Simple Touch Switch Circuit

Description
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. 
Circuit Diagram

Parts


Part

Total Qty.

Description

Substitutions
R1110 Meg 1/4W Resistor
R2147K 1/4W Resistor
R31120k 1/4W Resistor
R41470 Ohm 1/4W Resistor
C1115uF Electrolytic Capacitor
D111N4007 Silicon Rectifier Diode
Q112N5458 N Channel Field Effect Transistor
Q212N2222 NPN Transistor2N3904
Q312N3906 PNP Transistor
K11Relay w/12V Coil, Contacts To Suit Application
MISC1Board, 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.
Source - http://www.aaroncake.net/circuits/Simple_Touch_Switch_Circuit.asp
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