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Mystery circuit needs solving!

  1. Feb 12, 2012 #1
    I've built a Darlington pair from 2N2222 NPN transistors on my breadboard, and I find that just my touching of the base lead in the circuit cascades both transistors and lets current flow in the circuit. What's causing this? See the video below for details.

    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Feb 12, 2012 #2
    Your video don't work. But my guess is since you put them in darlington, β of 2n2222 is about 100, two of them darlington give 100000. So if your body is charged up to so voltage and touch the base, you provide enough current to make the pair conduct enough to light up the LED. Just a guess.

    Think of if you put a cap from the base to ground, just a small 0.1uF. If you apply a small voltage enough to light up the LED, then you remove the voltage. The voltage from the cap will hold the LED on for a while also. Just static charge.
    Last edited by a moderator: Sep 25, 2014
  4. Feb 12, 2012 #3


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    Science Advisor

    Your body picks up noise which is mainly 60 Hz voltage and your hand applies this voltage to the base of the transistor.

    This turns the LED on and off rapidly and you see light from when it is on.

    It is similar to the effect you get if you touch the microphone input of an amplifier. In that case, you get a loud burst of a buzzing noise.

    This comes from living in a city where mains noise is everywhere.
  5. Feb 12, 2012 #4
    Is it a result of the DC transformer being plugged into the wall then?
  6. Feb 12, 2012 #5


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    Staff: Mentor

    It could be. Any alternating magnetic field, and the nearer it is, the worse, usually.
  7. Feb 12, 2012 #6
    That seems to be the case. If I completely isolate the circuit and use charged capacitors as the power source, I can no longer light the bulb by touching only the base, I can by touching the base and positive capacitor lead, though.
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