Darlington circuit

The darlington circuit increased by an additional transistor, so that the gain achieved is now in total about 1,000,000 times. That is so high, that the AC current through the capacitor is sufcient to let the red LED light up. We have already used the capacitor in DC circuits. When the capacitor has completed charging, no current ows because of its innite resistance. What happens, when AC voltage is applied? The rapid changes of charging and discharging allows a current ow. This looks like a normal resistor, and the value of this resistiv part of a capacitor is depending on its capacity and the frequency of the AC voltage source. The higher the frequency and capacity value, the lower the resistance. The overall gain of all three transistors is large enough to amplify the current of a few nano-amps, so that the blue LED lights up, when the open side of the capacitor-brick is touched by a nger, and AC sources are around (powerlines for example). The darlington circuit can also be triggered by spontaneous electrostatics, then the LED light up only briey. Again there is no health hazard, because the voltages are very low.