Audio Level Detector w/ electret mic and 2 bjt's

Discussion in 'The Projects Forum' started by gizmoman0, Oct 24, 2010.

  1. gizmoman0

    Thread Starter New Member

    Jan 21, 2010
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    So I made an audio level detector with an electret mic fed into an lm386 amplifier which fed into an lm339 comparator to act as switch when an audio threshold was met. This worked well but I'm convinced I can make the circuit much smaller now by utilizing just 2 bjt's but I'm not sure. This is my idea but I could use some input.

    The electret mic requires the DC source and outputs only a small voltage and not enough current to turn on a bjt so I was thinking that I could use the first bjt as an amplifier to supply enough current to drive a second bjt into saturation or switching function.

    Since I can only get like 5mV out of the electret mic I would have to bias the base of the first transistor by 1 volt or so DC + the output of the electret mic. I could then take this output signal and put a DC blocking cap to allow only the ac to pass to the next transistor and attenuate it as necessary to create a threshold reference which turns on the bjt when this loudness is reached.

    Now thinking about it this might make the first bjt unnecessary. Does this circuit sound like it would work? Input appreciated thanks.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A single transistor can have an AC voltage gain of as much as 180 but its output is extremely distorted and compressed. You don't want compression in a level detecting circuit so use an opamp as the amplifier.
    Then a second opamp can be a precision rectifier circuit.
     
  3. gizmoman0

    Thread Starter New Member

    Jan 21, 2010
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    Hm well I've already achieved the functionality I want with the initial circuit I was just hoping I could make it way simpler and smaller with just 2 bjt's. Is a rectifier necessary for level detection or does it just provide a cleaner level to compare with a reference level?
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    If you use an AC signal into a comparator then the output will be switching quickly on and off.
    If you compare DC voltages (from a rectified signal) then the comparator output is switched DC on or off.
    Your choice.
     
  5. gizmoman0

    Thread Starter New Member

    Jan 21, 2010
    26
    1
    yea I've noticed this. Would it be easier just to add a little bit of hysteresis with a feedback resistor? Also I've noticed the output of the comparator stays high for about a second usually but not sure why.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    You didn't post your schematic so we don't know what you are talking about.
     
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