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Simple Touch Switch Circuit

2015-01-10 10:19  
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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 for touch switch



Total Qty.
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 



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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