GB1529862A - Proximity switch - Google Patents

Abstract

1529862 Proximity switches THORN DOMESTIC APPLIANCES (ELECTRICAL) Ltd 19 Jan 1976 [17 Jan 1975] 2150/75 Heading G1N [ Also in Division H2] A touch or proximity switch includes an insulating plate 1 on one face of which are spaced electrodes 2, 4 such that when a finger touches or comes into proximity with the other face of the plate a signal capacitively coupled between the electrodes is changed. A 50KHZ signal from oscillator 18 is applied to square electrode 2a, 2b surrounding an earthed screen electrode and the second electrode 4, the A.C. signal being capacitively coupled to electrode 4 connected to circuit 5 which provides a positive D.C. voltage proportional to the alternating voltage at electrode 4. A circuit 11 is also provided to convert the alternating voltage supplied from oscillator 18 to a negative D.C. voltage. The two D.C. voltages are summed by resistors 16, 17 and potentiometer 15 is adjusted so that in the absence of an operation finger at the other face of the plate the voltage at the junction of 16, 17 is zero. When the operator touches the other face the capacitive coupling between the electrodes is reduced and the voltage at electrode 4 is also reduced resulting in the output becoming negative. By reversing the polarity of the diodes a positive output can be obtained when the plate is touched. In an alternative arrangement a further electrode may be provided on the other face of the plate, Fig. 4 (not shown). A plurality of detectors may be used with a single oscillator and circuit 11. Three proximity switches 19, 20, 21 may be used to operate a power controller for an electric cooker, Fig. 3, the switches being connected to inputs 23, 24, 25 of counter 22, 23 causing the counter to count upwards, 24 causing it to count downwards and 25 resetting it. The counter output is applied to a circuit 28 which gates triac 29, controlling heating element 31, on at zero cross-over points of the mains waveform and may be a D/A converter the D.C. output of which is compared with a ramp voltage to give an output pulse of length proportional to the count stored and which controls the number of cycles for which the triac is gated on. Alternatively circuit 28 may be digital in which the count is compared with the output of a further counter to produce output pulses whose length corresponds to the count stored by counter 22. Lines 33 are also connected to a display 32 which indicates the count stored by counter 22. In operation if the count displayed corresponds to too low a heating the switch 19 is touched and then removed when the displayed count is the required value. Similarly if the count displayed is too high switch 20 is touched.