Darlington Transistor Touch Switch Simple Science Project

How to build a Sensitive Darlington Touch Switch

That earlier project shows you how you can use the gain of a single transistor to switch on an LED simply by touching your finger across a pair of wires.

The circuit works pretty well but the transistor that I used only has a current gain or Beta of 100 which is typical of general purpose bipolar junction transistors. This means that the circuit isn't as sensitive as I would like it to be. To make the LED glow brightly you may need to wet your finger or press harder.

I thought that I would try building the same touch switch again but this time I used two transistors in a Darlington Pair configuration to get better gain from the circuit.

In a Darlington Pair circuit the first transistor feeds the second and effectively multiplies the two Betas or gains together. Instead of a gain of 100 it should now have a gain of 10000. That sounds much better.

If you have previously built the single transistor version then you should see a big improvement with this one.