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I made a circuit that looks something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

If I touch the loose wire with my finger, the LED will light up. Could someone explain what is going on? Is there a specific name for this effect?

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  • \$\begingroup\$ Presumably you are near AC mains wiring. Mains "hum" is readily picked up by your body - there is capacitance bewteen you and high voltage (110 or 230 VAC typically) mains wiring. Also from your body to ground. The two aqct a a divider and cause your body to attain a voltage somewhere between mains voltage and ground but at VERY high impedance. Connecting this voltage to Q1 base causes it to be partially turned on and pass current to Q2 base. so Q2 is partially on so LED conducts. \$\endgroup\$
    – Russell McMahon
    Commented Dec 28, 2015 at 12:29

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I don't see a bias network on the base of the driver 3904, so your finger, and the resistance of your body, is giving the base of the 1st 3904 the .7 volts it needs to saturate and turn on the junction, allowing current to flow through the collector-emitter junction and driving the 2nd 3904 on, doing the same, passing current through the diode and lighting it up.

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  • \$\begingroup\$ As Russel pointed out, the source of the bias voltage is probably the stray 60 Hz from the house wiring that is being picked up by your body and filtered enough to provide the base of Q1 voltage to turn on. \$\endgroup\$ Commented Dec 29, 2015 at 21:41

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