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

Schematic for touch switch

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.

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