CH639228A5 - Switching panel of the sensitive type and its method of manufacture - Google Patents

Description

The present invention relates to a sensitive type switch panel comprising a dielectric sheet carrying at least one sensitive type switch, the or each switch comprising a first contact electrode on one side of said sheet and on its other side, second and third electrodes in spaced relationship, the second 2o and third electrodes both in capacitive relationship with the first electrode.

The three electrodes combine to form two capacitors connected in series. In use, an AC signal is applied to one of the second and third electrodes while the other electrode is connected to a detection circuit. When the first electrode is touched with the finger, the signal at the input of the detection circuit is significantly weakened and this weakening of the signal is used to carry out the control.

Such switches can be used in many fields, for example as elevator call switches, as switches for household appliances (e.g. cookers) and in switchboards for example such as telephone keypads sensitive or computer control keyboards.

To obtain reliable switching, such a switch must have as high a capacity as possible. The reason is that the signal sent to the electrode by a pulse generator must have the highest possible level at its entry into the detection circuit, taking into account the capacity io of the input impedance of the circuit . The higher this level, the easier it will be to reject the noise signals by means of a threshold circuit incorporated in the detection circuit. A typical value of capacitance to reach is 3 pF for certain commercial detection circuits which have been specially developed for capacitive switching systems of the sensitive type.

It is desirable that each switch occupies a small area, especially in the case of panels carrying rows of switches. This desideratum is at some point in contradiction with the need for a capacity greater than a given threshold value since, according to the well-known conventional formula, the capacity of a flat plate capacitor is proportional to its surface.

According to the same formula, the capacity is inversely proportional to the thickness of the dielectric material; in this way it would obviously be possible to reduce both the surface of the switch and the thickness of the panel while maintaining the same capacity.

It is also possible to modify the capacity by the 6o choice of the dielectric material, but this can lead to an apparent contradiction with other requirements. For example, natural dielectric materials such as mica are not suitable for mass production. Plastics are easily eroded and are generally flexible when it comes to thin sheets, so that a sensitive switch panel having a contact electrode deposited on a dielectric sheet of plastic would deteriorate quickly. Ordinary soda-lime glass em

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bent as a dielectric is satisfactory from the point of view of its wear resistance, but the glass sheet must be thick enough to resist impact. This limits the dimensions to which a switch can be reduced while keeping the capacity value above a threshold value.

One of the objects of the present invention is to remedy this difficulty.

According to the present invention, the sensitive type switching panel comprises a dielectric sheet carrying at least one sensitive type switch, the or each switch comprising a first contact electrode on one side of said sheet, and on its other side, second and third electrodes being in capacitive relation with the first electrode, the panel being characterized in that said dielectric sheet consists of a glass sheet having undergone a chemical toughening treatment, and in that at least one electrode is constituted a layer deposited on the sheet.

A sensitive type switching panel according to the invention resists impact better than a known type panel, due to the toughening treatment which the glass dielectric underwent, so that the thickness can be reduced. of the glass sheet while retaining sufficient resistance to impact. This characteristic in turn makes it possible to reduce the surface area of the or each switch while maintaining the value of its capacity at a sufficient level.

Preferably each said electrode consists of a coating deposited on said sheet.

The chemical toughening of glass is well known per se. The most common type of tempering involves an exchange of alkaline ions in the glass with alkali metal ions from a medium in contact with the glass when the latter is at elevated temperature. Ion exchange takes place in the surface layers of glass over thicknesses of a few microns or a few tens of microns. In one of the methods, the ion exchange is carried out at a temperature high enough for the relaxation of the tensions to take place and the ions penetrating into the glass are such that they confer a coefficient of thermal expansion lower than the surface layers of the glass. By way of example, lithium ions are substituted for the sodium ions in the glass. Glass with a high lithium content has a lower coefficient of expansion so that when the treated glass is cooled surface compressive tensions are created. In another method, ions on the surface of the glass are replaced by larger ions and the ion exchange is carried out at a temperature below the upper annealing temperature (corresponding to a viscosity of 1013-2 poises) and so such that there is practically no relaxation of tensions. In one example, potassium ions are substituted for the sodium ions in the glass, for example from a bath of molten potassium nitrate maintained at 470 ° C. Since potassium ions are larger than sodium ions, surface compression tensions are generated in the treated glass.

Advantageously, said dielectric sheet has a thickness of 3 mm at most, and preferably its thickness does not exceed 1.5 mm. A 1mm thick tempered glass sheet is especially suitable for use as a dielectric sheet for the present invention.

In preferred embodiments of the invention, the maximum dimension of the sheet surface occupied by the or said switch is 30 mm or less, and advantageously this dimension is 25 mm or less.

Preferably, the sheet surface occupied by the or a said switch is 450 mm2 or less and advantageously this surface is 250 mm2 or less.

Different coating materials can be used to form a said electrode, such as metals, conductive metal oxides and lacquers and conductive enamels (or containing metal).

With regard to the first contact electrode, it is preferred that the coating be a conductive metal oxide 5 consisting of tin oxide or indium oxide and containing a doping agent. Tin oxide is especially preferred because of its hardness, chemical stability and also for reasons of economy. The reason why metal oxide (especially tin) is preferred is as follows. It is extremely difficult, if not impossible, to chemically uniformly toughen a sheet of glass on which a coating has been deposited. For this reason, in practice, when carrying out the present invention, the sheet is quenched before the electrodes are deposited there. If the toughened sheet is then heated to a temperature at which substantial stress relief can occur, much of the strength imparted to the glass will be lost. Coatings of metal oxides of satisfactory hardness and conductivity can easily be deposited at temperatures not too high. It would obviously also be possible to produce the first contact electrode in a conductive enamel (for example containing silver) of chosen composition so as to obtain a relatively low melting point. However, in general, the lower the melting point of an enamel, the lower its hardness and therefore its resistance to abrasion; its corrosion resistance is also generally lower.

Abrasion resistance is not required for the second and third electrodes since normally they will not be exposed to contact and in fact it is preferred that said second and third electrodes are formed of conductive enamel deposits. The use of conductive enamel coatings on the unexposed side of the switch panel has a number of advantages. First an enamel composition can easily be applied to the dielectric 35 as a desired cannevas, for example using the silk screen printing technique. Interconnection conductors can be removed at the same time, if desired. In addition, soldered connections can easily be made on the enamel.

In certain embodiments of the invention, an opaque coating is deposited on one face of the tempered sheet before depositing the electrode or electrodes on this face. Such an opaque coating masks the electrical circuit located below the panel and is preferably ap-45 plicated on its unexposed face. If desired, such a covering can be provided with windows associated with the switches in front of which signaling means can be placed to indicate the state of the switching circuit.

Preferably, a reinforcement support is applied and secured to the unexposed face of the panel. This provides additional rigidity to the panel and thus makes it more resistant to bending. Advantageously, said reinforcing support consists of a synthetic plastic material. Such a support can for example be molded on 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 structure. For example, the support may have a honeycomb structure. The support can also be made of an expanded material such as foam glass. It can also include or be constituted by a printed circuit board.

The present invention also comprises a method for manufacturing a panel defined by claim 13. This method makes it possible to produce a sensitive switching panel according to the invention in a simple manner and to avoid most of the difficulties which could arise. during tempering of the glass sheet after deposition of the conductive coating (s)).

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Preferably said first contact electrode is formed by depositing a coating of metal oxide on a said face of the sheet. Advantageously, said metal oxide coating comprises a doping agent in order to increase its conductivity. Suitable doping agents are the ions of one or more of the following: antimony, arsenic, cadmium, chlorine, fluorine and tellurium. Said metal oxide can for example be indium oxide, but tin oxide is preferred.

In certain preferred embodiments of the invention, a said oxide coating is formed by pyrolysis of a metal salt, for example sprayed onto said sheet in the form of a solution in an organic solvent. As an example of such a solution, mention may be made of tin dibutylacetate in ethyl alcohol and SnCl4 • 5H20 in dimethylformamide with optionally a certain amount of tri-fluoracetic acid which provides ions doping fluorine.

In other preferred embodiments, a said oxide coating is formed by vacuum deposition p. ex. by sputtering.

In order to limit the coating to the desired surfaces, different techniques can be used.

According to a first preferred method, a mask is applied to the sheet by screen printing to cover the surfaces which should not be coated in the finished panel. This mask is removed after application of said conductive coating over the entire surface of the tempered sheet. This method leaves the coating only on the desired surfaces.

According to a second preferred method, an oxide coating is deposited over the entire surface of the tempered sheet. An acid-resistant mask is then applied by screen printing to mask the surfaces of the or each electrode and the unwanted coating is stripped.

Said second and third electrodes are preferably formed by depositing a conductive enamel on said opposite face of the glass sheet. Such a conductive enamel is preferably deposited by applying an enamel composition in the form of a paste to the tempered glass sheet and by melting in situ. Such enamel paste is preferably applied according to a screen printing technique. An enamel containing silver, in particular an organic type enamel, is especially suitable.

Advantageously, said tempered glass sheet is opacified, preferably by applying to one of its faces a coating of non-conductive opaque enamel. Such a coating is preferably applied to the unexposed face of the sheet, advantageously before the deposition of said second and third electrodes.

Said first electrode is preferably deposited on said sheet before the deposition of said second and third electrodes, and also before the deposition of said coating of opaque enamel when the latter is present. This avoids the problems due to the remelting of the enamel coatings during the deposition of the oxide coating.

Preferably a reinforcement support is applied and secured to the unexposed face of the panel. Such a support can for example consist of a synthetic plastic material and it can be formed in situ by molding.

According to certain embodiments of the invention, one of said second and third electrodes is shaped so as to surround at least the major part of the periphery of the other. Preferably, the surface of the internal electrode is at least a quarter of the surface of the external electrode. Such electrode arrangements allow a beneficial compromise to be reached between the total capacity of the system, the variation in capacity when the first electrode is touched and the amount of material to be used to form the second and third electrodes.

In this regard, the applicant draws attention to its patent application entitled: “Capacitive switching system of the sensitive type” filed on the same date as the present application, which describes a capacitive system comprising a dielectric sheet carrying a first electrode on one of its faces and second and third electrodes in spaced relation on its other face, the second and third electrodes 10 being in capacitive relation with the first electrode which is characterized in that said second and third electrodes, the one of them (hereinafter called "external electrode") is shaped so as to surround most of the periphery of the other (hereinafter called "internal electrode") and i5 in that the ratio of the surface of the internal electrode at the surface of the external electrode is greater than 0.25: 1.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a detailed perspective view of a sensitive type switching panel;

- Figure 2 is a cross section through part of the panel of Figure 1;

FIG. 3 is a cross section of another embodiment of the invention;

- Figure 4 is a plane of the underside of the panel of Figure 3;

- Figure 5 is a cross section of a third embodiment of a panel according to the invention; and

FIGS. 6 to 9 are plan views of the underside of switches, showing different arrangements of the electrodes.

In FIGS. 1 and 2, a sheet 1 of chemically toughened soda-lime glass carries on its upper face or ex-35 face a first square electrode 2 of doped SnO 2. The first electrode is surrounded by a decorative border 3 and bears a symbol 4, here represented in the form of the number 1, serving to indicate the function of the switch of which the electrode 2 is a part. The underside or unexposed face of the tempered glass sheet 40 is covered with a layer 5 of non-conductive opaque enamel provided with a window 6 in register with the symbol 4. Second and third electrodes 7,8 are deposited on the unexposed face of the tempered glass sheet above the opaque enamel layer 5 on either side of the window 6. 45 These electrodes are in capacitive relationship with the first electrode 2 and are made up of a silver enamel.

Figures 3 and 4 show a sensitive switch panel generally indicated at 10 which is provided with 10 sensitive switches SI to S0 deposited on a sheet of glass so chemically toughened 11. As they are represented with reference to Figures 1 and 2 , each switch comprises a first contact electrode 12 deposited on the exposed face of the glass sheet 11 and on which in turn a border 13 and a symbol 14 have been deposited. Each symbol 14 can for example constitute the numerical reference of the switch S associated with it. On the rear face of the toughened glass sheet is deposited an opaque layer 15 of material of non-conductive material. This layer can, if desired, be provided with windows such as window 6 shown in FIGS. 60 1 and 2. Each switch further comprises second and third electrodes, respectively 17 and 18 spaced apart by an interval 16. As shown in FIG. FIG. 4, the third electrodes 18 of the switches SI to S0 are connected to a common source AC, while the second electrode 17 of 65 each switch SI to SO is connected to an output circuit respectively OC1 to OCO.

The second and third electrodes are made of a material containing silver and can be applied

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quées in a single operation by screen printing techniques. Conductive wires for connecting the electrodes to the AC source and to the output circuits can be soldered to the electrodes, or printed conductors can be applied by screen printing simultaneously or after the application of the electrodes.

After completion of the necessary electrical connections, a reinforcement support 10 is applied to the rear face of the panel. This protects the second and third electrodes and the conductors associated with them against wear and corrosion during handling before installing the panel. However, its main function is to stiffen the panel against bending.

To manufacture a panel as shown in the drawings, a sheet of glass 1 or 11, for example of ordinary soda-lime glass of the desired thickness, after having been optionally polished, tempered by any suitable known process. For example, the sheet can be immersed for 8 hours in a bath of molten potassium nitrate maintained at 470 ° C to allow the potassium ions to diffuse inside the surface layers of the glass and to replace the sodium ions of the glass.

After tempering, the glass is washed and the first electrodes 2 or 12 are deposited on one of its faces. It is particularly advantageous to produce the first electrodes from a metal oxide, for example SnO2. Such an oxide coating can be deposited by a sputtering technique. Preferably, the coating is deposited by pyrolysis of a solution of the salt in an organic solvent which is for example sprayed onto the glass. Appropriate solutions are: tin dibutylacetate in ethanol with ammonium bifluoride for doping or tin tetrachloride in dimethylformamide with amounts of tri-fluoracetic acid for doping. Such a solution can be sprayed onto the tempered glass sheet heated to 450 ° C to form a uniform coating of desired thickness, for example 50 to 70 mm. Such a coating has a gray shade in reflection. Observing its color can be used as an indication of when to stop spraying. A coating formed in this way is as resistant to abrasion as glass.

Alternatively, the coating can be deposited by pyrolysis of a salt in the vapor phase. The coating can also be obtained by immersing the sheet soaked in an alcoholic solution of a tin compound, followed by a baking operation. Another possible way of proceeding is to form a coating from a paste containing an organic tin compound deposited by screen printing followed by baking.

After application of such a uniform coating, an acid-resistant mask is applied by screen printing to the surfaces which the electrodes will occupy and the remaining surfaces of the coating are etched.

The front surface of the tempered glass is then coated with a decorative material to form the edges 3.13 and the symbols 4.14. Optionally, the rear face of the tempered glass sheet can then be coated with a layer of opacifica-tion 5,15. In both cases these coatings can consist of an email with a low melting point. However, it is preferable to use a synthetic plastic material polymerized in situ to form the opaque coating. One can for example use a paint of the epoxy type whose polymerization temperature is lower than 120 ° C.

The second and third electrodes are then applied to the rear face of the panel. The second and third electrodes can consist of a conductive enamel or paint. When an opacifying polymeric layer is present, these electrodes must be deposited from a paint. These materials can be applied by screen printing and heated to melt the enamel on the glass or to cause the paint to set quickly. In a specific case, paint No. 4929 by Dupont de Nemours containing silver was used to form the second and third electrodes. After coating the paint was dried for one hour at 100 ° C. Another paint that can be used for the same purpose is paint n ° 245 by Degussa.

The conductors necessary for the electrical connections 10 can be made during the same screen printing step, but in order to keep the dimensions of the panel small, it is preferred to solder connection wires to the second and third electrodes. This welding operation can for example be carried out by applying a hard sou-15 paste and flux on the electrodes and by its fusion by means of a jet of hot air. The solder used can have the following composition: Sn: 62%; Pb: 36%; Ag: 2%. When using a soldering iron, its temperature should be limited to 250 ° C.

After the necessary electrical connections have been made, the panel can be stiffened (and the conductors on its rear side protected) by molding a reinforcement support 19 on its unexposed side. This can be achieved by having a fluid resin polymerized in situ.

A suitable fluid medium is a resin having the following composition by weight:

Pleximon 705 or 706 98.6%

0.2% butyl maleinate maleinate

Benzoyl peroxide 0.1%

30 Triethyl phosphate 0.7% Rohm activator 17

(maleic naphthenate) 0.4%

Pleximon 705 and 706 (brands) are methyl methacrylate resins produced by Rohm 35 This resin composition will polymerize in 8 hours at a temperature between 20 and 30 ° C.

Another resin which can be used to form the reinforcing support is a polyester resin such as "Polyester GTS" sold by the company Vosschemie.

FIG. 5 illustrates a variant of the embodiment shown in FIGS. 3 and 4 in which the same elements are designated by the same reference numbers. As described with reference to FIGS. 3 and 4, the switch panel, now indicated at 20, comprises a sheet of chemically tempered glass 45 on which a plurality of switches such as S4, S5, S6 is formed. comprising first contact electrodes 12, borders 13 and symbols 14 as well as an opacifying layer 15.

Each switch comprises second and third 50th electrodes 22,23 spaced from one another by an interval 21. These are deposited not on the opacifying layer 15, but on a printed circuit board 24 which is secured to the opacifying layer 15 by a layer of adhesive 25. This plate serves as a support for the glass sheet 11. The two-third and third electrodes 22, 23 are formed by a deposit of metallic copper according to the techniques known in the art of printed circuits. The printed circuit board 24 is pierced with a plurality of holes 26 through which different second and third electrodes are connected by means of wires 27 welded at 28 to the appropriate parts 29 of a printed circuit.

FIG. 6 illustrates a sensitive switch similar to those shown in FIGS. 1 to 4. In FIG. 6, a sheet of tempered soda-lime glass 61 carries on its unexposed face 65 a pair of electrodes (the second and third electrodes) 62 , 63 of rectangular shape and separated by an interval 64. The electrodes 62, 63 and the interval 64 together occupy a square on the unexposed face of the sheet 61. A

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first electrode is deposited on the exposed face of the sheet 61. Part of the edge of the first electrode is indicated at 65. The first electrode is square and in register with the square provided by the second and third electrodes 62,63.

FIG. 7 illustrates a circular sensitive switch formed on a toughened glass sheet 71. The second and third semi-circular electrodes 72, 73 separated by a diametral interval 74 are deposited on the unexposed face of the sheet 71. A first circular electrode , of which a part of the circumference at 75 has been shown is deposited on the exposed face of the sheet 71 in register with the circle formed by the second and third electrodes 72, 73 and the interval which separates them.

FIG. 8 illustrates a sensitive switch in which a tempered glass sheet 81 carries a second square electrode 82 surrounded by a third square annular electrode 83. The second and third electrodes are separated by a square annular interval 84. A first electrode (not shown) is deposited on the other face of the sheet 81 with its periphery in register with the periphery of the third electrode 83.

FIG. 9 illustrates a sensitive switch in which a toughened glass sheet 91 carries a second circular electrode 92 surrounded by a third annular electrode 93. The second and third electrodes are separated by an annular interval 94. A first electrode (not shown) is deposited on the other face of the sheet 91 so that its periphery is in register with the circumference of the third electrode 93.

Various particular sensitive switches will now be described by way of example.

Example I (Figure 6)

A sheet 61 of glass 1 mm thick is chemically toughened and a first square electrode 12 mm in side is deposited on one of its faces. This first electrode consists of doped tin oxide. Second and third electrodes 62,63 each measuring 12 × 5 mm are then deposited on the opposite face of the glass sheet 61. These electrodes are spaced from one another by an interval 64 of 2 mm and they consist of an enamel containing silver. The total area of the sheet occupied by the switch is 144 mm2.

In a first variant, the tempered glass sheet 61 has a thickness of 2.8 mm, and in a second variant, its thickness is 4.9 mm.

Example II (Figure 8)

A glass sheet 81 1 mm thick is tempered and a first square electrode 12 mm in side made of doped tin oxide is deposited on one side. Second and third electrodes 82, 83 are deposited on the opposite face in register with the first electrode. The second electrode 82 is a square of 6 mm side and it is separated by a square annular interval 84 of 1 mm wide from the third electrode 81 which is a square ring of 2 mm wide. The second and third electrodes consist of an enamel containing silver. The total surface is 144 mm2.

In variants, the tempered glass sheet 81 has a thickness of 2.8 mm and 4.9 mm.

Example III (Figure 8)

In variants of Example II, the second electrode 82 is a square with a side of 4 mm and it is separated by a square annular gap of 2 mm in width from the third electrode 83 which is 2 mm in width. Switches having these dimensions are formed on tempered glass having the following thicknesses: 1 mm, 2.8 mm and 4.9 mm. The total surface of the switch is again 144 mm2.

Example IV (Figure 7)

A glass sheet 71 1 mm thick is chemically toughened and a first circular electrode 13.5 mm in diameter made of doped tin oxide is placed on one of the faces. On the opposite side are placed second and third hemicircular electrodes made of enamel containing silver. These electrodes are separated by an interval 74 of 2 mm wide. The second and third electrodes together with the interval which separates them occupy a circular surface of 13.5 mm in diameter in register with the first electrode. The total area of the switch is 143 mm2.

In variants, the switch is formed on 2.8 mm and 4.9 mm thick tempered glass.

15

Example V (Figure 9)

A glass sheet 91 1 mm thick is chemically toughened and a first circular electrode 13.5 mm in diameter made of doped tin oxide is deposited on one side. On the opposite side are deposited, in register with the first electrode, second and third electrodes 92, 93 made of enamel containing silver. The second electrode occupies a circular surface of 7.5 mm in diameter and is separated by an annular gap 94 1 mm wide from the third electrode which is a ring 2 mm wide. Again the total area of the switch is 143 mm2.

In variants, the switch is formed on 2.8 mm and 4.9 mm thick tempered glass.

30 Example VI (Figure 9)

In variants of Example V, the second electrode 92 has a diameter of 5.5 mm and the annular gap 94 is 2 mm wide. Again the switch is formed on tempered glass having the following thicknesses: 1 mm, 2.8 mm and 4.9 mm.

Since the total surfaces occupied by these switches are similar, their capacities in non-contact state, and the change in their capacity in contact state, can be compared directly. The following table gives the different values.

Ex- Capacity Change Total floor area active capacity state non contact contact state

45

glass glass glass glass glass glass

1 mm

2.8

mm 4.9 mm 1 mm

2.8mm

4.9 mm

pF

pF

pF

pF

pF

pF

%

I

3.1

1.9

1.4

* 2.4

1.0

0.6

83.3

II

3.0

2.2

2.2

2.1

0.7

0.5

80.6

50 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

0.6

81.2

V

3.6

3.0

2.5

2.5

0.9

0.5

81.0

VI

2.4

2.0

1.6

1.6

0.7

0.5

67.1

The active surface column indicates the proportion of the first electrode which is in register with one or the other of the second and third electrodes.

The capacity values, when indicated in the 6o present text, are obtained by measurement by means of a universal bridge WAYNE KERR, type B 224. These measurements are carried out under normal ambient conditions. The capacitive switching system is arranged horizontally, the coating of SnO2 being directed upwards. Standard cone-65 xion wires are connected by means of terminal clamps fixed to 5 mm long wires soldered to the second and third electrodes.

VS

3 sheets of drawings

Claims (18)

639,228 1. Sensitive type switching panel comprising a dielectric sheet carrying at least one sensitive type switch, the or each switch comprising a first contact electrode on one face of said sheet, and on its other face, second and third electrodes in spaced relationship, the second and third electrodes being in capacitive relationship with the first electrode, characterized in that said dielectric sheet consists of a glass sheet having undergone a chemical toughening treatment, and in that at least one electrode consists of a layer deposited on the sheet. 2. Panel according to claim 1, characterized in that each electrode consists of a coating deposited on said sheet. 2 3. Panel according to one of claims 1 or 2, characterized in that the thickness of said dielectric sheet is 3 mm at most. 4. Panel according to claim 3, characterized in that the thickness of said dielectric sheet is of, 5 mm at most. 5. Panel according to one of claims 1 to 4, characterized in that the maximum dimension of the surface of the sheet occupied by the or a switch is 30 mm or less. 6. Panel according to claim 5, characterized in that the maximum dimension of the surface of the sheet occupied by the or a switch is 25 mm or less. 7. Panel according to one of claims 1 to 6, characterized in that the surface of the sheet occupied by the or a switch is 450 mm2 or less. 8. Panel according to claim 7, characterized in that the surface of the sheet occupied by the or a switch is 250 mm2 or less. 9. Panel according to one of claims 1 to 8, characterized in that the or at least one said first contact electrode consists of a conductive coating of metal oxide formed of tin oxide or indium oxide and containing a doping agent. 10. Panel according to one of claims 1 to 9, characterized in that a reinforcing support is applied and secured to the unexposed face of said panel. 11. Panel according to claim 10, characterized in that said reinforcing support consists of a synthetic plastic material. 12. Panel according to one of claims 10 or 11, characterized in that said reinforcing support comprises a printed circuit board. 13. A method of manufacturing a switching panel according to claim 1, characterized in that one selects as a dielectric sheet a glass sheet which is subjected to a chemical tempering treatment at high temperature and in that at least one conductive coating is deposited on the tempered sheet in order to constitute one or more so-called electrode (s), while the temperature of the sheet is lower than its upper annealing temperature corresponding to a glass viscosity of 1013-2 poises in order to avoid a substantial relaxation of tension. 14. Method according to claim 13, characterized in that the or at least one said first contact electrode is formed by depositing a coating of metal oxide on a said face of the sheet by pyrolysis of a metal salt. 15. Method according to one of claims 13 or 14, characterized in that a mask is applied by screen printing on the sheet to cover the surfaces which must not be coated in the finished panel, this mask being removed after application of a said conductive coating over the entire surface of the toughened sheet. 16. Method according to one of claims 13 to 15, characterized in that said second and third electrodes are formed by depositing a conductive enamel on said opposite face of the glass sheet. 17. Method according to claims 14 and 16, characterized in that said first electrode is deposited on said sheet before the deposition of said second and third electrodes. 18. Method according to one of claims 13 to 17, characterized in that a reinforcing support is applied and soli- io darisé on the unexposed face of the panel.