CA1157925A

CA1157925A - Capacitive touch control switch panels and method of manufacturing them

Info

Publication number: CA1157925A
Application number: CA000357116A
Authority: CA
Inventors: Robert Posset
Original assignee: BFG Glassgroup GIE
Current assignee: BFG Glassgroup GIE
Priority date: 1979-09-28
Filing date: 1980-07-24
Publication date: 1983-11-29
Also published as: IT8068428A0; ATA482480A; JPH0222488B2; ES8200507A1; DE3036049C2; IT1130507B; JPS5657228A; FR2466909A1; ES253277U; DE3036049A1; BE885097A; SE447435B; FR2466909B1; SE8006741L; ES495563A0; ES253277Y; NL8005281A; AT377399B; CH639228A5

Abstract

ABSTRACT
In a touch control switch panel comprising a dielectric sheet carrying a touch switch which comprises a first touchable electrode on one side of the sheet and second and third mutually separated electrodes on the opposite side of the sheet and in capacitive relation to the first electrode, there are conflicting requirements as to high switch capacitance, small switch area and scratch and impact resistant dielectric sheet. This difficulty is remedied by forming the dielectric sheet of glass which has been chemically tempered and by constituting at least one electrode by a coating deposited on the sheet.
The electrode is preferably a tin oxide or indium oxide coating while the electrodes are preferably of conductive enamel. The dielectric sheet may be provided with an opaque coating layer and/or bonded to a backing support e.g. of synthetic plastics material.

Description

- 2 -This invention relates to touch control switch panels co~nprising a dielectric sheet carryin~ at least . ' one touch switch~ the or each switch comprising a first, touchable electrode on one side of said sheet, and second and third mutually separated electrodes on the opposite side of the sheet in capacitive relation to the first electrode.
The three electrodes combine to form two capacitors connected in scries. In use, an AC signal is applied ~0 to one of the second and third electrodes whi.le thc other of them is connected to an output monitoring circuit. When the first electrode is touched with a finger, the s;.gnal passed to the input of the monitoring circuit is significantly decreased, and this decrease ln signal is used to effect switching.
Such switches may be used in many fields, for example as elevator control switches, as domestic appliance switche.s (e~g. for cookers) and in switch arrays for example as a telephone ~'touch dial" or as a computer input keyboard.
To effect reliable switching such a switch should have a capacitance which is as high as possible. The reason for this is that the signal applied by a pulse generator - to the electrode should keep the highest leve]. possible when entering the detection circuit talcing account of the capacity of the input impedance. The higher this level the easier it will be to reject noise signals by ~eans of a thresllold circuit incorporatcd into the detection circuitry.
For some commercially available detcction circuits which have

3 been especially developed for capacitive touch control r~

~LS7~S

switching systems a typical value to be achieved for the capacitance o~ the system of 3 p~ may be cited;
Especially in the case of panels bearing an array of switches, it is desirable that each switch should occupy a small area. This desideratum is to some extent in conrlict with the requirement for a capacitance above a given threshold value since, according to the well l~nown classical formula, the capacitance of a flat plate capacitor ;s proportional to its area.
~ccordin~ to the same formula, the capacitance is inversely proportional to the thickness of the dielectric material, so it would of course be possible to reduce both the area of the switch and the thickness of the panel w~lile maintaining the same capacitance, rt is also possible to affect the capacitance by a choice of t~le dielectric material, but this may lead to an apparent conflict with other requirements. ~or example, natural dielectric materials such as mica do not lend themselves to high volume series production.
Plastics materials are easily abraded and are generally flexible when thin, so that a touch control switch panel incorporating a touchable electrode deposited on a plastics dielectric sheet could soon wear to the point where it became unreliable in operation. The use of ordinary soda-lime glass as dielectric is satisfactory for scratch-resistance, but the glass sheet must be made sufficiently thick for impact resistance. This limits the size to which a switch can be reduced while maintaining its capacitance above a -threshold value, It is an object of the present invention to remedy ~7~

tllis difficulty.
According to the present invention there is provided a touch control switch panel comprising a dielectric sheet carrying at least one touch switch, the or each 5 switch comprising a first~ touchable electrode on one side of said sheet and second and third mutually separated electrodes on the opposite side of the sheet in capacitive relation to the first electrode, characterised in that said dielectric sheet is of glass wllicll has been subjected to a chemical temperirlg treatment, and in that at least one said electrode is constituted by a coating deposited on the sheet.
A touch control switch panel accordlng to the invention ;s more impact resistant than a known panel, by virtue of the tempering treatment to which the glass dielectric has been subjected, so that the thiclcness of the glass shiet can be reduced while retaining sufficient impact resistance.
This in turn enables the area of the or each switch to be reduced while maintaining its capacitance at a sufficient level.
Preferably each said electrode is constituted by a coating deposited on said sheet.
The chemical tempering of glass is well known per se.
The most current type of tempering involves an exchange of alkali metal ions from a contacting medium with alkali ions from the glass while the glass is heated. This ionic interchange takes place in surface layers of the glass which are several microns or tens of microns thick. In one process, the ion exchange is performed at a temperature sufficiently high for stress relaxation to occur in the glass and the ions ~ ~L579~5 entering the glass are such as to confer a lower co-efficient of thermal expansion on the surface layers of the glass. As an example, lithium ions are substituted into the glass for sodium ions. High lithium glass has a lower coefficient of expansion so that when the treated glass is cooled, compressive surface stresses are induced therein. ~n another process, ions in the surface of the glass are replaced by larger ions and the ion exchange is effectcd at a tempcrature belo-~ the annealing point (corresponding to a viscosity of 10 3 poises) so that stress relaxation will not occur. In an example, potassium ions are substituted into the glass e.g. from a bath of molten potassiurn nitrate maintained at 470 C. Potassium ;ons are larger than sodium ions, so again compressive surface stresses are induced in the treated glass.
Advantagcously, said die]ectric sheet is at most 3mm thicl~, and preferably it is at most 1.5mm thick. A
lmm thick tempered glass sheet is especially suitable for use as a said dielectric sheet.
In preferred embodiments of the invention, the maximum dimension of the sheet area occupied by the or a said switch is 30mm or less, and advantageously such dimension is 25mm or less.
~referably, the sheet arca occ~lpied by the or a said switcll is 450mm or less~ and advantagcously such area is 250mm or less.
Various coating materials may be used to form a said electrode, such as metals~ conductive metal oxides, and conductive (or metal containing) enamels.
As far as the first, touchable electrode is concerned it is preferred that the coating be a conductive metal oxide coating formed of tin oxide or indium oxide and containing a doping agent. Tin oxide is especially preferred because of its hardness and chemical stability, and also for reasonq of economy. The reason for preferring a metal (especially tin) oxide is as follows. It is extremely difficult, if not impossible, to uniformly chemically temper a glass sheet on which a coating has been deposited. Accordingly, in practice when performing the ~resent invention, the sheet must be tempered before the electrodes are deposited. If the tempered sheet i5 subsequently heated to a temperature at which any substan-tial stress relaxation may occur, much of the additional strength imparted to the glass will be lost. Metal oxide coatings of satisfactory hardness and conductivity can readily be deposited at lower temperatures. It would of course be equally possible to make the first, touchable electrode from a conductive ~e.g. silver containing) enamel of a composition selected to have a relatively low melting point. However, in general, the lower the melting point of an enamel, the lower is its hardness and thus its reslstance to abrasion and its resistance to corrosion is also generally lower.
- The requirement of abrasion resistance is not marked for the second and third electrodes since these will not normally be exposed for touching, and it is in fact preferred that said second and third electrodes be formed by deposits of conductive enamel. The use of conductive enamel coatings on the unexposed surface of the switch panel has a number of advantages. Firstly, a paste of enamel 75~;~S

forming ingredients can readily be applied to the dielectric in a desired pattern for e~cample by a silk-screen printin~
technique . Interconnecting conductor leads can also be deposited at the same time if this is desired. Secondly, 5 soldered connexions can easily be made to the enamel.
In some preferred embodiments of` the invention an opaque coating is deposited on a face of the tempered glass shee t prior to the deposition of the electrode or electrodes on that face. Sucil an opaque coating maslcs 10 any electri cal circuitry located behind the panel and is preferably applied to the unexposed face thereof. If desired such coating may be provided with windows associat ed witl a said switch and bellind which a light rnay be provided to indicate the state of the switcll circuit.
1 5 rreferably, a baclcing support is applied and bonded to the unexposcd face of said panel. This providcs added rigidi ty to t;he panel so that it becomes more resistant to flexure. itdvantageously said backin~ support is of synthetic plastics material. Such a support may for example 20 be moulded on to the panel. In some embodiments of the invention, the support is a solid block, but in other embodiments of the invention the support is of cellular form. F`or e~;amp~ e the support has a honeycomb structure.
The support may also be made Or expanded materials, such 25 as foamed glass, and it may comprise a printed circuit board.
The p re s ent invent i on inc l ude s a me thod of manufacturing a touch control switch panel comprising a dielectric sl eet carrying at least one touch switch, the or each swi tch comprising a first, touchable electrode on ~0 one side of said sheet and second and third mutually separated electrodes on the opposite side of the sheet , .

~ ~'7~

in capacitive relation to the first electrode, character-ised in that a sheet of glass is selected as said dielec-tric sheet and is ~ubjected to a chemical tempering treat-ment at elevated temperature and in that at least one conductive coating i~ deposited on the tempered sheet to constitute one or more said electrode while the tempera-ture of the sheet is below the annealing point (correspon-ding to a glass viscosity of 10 3- poises) and so as substantially to avoid stress relaxation.
This is a very simple method of forming a touch control s~itch panel according to the invention, and it avoids any difficulties which could arise in tempering the glass sheet after deposition of the conductive coating(s).
Preferably, said first, touchable electrode is formed by depositing a metal oxide coating on said one side of the sheet. Advantageously said metal oxide coating includes a doping agent in order to increase its conductivity. Suitable doping agents are ions of one or more of antimony, arsenic, cadmium, chlorine, fluorine and tellurium. Said metal oxide may for example be indium oxide, but it is preferably tin oxide.
In some preferred embodiments o~ thejn~ention, a said oxide coating is formed by pyrolysis of a metal salt for example sprayed onto said sheet as a solution in an organic solvent. As examples of such a solution may be cited tin dibutyldiacetate in ethyl alcohol and SnC14.5HzO
in dimethylformamide with optional amounts of trifluoracetic acid to provide doping fluorine ions.
In other preferred embodiments, a said oxide coating is formed by sputtering.
In order to limit the coating to the required areas dlfferent techniques may be used.

~ ~57"3~5 g According to a first preferred method a mask is serigraphically applied to the sheet to cover the areas which are not to be coated in the finished panel and the mask is removed after applyin~ a said conductive coating to the whole area of the tempered sheet. This leaves only the required areas with a coating.
According to a second preferred method, an oxide coating i9 deposited overthe whole surface of the tempered sheet. Then an acid resist mask is serigraphi-cally applied to mask the areas of the or each electrode,and the unwanted coating is etched away.
Said second and third electrodes are preferably formed by depositing a conductive enamel on said opposite side of the glass sheet. Such conductive enamel is preferably deposited by applying an enamel paste to the tempered glass sheet and melting it in situ. Such enamel paste is preferably applied by a serigraphic technique.
A silver containing enamel, in particular an enamel of the organic type~ is especially suitable.
Advantageously, said tempered glass sheet is ; opacified, preferably by the application to one of its faces of a non-conductive opaque enamel coating. Such a coating is preferably applied to the unexposed face of the sheet, advantageously prior to the deposition thereover of sald second and third electrodes.
Said first electrode is preferably deposited on - said sheet prior to the deposition of said second and third e~ectrodes, and also prior to the deposition of a said ~- opaque enamel coating when the latter is present. This avoids problems due to remelting of the enamel coatings during deposition of the oxide coating.

-lo- ~79~S

Preferably a backing support is applied and bonded to the unexposed face of the panel. Such support may for example be of a synthetic plastics material and it may be formed in situ by moulding the panel.
In some embodiments of the invention, one of said second and third electrodes is shaped so as to surround at least the greater part of the periphery of the other. Preferably, the area of the inner of such electrodes is at least one quarter of the area of the outer electrode. Such electrode arrangements allow a beneficial compromise to be reached as regards the total capacitance of the system, the change in capacitance when the first electrode is touched, and the amount of material to be used in forming the second and third electrodes.
In this connection, attention is drawn to Applicant's copending Canadian application N~ 357,113, filed July 24, 1980, entitled: "Capacitive systems for touch control switching", in which there is described a capacitive system comprising a dielectric sheet having a first electrode on one side thereof, and on the other side thereof in capacitive relation with the first electrode, second and third electrodes which are mutually spaced, characterised in that of said second and third electrodes, one (hereinafter called "the outer electrode") is shaped to surround at least the major part of the peri-phery of the other (hereinafter called "the inner elec-trode") and in that the ratio of the area of the inner electrode to the area of the outer electrode is greater 30 than 0.25 to 1.
Preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:
Figure 1 is a detail perspective view of a touch 3~S7~'~S

control switch panel, Figure 2 is a cross section through part of the panel of Figure l;
Figure 3 is a cross section of another embodiment of panel;
Figure 4 is an underplan of the panel of Figure 3:
Figure 5 is a cross section of a third embodiment of panel; and 10Figures 6 to 9 are underplan views of switches showing various arrangements of electrodes.
In Figures 1 and 2, a sheet 1 of tempered soda-lime glass has applied to its upper or exposed face, a square first electrode 2 of doped SnO2. The first electrode is surrounded by a border 3, and carries an index 4, here sho-~n as the digit "1", serving to indicate the function to be performed by the switch of which the electrode 2 - is a part. The lower or unexposed face of the tempered glass sheet 1 is covered with an opaque, non-conducting enamel layer 5 haYing a window 6 in register with the index

4. Second and third electrodes 7, 8 are deposited on the unexposed face of the tempered glass sheet 1 over the opaque enamel layer 5 to either side of the window 6 in capacitive relation with the first electrode 2. These electrodes are of a silver containing enamel.
Figures 3 and 4 show a touch control switch panel generally indicated at 10 which has ten touch control switches Sl to S0 deposited on a tempered glass sheet 11. As described with reference to Figures 1 and 2, each switch includes a first, touchable electrode 12 deposited on an expo~ed face of the glass sheet 11 and on which in turn are deposited a border 13 and a reference index 1l~. Each index 14 may for example be 7~
-- 1~

co~lstituted as the reference digit of the switch S with which it is associated. On the rear face of the ~e~pered glass sheet 11 ~here is deposited an opacifying layer 15 o~` a non_collductive ~aterlal. lhis layer may if de3ired be pro~i~ed wi~h windo~ such as t~e ~indow shown in Fi~ures 1 and 2. Each ~Witch further includes second and chird electrodes re4pectively 17, 1~ ~pacsd apar~ ~y a gap 16. As will be seen in ~igure 4, the : third electrodes 18 of the switches S1 to SQ are connected to a common A.C. source, while the second electrode 17 of each switch S1 to SO i~ connected to an output circuit respectively OC1 to OCO.
The qecond and third electrode~ are suitably of silver-containing material, and they may all be applied in a single operation by qerigraphic techniques. Wire conductors for connecting the electrode~ to the A.C.
source and the Output Circuits may be soldered to the electrodes~ or printed conductors may be applied in the same or ~ub~equent ~erigraphic technique.
,'0 After the necessar~ electrical conne~ions have been made, a backing support 19 is applied to the rear of the panel~ This serves to protect the ~econd and thlrd electrodes and their a3~0ciated conductor~ against wear or corrosion during handlirg prior to installation, but its main function is to brace the panel against flexure while it is being used.
In order to manufacture a panel as shown in the drawing3, a glass sheet 1 or 11 e.g. of ordinary soda-lime glass of a desired thickness is polished and then tempered ~0 in any convenient known manner, for e~ample by immersing ~.~57~5 it for 8 hours in a bath of molten potas~ium nitrate maintalned at 470 C to allow potassium ions to diffu~e into surface layers of the glass in substitution for ~odium ion~ pre~iousl7 forming part of the glass.
After tempering the glass is wa~hed and the first electrodes 2 or 12 are dapo3ited on one ~urface thereof.
It is especially suitable to form the first electrodes of a metal oxide, e.g. SnO2. Such an o~ide coating may be deposited by a cathode sputterin~ technique, but it is preferred to deposit such a coating by pyrolysi3 of a salt e.g. by a ~praying technique using a solution in an organic sol~ent. Suitable solutions ares tin dibutyldiacetate in ethanol ~ith doping amount3 of ~mmonium bifluoride or tin tetrachloride in dlmethylformamide with doping amounts of trifluoroacetic acid. Such a solution may be ~prayed onto the tempered glass ~heet heated to 450 C to form a uniform coating of the de~ired thickne~, for e~ample 50 to 70nm.
Such a coating has a grey tint in reflection, and indeed it~
colour may be used to indicate when to stop spraying. A
coating formed in thiY way is as abraslon resistant a~ glass.
According to P ~ariant, the coating may be deposited by pyrolysis of a -~alt in the vapour phase. The coating may al~o be obtained by immersion of the tempered sheet in an alcoholic solution of a tin compound followed by a baking operatlon. A further possible way i8 to form a coating ~tarting from a paste containing a tin organic compound deposited by serigraphy and followed by baking.
After the application of such a uniform coating, an - acid resist ma~k is applied serigraphically to the areas to ~ be occupied by the electrodes 2 or 12, and the remaining ~7~;~5 areas of the coating are etched away, The front face of the tempered glass is then ; coated ~iith decorating material to form the borders 3, 13 and the indexes 4, 14~ As an optional step the rear face of the tempered glass sheet is then covered with an opacifying layer 5 or 15. In both cases these coatings may be of a low melting point enamel. Preferably however use is made of a synthetic plastics material which is polymerised i~ situ to form an opaque coating. For example use may be made of an epoxy type lacquer the polymerisation temperature of which is below 120 C.
The second and third electrodes are then applied to the rear face of the panel. The second and third electrodes may be of a conductive enamel or lacquer unless there is a polyrneric opacifying layer present, in which case those electrodes should be of lacquer. These materials may be applied serigraphically and heated to fuse the enamel to the glass or to promote rapid setting of the lacquer, In a specific case silver containing lacquer No. ~929 of Dupont de Nemours was used for the second and third electrodes. After coating the lacquer was baked for one hour at 100 C. Another lacquer which may be used for the same purpose is lacquer No. 245 of Degussa.
Conductors may also be applied for the necessary electrical connections in the same serigraphic printing step, but in order to keep the size of the panel small, wire connectors are preferably soldered to the second and third electrodes. This soldering may for example be effected by applying a paste of solder and flux to the electrodes and melting it with a jet of hot air. The solder used may have the following composition: Sn : 620/~;
Pb : 360,~,; Ag : 2~/~. When use is made of a soldering iron - its temperature should be limited to 250 C.

~ ~7 After making the necessary electrical connections the panel may be rigidified ~and the conductors on it~ rear face protected) by moulding on a backing support 19. This may be done by cau~ing a fluid resin to polymerise in situ.
One suitable fluid medium is a resin having the following composition by weight:
P~EXIMON 705 or 706 98.67~
Butyl monoterpermaleinate 0~ 20,b Benzoyl peroxide0.1~' 10 Triethyl phosphate0.7/~
~ohm activator 17 (~aleic naphthanate) 0.4~o PLEXIMON 705 and 706 (Trade Marks) are methyl methacrylate resins produced by Rohm.
This resin ccmposition will polymerise in 8 hours at a temperature of between 20 C and 30 C.
- Another resin which may be used for forming the backing support i5 a polyester resin such as "Polyester GTS"
sold by the firm Vosschemie.
~igure 5 illustrates a variant of the embodiment shown in Figures 3 and 4 in which like elements are designa-ted by like reference numerals. As described with refer-ence to ~igures 3 and 4, the switch panel, now indicated at 20, comprises a tempered glass sheet 11 on which are formed a plurality of switches such as S~, S5, S6 comprising first touchable electrodes 12, borders 13 and reference indices 14, and on which is deposited an opacifying layer 15.
Each switch includes second and third electrodes 22, 23 spaced apart by a gap 21, but these are deposited not on the opacifying layer 15, but on a printed circuit board 2~ which is bonded to the opacifying layer 15 by a layer of adhesive 25 to serve as a support for the glass sheet llo The second and third electrodes 22, 23 are conveniently of metallic copper deposited as is well known in the printed circuit art. The printed circuit board ~ ~7 ~ ~5 - 16 _ 24 is pierced with a plurality of holes 26 via which the various second and third electrodes 22, 23 are connected by wires 27 soldered at 2~ to appropriate parts 29 of a printed circuit.
~igure 6 illustrates a touch control switch similar to that shown in Figures 1 to 4. In Figure 6, a sheet 61 of tempered soda-lime glass has applied to its une~posed face a pair of electrodes (the second and third electrodes) 62, 63 which are rectangular in form and are separated by a gap 61l. The electrodes 62, 63 and the gap 61! between them together occupy a square on the unexposed face of the sheet 61. A first electrode is deposited on the exposed face of the sheet 61~ Part of the boundary of the first electrode is indicated at 65. The first electrode is square and in register with the square formed by the second and third electrodes 62, 63.
Fisure 7 illustrates a circular touch control switch formed on a tempered glass sheet 71. P~rt circular second and third electrodes 72, 73 separated by a diametral gap 74 are deposited on the unexposed face of the sheet 71.
A circular first electrode, part of whose circum~erence is shown at 75 is deposited on the exposed face of the sheet 71 in register with the circle formed by th0 second and th~rd electrode~i 72, 73 and the gap 74 between them.
Figure 8 illustrates a touch control switch in which a tempered glass sheet 81 carries a square second electrode 82 surrounded by a square annular third electrode 83. T~e second and third electrodes are separated by a square annular gap 84. A first electrode (not shown) is deposited on the other face of the sheet 81 with its boundary in register with the periphery of the third electrode 83.
Figure 9 illustrates a touch control switch in which a tempered glass sheet 91 carries a circu~r second electrode 92 surrounded by an annul~r third electrode 93.

~S7'3;~

The second and third electrodes are separated by an annular gap 940 A first electrode (not shown) is deposited on the other face of the sheet 91 with its boundary in register with the circumferences of the third electrode 93.
~arious particular touch control switches will now be described by way of e~ample.
E~ample I (Figure 6) ~ sheet 61 of glass lmm in thickness is tempered and a first electrode 12mm square is deposited on one of its faces. This first electrode is of doped tin oxide.
Second and third electrodes 62, 63 each measuring 12x5mm are then formed on the opposite face of the glass sheet 61.
These electrodes are spaced apart by a 2mm gap 64, and they are formed of a silver containing enamel. The total area of the sheet occupied by the switch is 144mm2.
In a flrst variant, the tempered glass sheet 61 is 2.8mm thick, and in a second variant it is 4.9mm thick.
Example II (Figure 8) A sheet of glass 81, of thickness lmm, i~ tempered ~0 and a first electrode 12mm square of doped tin oxide is doposited on one face. Second and third electrodes 82,83 are deposited on the opposite face of the sheet in register with the first electrode. The second electrode 82 is 6mm square and it is separated by a square annular gap 84 which is lmm in width from the third electrode 83 which is a s~uare annulus ~mm in width. The second and third elec-trodes are formed from silver containing enamel. The total switch area is 144mm2.
In variants, the tempered glass sheet 81 is 2.8mm thic~c and 4.9mm thick.
Example III (Figure 8) In variants of Example II the second electrode 82 is 4mm square and is separated by a 2mm wide square annular gap 84 from the third electrode 33 ~hich is 2mm wide. Such switches are formed on tempered glass having the following thicknesses: lmm, 2.8mm and 4.9mm. The ~7~
- 18 _ total switch area i~ again 144mm .
Exam~le IV (Figure 7) A sheet of glass 71, of thickness lmm, is tempered and a first circular electrode 13.5mm diameter of doped tin oxide is deposited on one face. On the opposite face are deposited part circular second and third electrodes 72, 73 of silver containing enamel.
These electrodes are separated by a gap 74, 2mm wide.
The second and third electrodes together with the gap separating them occupy a circular area 13.5mm in diameter which is in register with the first electrode. The total switch area is 143mm .
In variants, the switch is formed on tempered glass 2.8mm and 4.9mm thick.
Example V (Figure 9) A sheet of glass 91, of thickness lmm, is tempered and a first circular electrode 13.5mm in diameter of doped tin oxide is deposited on one face. On the opposite face are deposited, in register with the Mrst zo electrode, second and third electrodes 92, 93 of silver containing enamel. The second electrode occupies circular area 7.5mm in diameter and is separated by an annular gap 94, lmm wide from the third electrode which is an annulus 2mm wide. I\gain the total switch area is 143mm2.
In variants, the switch is formed on tempered glass 2.8mm and 4.9mm in thickness.
- Example VI (Figure 9) In variants of Example V, the second electrode 92 is 5.5mm in diameter, and the annular gap 94 is 2mm wide. Again the switch is formed on tempered glass of the following thicknesses: lmm, 2.8mm and 4.9mm.
Since the total areas occupied by these switches are similar, their capacitances when not touched, and the change in their capacitances when touched, may be directly compared, as in the following table.

E.Yample Total capacitance Capacitance change Active when untouched when touched surface lmm 2.8mm 4.9mm lmm 2.8mm 4.9mm area glass glass glass glass glass glass pF pF pF pF pF pF ~/o I 3.1 1.9 1~4 2.4 1.0 o.6 83.3 II 3.0 2.2 2.2 2.1 0.7 0.5 80.6 III 2.1 1.8 1.3 1.6 0.7 0.3 66.7 IV 3.2 1.9 1.5 2.4 1.0 o.6 81.2 10 V 3.6 3.0 2.5 ~.5 0.9 0.5 81.0 VI 2.4 2.0 1.6 1.6 0~7 0.5 67.1 The column active surface area indicates the proportion of the first electrode which is in register with one or other of the second and third electrodes.
15 The capacitance values where indicated in this application are obtained by measurement with a universal bridge WAYN~ KERR, type B224. The measurements are effected under normal ambient conditions. The capacitive switching system is disposed horizontally with the SnO2 coating located on top. Standard connecting wires are connected by means of terminal clamps fi~ed to wires 5mm in length soldered to the second and third electrodes.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A touch control switch panel comprising a dielectric sheet carrying at least one touch switch, the or each switch comprising a first, touchable elec-trode on one side of said sheet and second and third mutually separated electrodes on the opposite side of the sheet in capacitive relation to the first electrode, characterised in that said dielectric sheet is of glass which has been subjected to a chemical tempering treat-ment, and in that at least one said electrode is cons-tituted by a coating deposited on the sheet.

2. A panel according to claim 1, characterised in that each said electrode is constituted by a coating deposited on said sheet.

3. A panel according to claim 1, characterised in that said dielectric sheet is at most 3 mm thick.

4. A panel according to claim 3, characterised in that said dielectric sheet is at most 1.5 mm thick.

5. A panel according to claim 1, characterised in that the maximum dimension of the sheet area occupied by the or a said switch is 30 mm or less.

6. A panel according to claim 5, characterised in that the maximum dimension of the sheet area occupied by the or a said switch is 25 mm or less.

7. A panel according to claim 1, characterised in that the sheet area occupied by the or a said switch is 450 mm or less.

8. A panel according to claim 7, characterised in that the sheet area occupied by the or a said switch is 250 mm2 or less.

9. A panel according to claim 1, characterised in that the or at least one said first, touchable elec-trode is constituted by a conductive metal oxide coating formed of tin oxide or indium oxide and containing a doping agent.

10. A panel according to claim 1, characterised in that a backing support is applied and bonded to the unexposed face of said panel.

11. A panel according to claim 10, characterised in that said backing support is of synthetic plastics material.

12. A panel according to claims 10 or 11, charac-terised in that said backing support comprises a printed circuit board.

13. A method of manufacturing a touch control switch panel comprising a dielectric sheet carrying at least one touch switch, the or each switch comprising a first, touchable electrode on one side of said sheet and second and third mutually separated electrodes on the opposite side of the sheet in capacitive relation to the first electrode, characterised in that a sheet of glass is selected as said dielectric sheet and is sub-jected to a chemical tempering treatment at elevated temperature and in that at least one conductive coating is deposited on the tempered sheet to constitute one or more said electrodes while the temperature of the sheet is below the annealing point (corresponding to a glass viscosity of 1013.2 poises) and so as substantially to avoid stress relaxation.

14. A method according to claim 13, characterised in that the or at least one said first, touchable elec-trode is formed by depositing a metal oxide coating on said one side of the sheet by pyrolysis of a metal salt.

15. A method according to claims 13 or 14, cha-racterised in that a mask is serigraphically applied to the sheet to cover the areas which are not to be coated in the finished panel and the mask is removed after applying a said conductive coating to the whole area of the tempered sheet.

16. A method according to claim 13, characte-rised in that said second and third electrodes are formed by depositing a conductive enamel on said opposite side of the glass sheet.

17. A method according to claims 14 or 16, charac-terised in that said first electrode is deposited on said sheet prior to the deposition of said second and third electrodes.

18. A method according to claim 13, characterised in that a backing support is applied and bonded to the unexposed face of the panel.

19. A touch control switch panel manufactured by a method according to claim 13.