Simple Touch Switch Circuit

Similar to the CMOS based Touch Switch also on this site, this transistor based touch switch can activate a load simply by the user touching a metal plate. It is designed to directly switch a relay to allow it to be used with large loads. As it uses only a few commonly available transistors and a 12V supply, it is ideal for hostile environments where mechanical switches would be damaged. Using a latching relay and two of these circuits, a simple two pad "touch on/touch off" arrangement can be made.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 10 Meg 1/4W Resistor R2 1 47K 1/4W Resistor R3 1 120k 1/4W Resistor R4 1 470 Ohm 1/4W Resistor C1 1 15uF Electrolytic Capacitor D1 1 1N4007 Silicon Rectifier Diode Q1 1 2N5458 N Channel Field Effect Transistor Q2 1 2N2222 NPN Transistor 2N3904 Q3 1 2N3906 PNP Transistor K1 1 Relay w/12V Coil, Contacts To Suit Application MISC 1 Board, Wire, Small Metal Pad For Touch Pad

Notes

1. The touch pad can be most easily made by cutting a small square of PCB material and then soldering on a single wire. Alternatively, something like a penny glued to a plastic backing will do the job.
2. As mentioned, a latching relay can be used so that a momentary touch activates the relay and it remains active. To turn off a latching relay, power must be interrupted. So a 2nd circuit with a normal relay can be used to cut power (use the NC contacts on the 2nd circuit). Placed side by side, two touch pads form an "on" and an "off" pad.