Simple Touch Switch Circuit

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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

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.

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.

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