Darlington circuit (touch sensor)
In this project, you'll use a touch sensor having a Darlington circuit to learn Darlington connections. A Darlington circuit is simple in connections, yet it has excellent features.
The current amplification factor of the circuit equals the product of the current amplification factors of the component transistors. For example, suppose each transistor has a current amplification factor of 500, then, ooh, you get 2,500.
Here's another math: if an LED carries a current of 10 mA, the base current needed to light the LED is 10/2,500, or a mere 4 ľA.
In this project, you are going to light an LED by the current flowing through your body (a resistor). You'll see that with Darlington connections, the current from your body is enough to turn on the LED.
Also, Darlington connections are commonly used as a high input impedance (AC input resistance) circuit.
When you finish wiring, turn power ON. Grip the two wires from the power switch and Q1's base with both hands. The very weak current passing through your body should have turned on the LED. Yes, you didn't feel the current a bit, but you did feel the power of the Darlington connections.