Make a Simple Touch Sensor

To make a simple touch sensor, you only need 3 components: a transistor, a resistor and an LED. Connect the resistor and LED in series from the positive supply to the collector of the transistor. Choose a resistor that sets the current of your LED to around 20 mA. Connect the emitter of the transistor to the negative terminal of the supply. Create two wires with exposed ends – so you can see the metal. 

One goes to the positive supply. The other goes to the base of the transistor. Now, try to touch these two wires with your finger – and the LED lights up! The reason is that your finger acts as a resistor and will give enough current to the base of the transistor to turn it on.