Touch Switch Using a SCR (ART314E)

Even the low current passing across the fingers of a human is enough to trigger sensitive SCRs such as the TIC106 (see Chapter 10 for more details).

The circuit illustrated by Figure 1 can be used to trigger loads up to 3 A by a single touch on two metal plates.




To turn off the circuit, S2 is pressed momentarily. The current in the sensor is less than 1 mA and is erd by Rl. The sensor is formed by two small, metal plates (1 X 2 cm), and the wire used to connect it to the circuit must be short to avoid hum. A 1 M potentiometer between the gate and the ground can be added as a sensitivity control.

This conhguration can be used with other resistive sensors, but the specific configurations are given in other articles in this site, where sensor blocks are described.

Warning: Don’t power this circuit from the ac power line using transformerless power supplies. Touching the sensor can be a severe shock hazard.