Electronic Touch Switch

Have you ever wondered how those elevator switches work that only require you to touch a button?

This project demonstrates an electronic relay switching circuit that will do it. Just a touch of a terminal or wire will cause the Relay to operate.

After wiring the circuit, connect one of the leads to some electrical appliance which is not grounded (such as with a 3-prong plug).

Make the other lead available so that you can touch it without touching any other metallic object at the same time.

Now, as you touch this "touch plate" lead, the Relay will buzz, indicating it has received energizing current. Obviously, to operate anything with the Relay, more circuitry is needed. What we are interested in at this time is how the "touch sensitive" circuit operates.

Circuit operation is as follows:

Without connections to the "touch plate" lead, the Q2 Transistor has no complete circuit to provide base-bias, so this Transistor is in the OFF state. Recall that a Transistor which is OFF has what acts like an open circuit between C and E of the Transistor. This opens the base-bias current for the Q1 Transistor so that it is OFF, also.

The Relay therefore cannot receive current, and is de-energized. When you touch the "touch plate" lead, the static charge on your body is enough to provide the small required base-bias current to the Q2. The Q2 turns ON, as does the Q1 which energizes the Relay. The Relay buzzes because it is connected like a doorbell buzzer.