A touch sensor is a useful circuit that can be used to detect humans or protect small objects, such as antiques. It can be used to turn on a lamp or as an indicator to sound a buzzer when someone comes near a door or table. The touch switch, or capacity switch, can also be used to start a moving display sign.

This touch switch uses the ubiquitous 555 chip. The circuit is configured as a monostable multivibrator. The load remains on for a time period determined by the R1/C1 combination.

Time Period = 1.1 x R1 x c1

After the time period elapses, the circuit turns off until triggered again. The sense plate is connected to a capacitor placed in series with pin 2 of the IC timer to increase the charge accumulation.

The touch switch relies on the "stray capacitance effect" of a human body from the sense plate to a lower potential, i.e., ground. By completing a path to ground through the human body, the switch magically appears to turn on a light or external load. Always power your touch switch either with batteries or with a power supply that uses a transformer to ensure you are not in the direct path to a AC line.

Parts list

NOs   Part        Description

1       R1            100-k,  resistor
1       C1            4.7-10MFD, 25-V  electrolytic capacitor
1       C2            0.1MFD, 25-V capacitor
1       C4            0.05MFD, 25-V capacitor (disk)
1       C3            50MFD, 25-V  electrolytic capacitor
1       DI            1N4001  silicon diode
1       Q1           2N2222  pnp transistor
1       UI            555 IC timer
1       Ry-1        6-V SPST relay
1       S1           Sense-plate copper circuit board.

   Schematic