The invention relates to pressure pads and methods of manufacturing such pressure pads. Pressure pads are used in a wide variety of fields as switches which respond to applied pressure to close a circuit so as to initiate a response. Pressure pads can constitute individual switches but often are provided in the form of arrays of switches. An example of one application of a pressure pad is described in WO 93/17764 which illustrates a toy or educational device in the form of a book or magazine in which a set of individual switches defined by pressure pads are embedded in the back cover of the book. When pressure is applied to a page in the book resting on the back cover in the vicinity of one of the switches, this pressure will cause the underlying switch to activate. US 4818827 discloses a membrane switch for use in a book. Other membrane switches are disclosed in FR-A-2324108 (corresponding to US-A-4017697) and US-A-4694126. In all these membrane switches, the membranes are formed by polymeric films. US-A-4420663 discloses a membrane switch in which the flexible component of the switch is formed of paper. In this case, the paper membrane is supported on a spacer having a number of openings through which conductors can move to make contact.

In all these known examples, the membrane switches and pads are bulky, rigid structures which are not well suited for incorporation into books and the like. In accordance with one aspect of the present invention, a pressure pad comprises a flexible sheet held in tension over a base, facing surfaces of the sheet and base carrying electrically conductive tracks, the sheet and base being held apart, so that the tracks are normally not in contact, by an array of spacers fixed to the sheet and/or the base, whereby when pressure is applied to the εheet, tracks on the base and sheet in the vicinity of the

applied pressure are brought at least into proximity of each other.

We have devised a new type of pressure pad which can be made thin enough to fit into the cover of a book and can use a material for the flexible sheet which has a sufficient flexibility to be provided with the electrically conductive tracks by methods such as silk-screen printing and is resilient enough to recover from flexing when pressed. This is achieved by holding the flexible sheet in tension over the base.

The invention also leads to a particularly convenient method of manufacturing a pressure pad in which a flexible sheet is mounted over a base, the method comprising: a) printing a pattern of electrically conductive tracks on the flexible sheet; b) printing an array of curable tactile ink pads on the flexible sheet and/or the base, the pads having a thickness greater than the ink pads; c) curing the ink pads; and, d) mounting the sheet in tension to the base, the base having cooperating electrically conductive tracks, with the ink pads positioned between the base and the sheet.

This method of manufacture is particularly advantageous since the provision of tracks and pads is carried out using a normal printing process, for example silk-screen printing.

Typically, the tactile ink will be UV curable while the electrically conductive tracks will be heat curable. The flexible sheet could be mounted in tension to the base in a variety of ways. However, conveniently, the flexible sheet is glued to the base and preferably, therefore, the method further comprises applying a heat activatible glue to the base (or to the flexible sheet) , step d) further comprising heating the glue so as to cause the flexible sheet to adhere to the base. Alternatively, where the pressure pad forms part of a book, the flexible

sheet could be glued to the base using normal glue as part of the book binding process.

The flexible sheet could be made from a variety of materials but most conveniently is made of paper, typically a relatively high density paper with, for example, a density in the range 180-200 gsm.

The use of paper is particularly advantageous for a number of reasons. Paper is a readily available and relatively cheap material, is particularly suitable for printing using techniques such as silk-screen printing, and is readily incorporated into books using conventional book binding techniques. In contrast to US-A-4420663, the inventors have found a way of incorporating paper into the pressure pad while maintaining a relatively thin construction. In the prior patent, it is necessary to use a relatively thick, rigid separating device having apertures through which contact can be made between tracks. In the case of paper, the invention avoids the need for a substantial spacer by holding the paper in tension on the base while using relatively thin spacers.

The arrangement of the electrically conductive tracks can be in any conventional form. In one example, the tracks on one of the sheet and base are in the form of interleaved conductors and on the other in the form of one or more conductive bars. Typically also the pressure pad defines an array of switches, each switch being defined by respective cooperating sets of conductive tracks on the sheet and base.

Some examples of pressure pads and a method for its manufacture for use in a book will now be described with reference to the accompanying drawings, in which:- Figure 1 is a plan of the book when opened; Figure 2 is a longitudinal section through the book shown in Figure 1 when closed; Figure 3 is a section taken on the line 3-3 in Figure 1 when the book is closed;

Figure 4 illustrates part of the bottom sheet of one example of the pressure pad;

Figure 5 illustrates a corresponding part of the top sheet; Figure 6 is a partial cross-section taken on the line 6-6 in Figure 4 through the pressure pad;

Figure 7 is a schematic diagram of apparatus for manufacturing the bottom sheet;

Figure 8 illustrates part of the top sheet of a second example of the pressure pad; and,

Figure 9 illustrates a corresponding part of the bottom sheet of the second example.

The book illustrated in Figures 1-3 comprises a front cover 1 and a rear cover 2, each formed of cardboard and hinged at 3. The front cover 1 has a pressure pad 4 mounted to it while the rear cover 2 has a pressure pad 5 mounted to it. The construction of the pressure pads 4,5 will be described in more detail below. The right-hand edge of the rear cover 2 supports an electronics housing 6 and a clasp 7 which is hinged to the housing 6 so that it can be moved between an open position (Figure 1) and a closed position (Figures 2 and 3) in which the front cover 1 is held closed over the rear cover 2.

The book has a number of pages 8 (not shown in Figure 1) which are bound into and hinged between the covers 1,2. In the closed position shown in Figures 2 and 3, these pages 8 are sandwiched between the covers 1,2.

The housing 6 is made of plastics and supports a printed circuit board 9 which is connected to the pressure pads 4,5. The printed circuit board 9 includes processing electronics which responds to the location of applied pressure on either of the pressure pads 4,5 to cause an appropriate sound to be generated by a sound generator 10. The electronics are powered by batteries ll mounted in the housing 6.

The construction of each pressure pad 4,5 will now be described. Each pressure pad comprises a base sheet 13

which may be made of rigid material such as cardboard or alternatively paper glued to a rigid support such as the book cover. This latter construction allows a relatively thin paper sheet to be used. Overlying the base sheet 13 is a flexible paper sheet 12 as can be seen in Figure 6. The sheet 13 carries sets of screen printed electrically conductive tracks arranged in a 4x5 array, each set defining a separate pressure switch area. Four such pressure switch areas are indicated by reference numerals 14-17 in Figure 1. Figure 4 illustrates the form of the electrically conductive tracks defining the areas 14-17. As can be seen in Figure 4, each set of tracks comprises a first set of four tracks 18 extending laterally in one direction and a second set of four tracks 19 interleaved between the tracks 18 and extending in an opposite direction. The tracks 18 of the area 14 are connected to a common conductor 20 and the tracks 19 of the area 14 to a common conductor 21. Although not shown, the conductors 20,21 are then connected to the printed circuit board 9. It will also be seen that the conductors 18 of the areas 14,16 are connected to the same conductor 20 while the conductors 19 of the areas 14,16 are connected to different conductors 21,22. This means that it is possible for processing electronics on the printed circuit board 9 to distinguish between the different areas when pressure is applied. In practice, the tracks 19 will overlap the conductor 20 with suitable insulation between them. For clarity, this is not shown in Figure 4.

The areas 15,17 are constructed in a similar manner to the areas 14,16. Thus, the tracks 18 of the area 15 are connected to a common conductor 20' and the tracks 19 of the area 15 are connected to a common conductor 21 . The tracks 18 of the area 17 are connected to the conductor 20' and the tracks 19 of the area 17 are connected to a common conductor 22' .

In addition, four spacers 23 are provided around each area 14-17. The spacers are formed by a screen printed UV

cured tactile ink and, as can be seen in Figure 6, have a height significantly greater than the thickness of the tracks 18,19.

The sheet 12 carries a single screen printed electrically conductive bar 24 overlying each area 14-17 as shown in Figure 5.

The paper sheet 12 is secured to the base sheet 13 under tension by gluing around its edge as shown at 25 in Figure 6. As can be seen in Figure 6, in a normal, rest condition, the sheet 12 is held spaced from the base 13 due to the spacers 23 and the fact that the sheet is held under tension. If pressure is applied, for example finger pressure, in one of the areas 14-17, this will urge the sheet 12 to flex towards the sheet 13 and bring the conductive bar 24 into proximity with and usually into contact with the conductors 18,19 of the underlying area. This will then cause a circuit to be completed between the conductors 18 and 19 in the respective area and this circuit can be detected in a conventional manner.

Typically, a voltage is continually applied to the common conductors 20,20' etc. and the existence of current on one of the conductors 21,22 etc. is monitored to determine which of the areas 14-17 has pressure applied to it.

Figure 7 illustrates schematically apparatus for preparing the bottom εheet 13.

A paper web 30 of photographic quality having a weight of 110-115 gsm (and a thickness of about 150 μm) is stored on a roll 31 and is then fed to a first printing station 32 where the tracks 18,20 are screen printed onto the web. The ink is a graphite based ink and following the screen printing step, the station 32 heat cures the ink.

The web 30 then passes to a coating station 33 which lays down one or two coats of a dielectric over portions of the conductors printed at the station 32. The reason for this is that in order to maximise the density of conductors

and pressure pad regions it is convenient to overlap the conductors 19 and 20 (although this is not shown in Figure 4) and at the positions of overlap the conductors need to be insulated from each other. Following the coating step, the dielectric is UV cured. The web then passes to a printing station 34 where the conductors 19,21,22 etc. are screen printed onto the web in a similar manner to the tracks 18,20. Again, this printing is heat cured. The web 30 then passes to a printing station 35 where the tactile ink pads 23 are screen printed onto the web and UV cured. Typical dimensions for the pads 23 are a diameter of 2-3mm and a height of about 180-220 μm. Finally, the fully printed web passes to a cutting station 36 which cuts the web into individual sheets which are stacked at 37. The top sheet having a weight of about 190 gsm and a thickness of about 195 μm is prepared in a similar manner except that there is only a single screen printing stage for printing the tracks 24 and heat curing them, followed by a single screen printing stage for printing adhesive 25 onto the sheet. The adhesive is printed in a predetermined pattern chosen so as to ensure a strong fixture between the sheets. The printed top sheet is then cut into similar sized sheets to those of the bottom sheet.

Pairs of top and bottom sheets are then assembled in register and passed to a heating station (not shown) having a platen shaped to conform with the adhesive. The platen engages the assembled sheets and heats up the adhesive so that the adhesive is reactivated and bonds the two sheets together. The resultant pressure pad having a thickness in the order of 525-565 μm is then adhered to the front or rear cover of the book.

A second example of a pressure pad is shown in Figures 8 and 9. As in the first example, the bottom sheet (Figure 9) carries sets of electrically conductive tracks arranged in a 4x5 array defining 20 pressure switch areas 40, only some of which are labelled. Each pressure switch area 40 comprises a generally W-shaped track 41 interleaved with

the arms of a U-shaped track 42. The two tracks 41,42 are electrically isolated from one another. Each track 41,42 is connected to respective tracks 43,44 in a similar manner to the first example. Overlying each area 40 is a corresponding track arrangement 45 provided on an upper sheet similar to the sheet 12 (Figure 8) . The track arrangement 45 comprises a set of parallel, electrically isolated tracks 46 defining a generally circular configuration and surrounded by a printed, solid, circular spacer ring 47 formed by printed, UV cured tactile ink. Only one ring 47 is shown in Figure 8 although each track arrangement 45 will be surrounded by such a ring. When the sheet shown in Figure 8 is overlaid on, and secured under tension to, the sheet shown in Figure 9, each track arrangement 45 is spaced from the tracks 41,42 underneath by means of the respective spacers 47. When pressure is applied in the area 40, however, the tracks 45 are pushed down into engagement or proximity with the tracks 41,42 thus completing an electrical connection between the tracks 41,42 via the tracks 46 which can be detected as previously described.

In order to optimise the manner in which the pressure pad is incorporated into a book, it is has been found particularly advantageous to adhere the lower layer, for example base sheet 13, into a book cover using a similar εystem to that used for assembling the inner sheet on the inside of a hard back book. The upper sheet, for example the sheet 12, is then treated in a similar way to the end sheets of the centre of a book, thus forming the book block.

This book block is then inserted into the cover complete with its circuit and a suitably constructed roller applies a glue pattern over the whole area of the switch pad, leaving holes around each area of switch contact. The glues used can be the same as those used in conventional book binding and will be water based with the result that some shrinkage of the paper will take place. This is

beneficial, however, as the effect is to produce a tightly tensioned εheet, rather like a drum skin, where the glue pattern is absent. Since the paper is subjected to moisture during the book binding process and the use of water soluble glues, the paper iε dried back to normal humidity conditionε after assembly so that it is held under tension and thus forms a resilient top surface that flexes and returns to make and break the switch.