Transistor Touch Switch Simple Science Project

How to make a simple one transistor touch switch

How to turn on an LED using nothing more that the tiny conductivity of you finger placed across two wires.

This project illustrates how you can use a single transistor as a solid state switch. No moving parts. Except for your finger of course.

If you were to wire up an LED to a battery and complete the circuit with your finger then you are going to be unlucky if you thing that the LED will start to glow.

It is true that modern LEDs require very low currents to flow in order to see light output but you do have to give them some current however and your finger isn't going to do that. Not today at any rate.

Circuit diagram for the simple touch switch

If you add a transistor to the circuit you can use it's gain or β to amplify the current flowing through your finger to illuminate the LED.

I used an LED that starts to glow faintly when as little as 10µA is passed through it. My general purpose NPN transistor has a typical gain of 100 which means that we only have to get a tiny 100nA to flow into the base of the transistor to start the LED illuminating when connected to the collector of the transistor.

Of course the LED won't be very bright at it's minimum but it is possible to make much more current flow just by pressing a finger between two contacts.

Lets go get the components and try it out.