GB2036330A - Circuit arrangement for operating a touch control - Google Patents

Abstract

Touch controls in which the active switching areas are provided as thin, preferably transparent, metal layers on glass plates etc., are used for electric controls, particularly for controlling dish washers. According to the invention, a pulse generator JG1 which is connected on the one hand directly and on the other hand via a series resistor r1 to the inputs of an AND-gate G, a capacitive CT or resistive touch control being also connected to the input with the preceding resistor, is used to operate this tough control. The gate produces a switching command when the two inputs of the gates receive different pulses, which is always the case when the touch area of the touch control is touched. <IMAGE>

Description

SPECIFICATION Circuit arrangement for operating a touch control.

It is known, for electric controls, particularly for the control of electric household appliances, to use so-called touch controls which are set to the required switching state either capacitively or by changing a resistance via a subsequent evaluation circuit, instead of rotary or push button switches.

For capacitive "touch" controls, it is known to provide the active switching areas in the form of thin metal layers on isolating plates, for example glass plates. It is known to use a square-wave pulse generator as the generator to operate such a capacitive switch, tp connect the output of the generator to the capacitive areas in such a way that two consecutively connected small capacitances are interconnected across the touch areas so that an operator forms a bypass capacitance to ground when he touches the areas.

The output of this capacitive touch control is connected to an integrating element which converts the peaks of the pulses, produced by the square-wave generator and connected across the capacitance of the touch control, into an equivalent d.c. voltage which controls the switching stages via an amplifying element.

Let it be assumed that the capacitance areas of the control switch are rated so that voltage peaks of a sufficient height are produced in the integrating member. If now the touch area of the touch control is charged via the body of the operator which then forms the capacitive quantity, the pulses are substantially short-circuited and the integrator is no longer capable of controlling the subsequent amplifier stage. Consequently, the switching stage receives a different signal and switches.

In the known embodiment the active switching area of the touch control switch must be relatively large in view of the requisite series connection of two capacitances and, in addition, the voltage peaks of the generator must also be relatively great.

It is an object of the invention to provide a switching arrangement for the operation of a touch control of the above-defined type which is superior to the known circuits as regards switching reliability, that is to say as regards the degree to which it is sensitive to foreign influences and for which, furthermore, considerably smaller areas are sufficient, particularly for capacitive switches.

According to the invention this is accomplished in that the output of a pulse generator, preferably a square-wave pulse generator is connected on the one hand directly and on the other hand via a resistor to the inputs of a AND-gate and that the input with the preceding resistor is, in addition, connected to the touch control.

The switching arrangement according to the invention is suitable for a capacitive as wEll as for a resistive touch control.

Details of the invention, its embodiments and its manner of operation will now be further described with reference to the drawings.

Figure 1 shows schematically a circuit arrangement for a capacitive touch control.

Figure 2 shows schematically an embodiment of a resistive touch control.

Figure 3 shows schematically the signals at the different points of the switching arrangements shown in Figures 1 and 2.

Figure 4 shows the equivalent circuit diagram for a capacitive touch control.

Figure 5 shows the equivalent circuit diagram of a resistive touch control.

In Figure 1 a square-wave pulse generator IG1 applies a square-wave pulse voltage to point B, this voltage being further applied to the input el 1 of the AND-gate G. From point B this same voltage is also applied to the input e1 2 of the gate G via the resistor r1. In addition, the capacitance CT constituted by the touch control is also applied to the input e12 via a resistor r2. The output voltage of this gate G is applied from output A via a diode D to a control section S, which applies its output signal in point D to the subsequent switching stages. When the square-wave pulse voltage shown in Figure 3 is applied to point B, the pulse shown in Figure 3 - second line - is applied to the input e12 via the series resistor r1.The rising edge of the pulse results from the RC-network formed by the resistor r1 and the self-capacitance of the gate G.

It will be apparent from Figure 3 that the threshold voltage denoted by Ss for the gate G is reached, so that gate G switches and applies a corresponding signal voltage to E via D. If now the control is touched -- Figure 3 right-hand side one side of the capacitor CD is substantially connected to ground. Consequently, the capacitance CD becomes operative and a RCnetwork is formed via r1 which considerably flattens the edges of the signal applied to e12 so that the threshold voltage Ss is no longer reached.

Consequently, the gate G can no longer produce a signal.

A diode D1 is arranged in parallel with resistor r1 so that the touch capacitor CT can be discharged via this diode after each pulse.

Figure 2 shows a switching arrangement comprising a resistive touch control in which the capacitive switch CD has been replaced by an ohmic switch RT: The voltage pulses shown in Figure 3 for the input e22 are now produced. The edge steepness of the pulse is equal to the input at el 1. After the area RT has been touched, the edge steepness atel2 is indeed maintained but the pulse voltage decreases to below the threshold voltage Ss, so that the gate G no longer supplies a 'signal.

Figure 4 shows the equivalent circuit diagram for the switching arrangement shown in Figure 1.

The voltage increase at point e 12 for the pulses which are inputted in point B with a frequency over 20 kHz and a pulse voltage of, for example, 5V depends on the value of the resistor r1 , for example 5 Megohm, on the capacitance CT of the touch control, which is approximately 5 to 10 pF when the touch area has a size of 300 m2 and the insulating glass plate is 2 mm thick, and on the body capacitance CB of the operator of approximately 200 pF, on the capacitance of the gate CG of approximately 2 to 5 pF, on the gate resistance RG of more than 1012 ohm, and the capacitance CA of the overall control circuit relative to the gate of more than 200 pF.It is apparent from this equivalent circuit diagram that the edge steepness of the pulses can be reduced to such an extent that perfect switchinq criteria are obtained for the gate G by simply arranging the small capacitance CD in parallel.

It will be apparent from the equivalent circuit diagram of Figure 5 that when the series resistor r1 has a value of several Megohms, the voltage at e22 them substantially collapses when the resistor RB is switched on by the operator. As this resistor has a value of some kOhms the voltage for e22 is reduced to such an extent that the gate G again becomes a perfect switching criterion.

The switching arrangement shown in Figure 2 is of a simpler principle, but has the drawback that the operator does not come into contact with the control circuit under no-load conditions so that, at least for devices and arrangements which also carry higher voltages, a capacitive circuit arrangement should be preferred as these are absolutely safe for the operator. In addition, a capacitive touch control has the advantage that it can substantially not be affected by dirt and, in particular, moisture on the operating areas.

Claims (9)

1. A circuit arrangement for operating a touch control for electric controls using a-pulse generator, at least a metallic touch area fund an electric control member by rrieans of which the pulses which are influenced by the touch control are evaluated, characterized in that the output of the pulse generator is connected on the one hand directly and on the other hand via a resistor to the inputs of an AND-gate and that the input with the preceding resistor is further connected to the touch control.

2. An arrangement as claimed in Claim 1, characterized in that the series resistor has a high resistance value, preferably exceeding 3 Megohm.

3. An arrangement as claimed in Claim 1 having metallic areas which are insulated from one another and constitute a capacitance (capacitive touch control), characterized in that the series resistor forms a RC network in combination with the untouched touch control, the time constant of this network being smaller than the pulse length of the generator.

4. An arrangement as claimed in Claim 3, characterized in that a leakage resistance is arranged between the gate and the capacitive touch control and the resistor is shunted by a diode which is not conductive for the voltage of the pulse generator.

5. An arrangement as claimed in Claims 1 to 4, characterized in that a square-wave generator is used as the pulse generator.

6. An arrangement as claimed in Claims 1 and 2 having at least one metallic area which is connected by an operator to a second area or to ground (resistive touch control), characterized inthat the series resistor is made highly' resistive relative to the resistance constituted by the operator.

7. An arrangement as claimed in Claims 1 to 6, characterized in that a diode which forms an itegrator in combination with the gate capacitance of the gate "S" is connected between gate "G" and gate "S".

8. A circuit arrangement for operating a touch control substantially as herein described with reference to Figures 1, 3 and 4 of the accompanying drawings.

9. A circuit arrangement for operating a touch control substantially as herein described with reference to Figures 2, 3 and 5 of the accompanying drawings.