Two transistors connected in direct engagement, so that the first emitter is connected to the base of the second collectors interconnected and form a Darlington pair or step.
The signal is applied to the base of the first transistor and removed from the second or emitter collector in common.
In our circuit, we use two BC548 transistors connected as a Darlington so that a very small current at the base of the former is sufficient to cause a much larger current in the second collector.
Thus, a very small current through the base of the first coming touch our fingers is sufficient to light the LED.
The current through the touch of our fingers is the order of millionths of amp which is insufficient to cause us any sense of shock. We do not feel anything.
This circuit is much more sensitive than the project of touch switch with a transistor, because the gain is much higher.
In the figure we have the full circuit of the touch switch with transistors in Darlington connection.
Simulation:
To download the simulation files and Netlist - click here (msb0060.zip)
