Test a Darlington-pair touch switch.
A Darlington pair refers to a couple of bipolar transistors connected so that the amplified emmitter signal of one is used to drive the second. This results a multiplication of the gain, so the circuit is hyper-sensitive to any input signal.
An ideal applications is a touch switch - the classic demonstration case.
The load in this circuit is just an LED with a current-limiting resistor. Two NPN transistors form the Darlington-pair. The circuit works over a wide-range of voltages - basically constrained by the specific transistors used and the size of the current-limiting resistor (to avoid blowing the LED).
The touch switch control comprises two leads:
- to the base of the Darlington-pair
- to positive power
A 100kΩ resistor is included to protect against bad things happening if the touch switch is completely shorted.
It doesn’t take much to activate the switch:
- touch the base lead only will turn on the Darlington-pair to a degree
- a single finger pressing across the base and power lead will turn the switch hard on
- touch one lead with one hand, and the other lead with the other hand (using your body as the conductor) will also turn the switch hard on