Table lamp touch switch. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting
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Modern mechanical electric switches are reliable, convenient and cheap. But sometimes you need a switch that works without the application of mechanical force. If you use the proposed device, a light touch on the metal sensor plate is enough to turn the load on or off.
The touch switch circuit is shown in the figure.
The master oscillator on the elements DD1.1, DD1.3 generates pulses with a frequency of approximately 12 kHz and a swing close to the supply voltage (12 V). From output 4 of the DD1.3 element, the pulses through the capacitors C4 and C5 are fed to the input of the DD1.4 element, which operates in a linear mode and serves as an amplifier.
Further, the pulses differentiated by the C6R5R6 circuit are fed through the DD1.2 element to the rectifier (diode VD1). The constant voltage at the output of the rectifier corresponds to the amplitude of the pulses.
Sensor E1 - a metal plate connected to the connection point of capacitors C4 and C5. Touching the plate connects this point to the "ground" through the active-capacitive resistance of the human body. As a result, the amplitude of the pulses at the inputs of the element DD1.4 decreases sharply. Accordingly, the voltage at the rectifier output changes: from approximately 8 V (before touching the sensor) to almost zero (after touching). The low logic level at the output of the element DD2.1 is replaced by a high one. This difference through the capacitor C1 enters the input 13 of the element DD1.2 and maintains a low level at its output and the output of the rectifier, regardless of the presence or absence of pulses at the input 12.
The duration of this state is determined by the circuit time constant R1C1 and is approximately 0,5 s.
The signal from the output of the element DD2.1 is fed to the input J, which is also inverted using the element DD2.2, to the input K of the trigger DD3.1. The counting input of this trigger receives pulses from the master oscillator (from pin 3 of the DD1.1 element). This avoids "bounce" and ensures a clear single switching and reset of the DD3.1 trigger with each touch on the sensor.
Trigger DD3.2 operates in the counting mode, changing the state to the opposite each time a pulse arrives from the output of DD3.1. A key on the transistor VT3.2 is connected to the output of the trigger DD1. It controls the relay K1, the winding of which is included in the collector circuit of the transistor and is shunted by the VD2 diode, which dampens the voltage surge at the moment the relay is turned off.
The relay contacts K1 switch the load. In the author's version, it is a lamp of a table lamp woven from willow twigs. Relay RES60 was used, passport RS4.569.435-02. The current consumed by the switch in the 12 V circuit is 12 mA in standby mode and 26 mA when the relay is activated. The switch can be powered from any source providing the above voltage and current.
Relay K1 should be selected with contacts that can withstand the current and voltage of the switch load, and capacitors C4 and C5 - for a voltage of at least 400 V. The latter guarantees the electrical safety of the device when it is powered from a transformerless source.
Author: O.Cherevan, St. Petersburg
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