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A Touch Sensitive Switch - Three Versions
LEFT - Wet finger - doesn't really work:
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Light an LED with current flowing through your finger!
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Your finger has a high resistance so the current will be tiny.
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The LED will not light or perhaps in a very dark room there might be the tiniest glimmer.
MIDDLE - BJT Switch:
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Use a bipolar transistor as a current amplifier or switch.
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The BC182L amplifies the Base current about 200 to 300 times.
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The LED lights, perhaps dimly. You might need to wet your finger.
RIGHT - Darlington Pair:
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Combine two transistors multiplying the current by up to 300 x 300 = 90 000.
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This works and it's posssibly too sensitive.
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Just touching the base wire without touching the resistor still lights the LED. Your body is acting like an antenna and picking up random signals from the environment.
Bipolar Transistor Switch - BC182L
Build this circuit. Use your fingers to bridge the gap as shown. The LED might light dimly. If you have wet or sweaty fingers, it might be brighter.
Darlington Switch - Two BC182L Transistors
This uses two transistors so the current multiplying effect is much greater
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