Here is a project of a simple touch switch circuit. This circuit and can also be used to activate a relay in the place of LED and resistor. The circuit is quite sensitive because we have used the two transistors as a Darlington pair. The gain is first amplified by the first transistor and then further
amplified and multiplied by the second and so as a result we get a very high current gain. Due to high current gain the circuit can detect the very small current passing through our skin and activate the LED.
The circuit can be operated with 6, 9 and 12 volts but make sure to use a proper LED current limiting resistor in the place of Rx. Use 330 ohms resistor for 6 volts, 390 ohms for 9 volts and 470 ohms for 12 volts
Working of the circuit is simple; both the transistors are working as a switch. When you touch the touching points of the circuit a very small current starts passing through our skin which completes the circuit due to which the Darlington pair becomes switched on and light up the LED.