Touch switch

A touch switch 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 annunciator 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. A touch switch is shown in Fig. 1, and it can be activated by touching a small metal plate connected to pin 2 of the 555 timer chip. Once triggered, the load remains on until reset. A low logic level applied to pin 4 resets the circuit. The output is on pin 3, which is used to drive an LED.
Another variation of the touch switch is depicted in Fig. 2. This touch switch also uses the ubiquitous 555 chip. The circuit is configured as a monostable multivibrator. The load remains on for a time period determined by the Rl/Cl combination. 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 1C 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 110-Vac line.


Touch switch with manual reset parts list
Quantity Part Description
1 R1 1-kΩ, ¼-W resistor
2 C1, C2 0.5-uF, 25-V capacitor (disk)
1 D1 Red LED
1 U1 555 timer 1C
1 S1 Sense-plate copper circuit board

Touch switch parts list
Quantity Part Description
1 R1 100-kΩ, ¼-W resistor
1 C1 4.7-10-μF, 25-V electrolytic capacitor
1 C2 0.1-μF, 25-V capacitor
1 C4 0.05-μF, 25-V capacitor (disk)
1 C3 50-(iF, 25-V electrolytic capacitor
1 D1 1N4001 silicon diode
1 Q1 2N2222 pnp transistor
1 U1 555 1C timer
1 Ry-1 6-V SPST relay
1 S1 Sense-plate copper circuit board
Thomas Petruzzellis

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