spring fingers from an non-conductive resilient material, for example acetal polymer, having a coating of conductive material on its upper surface, or being in laminated form with a layer of conductive material sandwiched between two layers of, say, acetal polymer.

An advantage of providing movable capacitor plates as described above is that the spring device can be made using precision techniques, similarly the disposition of the fixed plates on the printed circuit board can be controlled with high precision, and the plates can readily be accurately aligned even over the length of row of say 20 or more keys. Hence any dimensional variation in the spacing of key stations in a moulded housing, which variation, in prior art keyboards having the movable plate attached to the bottom of the plunger and in the form of a foam pad coated with a layer of foil or a layer of MYLAR (registered Trade Mark) with a coating of metal, results in mismatch between the movable and fixed plates, is now manifested as a tolerable mismatch between the plungers and the spring fingers.

Whereas in Figure 4 the anti-phase input is applied to region 62 on the upper surface of the board, it may be applied to a conductive region 70 on the lower surface of the board as shown in Figure 6, in which case the root part of the spring finger can have a flat portion 71 opposite region 70. If desired the row drive signal can be applied to region 70 to capacitively couple the signal to the spring device, in which case a different method of applying anti-phase signal will be used such as in Figure 4 or as described below.

Instead of capacitively coupling the row drive signal to the spring device by means of region 70, it may be applied to a similar region 72 on the upper surface of the board as shown in Figure 7. If there is resistive contact between region 72 and the spring device then the row drive signal will be directly coupled to it. However, there may be no resistive contact and in this case the signal will be capacitively coupled, for instance there may be intentional or an unintentional (due to dirt, say) gap between region 72 and portion 71; the fixed capacitor plate can be provided on the upper surface of the board and a layer of dielectric material can cover both the capacitor plate and region 72; or the undersurface of the spring device can be non-conductive as mentioned above.

Figure 8 shows a folded spring finger having its bottom finger portion 75 straight. The top finger portion 76 is about half the length of the bottom finger portion 75, alternatively, the top finger portion can be approximately the same length as the bottom finger portion 75 as shown in dashed lines 77. Not only is the make-point position and mechanical hysteresis improved by the use of folded springs which give a greater leverage ratio than the 2:1 ratio obtained with a simple spring finger such as finger 31, but as compared with a finger such as finger 31 there is less movement of the driven point of the spring (usually the free end region of the top finger portion) relative to the driving face of the plunger.

Furthermore, the stress in the spring is less than for springs of the form shown in Figure 1.

In Figure 9 the bottom finger portion 75 is bowed generally downwardly. The top finger portion 76 has a length of about two thirds that of the bottom finger portion 75, is bowed generally upwardly, and a turned over end 78 larger than end 67 in Figure 1 and constituting a transverse rib for pressing on the bottom portion to improve its contact with the board or the plate on the upper surface.

Figure 10 shows an example of a spring having more than one fold. The top finger portion 76 may have a downwardly directed transversely extending rib 79 (shown in dashed lines) formed at about its middle so as to press on the bottom finger portion via the intermediate finger portion 80. If desired, the intermediate finger portion 80 can be provided with the rib 79 instead of portion 76.

Figure 11 shows a spring having a bottom finger portion 75 as shown in Figure 9, and a top finger portion 77 as shown in Figure 8 but having a rib 79 as shown in 10.

Figure 12 shows a spring similar to spring 31 in Figure 1 but having a generally up ward bow. This form of spring would result in an abrupt change of Key capacitance as the spring flattens onto the board, as compared with springs having a generally downward bow such as spring 33 in Figure 1.

Figure 13 shows one form of the use of a magnetic element 81 to provide a snap action for the key. The element 81, which may be in strip form, is mounted on a wall 13 and magnetically attracts the free end 82 of the spring. It will be appreciated that the spring will be made of magnetic material and that the plunger will be shaped to provide a clearance relative to the element 81.

The plunger will press on the bowed upper surface 83 of the spring and cause the spring to deform while the free end 82 remains in contact with the element 81.

Eventually the magnetic attraction is overcome and the spring separates from element 81 with a snap action. Such an element 81 can similarly be used with other shapes of spring, for example the spring shown in Figure 9.

A strip-form magnetic element 81 can extend the whole length of the wall 13 and be common to all the keys in the row.

In an alternative construction, not shown, individual magnetic elements 81 can be respectively fixed to the bottom of the plunger, preferably in a recess near the free end of the spring.

Figure 14 shows an arrangement in which the fixed capacitor to which the anti-phase drive signal is applied is in the form of a metal strip 84 attached to the bottom of wall 13 adjacent the free end of the spring 31 (or the first fold of a folded spring).

By this arrangement the capacitance between strip 84 and the fixed capacitor plate on the board (on the upper or lower surface, as desired) reduces as the key is depressed due to the screening effect of the spring 31.

Figure 15 is a perpective view of a modified form of a spring such as the spring shown in Figure 9, in which the top finger portion 76 has a smaller width as compared with the bottom finger portion 75.

This spring provides an action similar to that of the double-finger spring 34 in Figure 1.

In a spring having more than one fold, such a reduced width finger portion can be the top portion or any intermediate portion. It may even be the bottom finger portion if the degradation in make-point position can be tolerated.

Figure 16 shows a modified form of a spring device shown in Figure 2. A first modification is the provision of a stretched or deformed region 85 at the root of the spring which becomes of opposite curvature and provides a click action as the key is depressed. There will be a respective region 85 for each spring, but only one region is show: in Figure 16. A second modification is the provision between adjacent springs of a portion 86 corresponding to the root part of a spring, particularly the flat portion 71. This permits improved coupling to the spring device since region 70 (or region 72) can be provided extending substantially the whole length of the spring device and the capacitive (or resistive) coupling will be improved as compared with when the spring device has no portions 86, as shown in Figure 2.

Figure 17 shows a low profile arrangement such as can be used in pocket calculators.

A button 87 is fixed to a spring having a shape similar to that shown in Figure 12. If desired, the button 87 can merely rest on the spring and not be attached to it.

As described above the row drive signal is coupled to the movable capacitor plate and the sensing circuit is connected to the fixed capacitor plate. If desired, the drive unit and the sensing circuit can be interchanged, i.e. the row drive being applied to the fixed capacitor plate and the signal to the sensing circuit being taken from the movable capacitor plate.

WHAT WE CLAIM IS: 1. A keyboard having a plurality of key stations and comprising a printed circuit board having a plurality of capacitor plates formed thereon, each key station including a key and means arranged to support the key for operative movement and wherein conduc tive metal spring means are provided having a plurality of separately movable portions of an integral spring strip, the separately movable portions bearing resiliently against the lower ends of respective keys for separately urging the various keys upwardly, said separately movable portions constituting electrically connected but separately movable key-operated capacitor plates, co-operating respectively with fixed capacitor plates on the printed circuit board.

2. A keyboard as claimed in claim 1 wherein at least one movable spring portion is generally a concertina-folded leaf spring having a plurality of finger portions includ ing a bottom finger portion remote from the key.

3. A keyboard as claimed in claim 2 wherein at least one concertina spring finger has at least one of its finger portions other than the associated bottom finger portion at least partly of reduced width.

4. A keyboard as claimed in either claim 2 or claim 3 wherein at least one concertina spring finger has its finger portions approximately of equal length.

5. A keyboard as claimed in any one of claims 2 to 4 wherein the or at least one concertina spring finger has a finger portion other than the associated bottom finger formed with a downwardly directed transversely extending rib disposed so as to exert, in use, a force on the associated bottom finger at about its middle either directly, or indirectly via an intermediate finger por tion.

6. A keyboard as claimed in claim 5 wherein the or at least one rib is formed by the rolled over free end of the associated top finger portion.

7. A keyboard as claimed in any one of claims 2 to 6 wherein the bottom finger portion of the or at least one concertina spring finger is turned having a generally downward bow.

8. A keyboard as claimed in any one of claims 2 to 6 wherein at least the bottom finger portion of the or at least one concer tina spring finger is substantially straight.

9. A keyboard as claimed in any one of claims 2 to 6 wherein the bottom finger por tion of the or at least one concertina spring finger is curved having a generally upward bow.

10. A keyboard as claimed in claim 1 wherein each movable spring portion com prises a curved spring finger having a gener ally upward bow such that in use the key contacts a spring finger at or near the middle of the spring finger when the key is up.

11. A keyboard as claimed in claim 1 wherein each movable spring portion com prises a straight spring finger.

12. A keyboard as claimed in claim 1 wherein each movable spring portion com prises a spring finger curved so as to be gradually flattened when the key is depressed in use.

13. A keyboard as claimed in any one of claims 1, 11 and 12 wherein each movable spring portion comprises two or more spring fingers staggered such that the spring re turn force acting on the key changes dur ing its travel.

14. A keyboard as claimed in any one of the preceding claims wherein each mov able spring portion comprises a spring finger formed so as to provide, in use, a snap action.

15. A keyboard as claimed in any one of claims 1 to 10 wherein the spring means is formed of magnetic material and includ ing magnetic means fixed to the support means and arranged to attract a part of the or movable spring portion such as to provide, in use, a snap action.

16. A keyboard as claimed in any one of the preceding claims wherein the support means for each key includes a wall carrying or having mounted on it at its bottom a further fixed capacitor plate which co operates in use with the fixed capacitor plate on the printed circuit board and with the spring means such as to provide a capacitance of a first value between said fixed

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. region 85 for each spring, but only one region is show: in Figure 16. A second modification is the provision between adjacent springs of a portion 86 corresponding to the root part of a spring, particularly the flat portion 71. This permits improved coupling to the spring device since region 70 (or region 72) can be provided extending substantially the whole length of the spring device and the capacitive (or resistive) coupling will be improved as compared with when the spring device has no portions 86, as shown in Figure 2. Figure 17 shows a low profile arrangement such as can be used in pocket calculators. A button 87 is fixed to a spring having a shape similar to that shown in Figure 12. If desired, the button 87 can merely rest on the spring and not be attached to it. As described above the row drive signal is coupled to the movable capacitor plate and the sensing circuit is connected to the fixed capacitor plate. If desired, the drive unit and the sensing circuit can be interchanged, i.e. the row drive being applied to the fixed capacitor plate and the signal to the sensing circuit being taken from the movable capacitor plate. WHAT WE CLAIM IS:

1. A keyboard having a plurality of key stations and comprising a printed circuit board having a plurality of capacitor plates formed thereon, each key station including a key and means arranged to support the key for operative movement and wherein conduc tive metal spring means are provided having a plurality of separately movable portions of an integral spring strip, the separately movable portions bearing resiliently against the lower ends of respective keys for separately urging the various keys upwardly, said separately movable portions constituting electrically connected but separately movable key-operated capacitor plates, co-operating respectively with fixed capacitor plates on the printed circuit board.

capacitor plates when the key is unoperated and to provide a reduced value of said capacitance when the key is depressed.

17. A keyboard as claimed in any of the preceding claims wherein said first capacitor plates are formed on the underside of the printed circuit board.

18. A keyboard as claimed in any of the preceding claims wherein said fixed capacitor plates are formed on the upper surface of the printed circuit board, and are covered by a layer of dielectric material.

19. A keyboard as claimed in claim 18 wherein each movable portion of the spring means comprises a layer of conductive material with a layer of electronically insulative material formed at least on its undersurface.

20. A keyboard as claimed in any one of claims 17 to 19 including a conductive region arranged on the printed circuit board adjacent each fixed capacitor plate so as to provide in use a predetermined capacitance to circuit ground.

21. A keyboard as claimed in any of claims 17 to 20, and including another conductive region arranged on the board adjacent each fixed capacitor plate for the application of an anti-phase drive signal.

22. A keyboard as claimed in claim 21 wherein said another region is on the oppo site side of the board to the adjacent fixed capacitor plate.

23. A keyboard as claimed in any one of claims 17 to 22 and including, for at least one key station, another fixed capacitor plate either formed on the printed circuit board, or formed on or attached to the support means, said another fixed capacitor plate being associated with the movable portion spring means such as to permit capacitive coupling, in use, of the spring means with an associated circuit.

24. A keyboard as claimed in any one of claims 17 to 22 and including, for at least one key station, a further conductive region formed on the upper surface of the printed circuit board and disposed under and in contact with the movable portion of the spring means to directly couple the spring means with an associated circuit.

25. A keyboard as claimed in any of the preceding claims and having key stations arranged in a matrix of rows and columns and wherein the respective fixed capacitor plates of a column (or row) are interconnected via conductive regions on the board.

26. A keyboard as claimed in claim 25 as appendant to either claim 21 or claim 22, and wherein said respective another conductive regions common to a row (or column) are interconnected.

27. A keyboard as claimed in claim 25 as appendant to claim 23, and wherein the respective other fixed capacitor plates common to a row (or column) are interconnected.

28. A keyboard as claimed in any one of claims 25 to 27 as appendant to claim 24, and the respective further conductive regions common to a row (or column) are interconnected.

29. A keyboard as claimed in any one of claims 25 to 28 having a plurality of rows of keys, and wherein the respective support means are together constituted by a single integral structure having a top from which depend elongate walls spaced apart and housing in use the keys therebetween.

30. A keyboard as claimed in any one of claims 25 to 28 having a plurality of rows, and wherein the respective support means are together constituted by a plurality of separate elongate modules, each having an upright wall and a top and being arranged to interlock in side to side relation, and a corresponding upright wall member arranged to interlock with a module such as to provide, in use, an end wall for the inter locked set of modules, the keys being housed in use between the walls.

31. A keyboard as claimed in either claim 29 or claim 30 and including spaced apart transverse walls, the keys being disposed in use in the respective gaps between adjacent transverse walls.

32. A keyboard as claimed in any one of claims 29 to 31, wherein said spring means includes an elongate element which is up right and lies against, or in a recess in, a respective wall.

33. A keyboard substantially as herein before described with reference to and as shown in the accompanying drawings.