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Homework Help: Understanding a Homemade Lie Detector

  1. Apr 15, 2012 #1
    Hey guys, I'm new to this forum, and I want to start off by asking about some help to understand the composition of this circuit.

    The circuit is made by Kip Kedersha, and is found here.

    This is the circuit:

    I'd like to know how the resistors and the capacitor effect the circuit, or what it does, and also how the transistors help to make the difference between dry and sweaty fingers the difference between sound and no sound. (The thing about this circuit is that what it really does is only to react depending on whether the person sweats, in which case it conducts electricity better.

    Any help is appreciated :)
  2. jcsd
  3. Apr 16, 2012 #2


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    Staff: Mentor

    Hi Norse! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    You have posted this to the homework sub-forum, which implies that the explanation of the circuit operation is a homework question. As you must be studying electronics, you will have to show you have made an attempt at the work before we can assist.

    To start you off, I can see a charging path through a sweaty finger: from the battery ⊕ through the E-B junction of Q1, through R2 and C1 and the speaker and returning to the battery ⊖.

    See how you go now.
    Last edited by a moderator: May 5, 2017
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