Circuit + Transistor Help Please

Discussion in 'General Electronics Chat' started by asdff1818, Dec 7, 2009.

  1. asdff1818

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    Dec 7, 2009
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  2. beenthere

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  3. asdff1818

    Thread Starter Member

    Dec 7, 2009
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    Thank you for your response! I kind of understand about the transistors, but how does the circuit work?

    Thank You!!!!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    It's basically just an oscillator circuit.

    See the attached.

    "Rbody" will change (decrease) when a person perspires (sweats). Usually, if someone's lying, they'll start to perspire, so the resistance of the skin will go down.

    This causes the oscillator to increase in output frequency.
     
  5. asdff1818

    Thread Starter Member

    Dec 7, 2009
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    Thank you! But I still don't understand how the circuit works. Please Help.


    Thank You!!!!
     
  6. asdff1818

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    Dec 7, 2009
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    And what is the difference between that circuit and just a ohm meter directly?

    Thank You!!!!
     
  7. SgtWookie

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    R1 and Rbody conduct a very small amount of current which charges the lower side of C1 to be more positive, until the base of Q1 rises above about 0.63v via R2. When that happens, Q1 turns on, and pulls the base of Q2 low, which charges the high side of C1 causing Q1 to momentarily turn fully on. At the same time, the voltage across SPK1 rises sharply.

    As C1 is discharged via R2 through the base of Q1, Q1's base current drops off, turning Q1 off. This turns off Q2, and the voltage on the speaker is removed. C1 then starts charging up again via R1 and Rbody, and the cycle repeats.
     
  8. SgtWookie

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    The circuit produces a sound that varies in frequency depending upon the resistance. It does not display the resistance.

    An Ohmmeter simply displays the resistance in Ohms, either with a needle or a digital display, depending upon the type of meter.
     
  9. asdff1818

    Thread Starter Member

    Dec 7, 2009
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    But if I wanted to measure it quantitatively, what would be the difference between that circuit, but instead of using the speaker, using a resistor + a ohm meter and just using an ohm meter directly?
     
  10. SgtWookie

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    An Ohmmeter would likely give a more accurate indication, as it is calibrated.

    You could use a frequency counter on the output of the oscillator to get relative indications, but not absolutes. The circuit is not calibrated.
     
  11. asdff1818

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    Dec 7, 2009
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    So is there a way to get even more accurate results than the ohm meter? How accurate would it be?

    Thank You!!!!
     
  12. SgtWookie

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    I don't know. I am not a medical person.
     
  13. asdff1818

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    Dec 7, 2009
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    Since I am doing this as part of my science fair, how can I make this more complex? I am going to talk about the nervous system which is complex but how can I make the circuit more complex (and useful).

    Thank You!!!!
     
  14. t06afre

    AAC Fanatic!

    May 11, 2009
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    You need a some sort of data logging system that can measure skin conductance as a function of time. In your other thread about this topic you said things was to complicated. But now you want a more advanced circuit?
     
  15. asdff1818

    Thread Starter Member

    Dec 7, 2009
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    More complicated than only the ohm meter, and less complicated than the advanced circuit.
     
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