A touch switch is a type of switch that only has to be touched by an object to operate. It is used in many lamps and wall switches that have a metal exterior as well as on public computer terminals. This touch sensitive switch is built around NAND gate IC CD4011 and transistor BC547. When someone touches plate 1 (which is connected between pin 1 of gate N1 and ground), the RS flip flop comprising gates N1 and N2 is set. The resulting high output at pin 3 of gate N1 energises the relay via relay driver transistor. The relay, in turn, switches on the load operating on mains.
Touch sensitive switch circuit
On the other hand, when someone touches plate 2 (which is connected between pin 6 of gate N2 and ground), the RS flip-flop is reset. The resulting low output at pin 3 of gate N1 de-energises the relay via relay driver transistor. The relay, in turn, switches off the load operating on mains. The diode across the relay coil protects the transistor from back e.m.f induced in the relay when it de-energises.
The circuit works off a 12V power supply. If you want to control heavier loads, the current rating of the relay should be increased accordingly.
The article was first published in October 2003 and has recently been updated.