Homemade Lie Detector Help

Discussion in 'The Projects Forum' started by NorseMan, Apr 15, 2012.

  1. NorseMan

    Thread Starter New Member

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

    The circuit is really the same as Kip Kedersha made here.

    The circuit looks like this:
    [​IMG]

    I'd like it if someone would tell me what the different component does to make the circuit react to sweaty fingers (Because all in all what the circuit really does is only to react depending on whether the person who is tested sweats or not...)

    (Notes: Both the voltage sources are 1,5 V. One wire is connected to a finger, and the other to another finger on the same hand)

    Any help is appreciated :)
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
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    The video is absurd. Who's skin goes from dry to sweaty and back to dry in less than a second?
     
  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,166
    A dirty low-down prevaricators skin does. ;)
     
  4. #12

    Expert

    Nov 30, 2010
    16,266
    6,777
    Let pnp = the left transistor.
    Let npn = the right transistor.
    Let the bottom line = "common".

    When fingers conduct, current flows through the pnp from emitter to base, through the fingers, through the 4k resistor, to the base of npn, out the emitter of npn to common.

    When current flows from emitter to base of pnp, much more current flows from emitter to collector of pnp. This increased current builds up some voltage on the speaker and that voltage is conducted to the base of the npn through the capacitor and the 4k resistor, accentuating the finger conductance which is turning it on.

    That current through the base to emitter of the npn causes more current to flow from emitter to base of the pnp which causes more current to flow to the speaker.

    This is a sort of avalanch of transistors helping each other turn on, and it can't last because the capacitor loads up and stops the avalanch. Then the capacitor discharges through the speaker. Then it all repeats. That is why the sound is a series of ticks.

    When the fingers are more conductive, the frequency should increase...I think. This is contrary to the video which shows a lower frequency after every answer.

    or perhaps I am wrong about the direction of frequency change.

    Anyway, it's an oscillator. The frequency changes according to conductance in the fingers. As noted already, it is not going to be as clean a response as the video shows.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    It's also a lazy design. There is no current limiter (R) between the base of the PNP and the collector of the NPN.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The circuit will not work if the person has dry skin.
    Some people NEVER sweat.
    Other people sweat when they tell the truth.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    A congenital liar will never sweat. I see the same guy lying on the news damn near every day. He'd be a challenge for Judge Judy, who's into the eye contact thing. ;)
     
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