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Need help to understand sound level meter circuit

Discussion in 'Electronic Design' started by kash, Jul 11, 2003.

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  1. kash

    kash Guest

  2. Ted Wilson

    Ted Wilson Guest

    Yuk - what a horrible circuit!

    The answer to your question is that C1 is there to bypass R1 and couple the
    output of the microphone onto the base of T1.

    Basically, circuit operation is as follows:

    T1 and its associated components provide voltage gain to the signal from the
    microphone and T2, together with C4 effectively provide half-wave
    rectification of the amplified signal on T1 collector.

    The larger the output from the microphone, the more positive the voltage on
    T2 emitter will be and hence the more LEDs will be illuminated, starting
    with LD1, turned on by T3, and progressively lighting each LED to the right
    until all are lit.

    Each of transistors T4 thru T7 has a threshold voltage one diode drop above
    the one to its left.

    I assume from your question that you are a relative newcomer to electronics
    and, if so, I would not recommend this circuit as an indication of how to do

    It is very poorly defined in terms of its DC bias conditions and both its
    zero level and sensitivity will be prone to temperature drift. Furthermore,
    RV1 which is there presumably to set the zero threshold level changes the
    sensitivity at the same time - awful!

    Hope this helps

    Ted Wilson
  3. Ted Wilson

    Ted Wilson Guest

    A good candidate for one of the 'bad circuits' sections in the next edition
    of A of A I'd say.

  4. kash

    kash Guest

    I have been thinking about all of this too, and there is one thing I was thinking
    we should have considered: the impedance of C1 can be found. If we assume
    the frequency to be 1000Hz, the 1/jwc is the impedance, and substituting in for
    values we get (1/(2*pi*1000*1e-6)) = 159ohms. Shouldn't we be substituting
    this into our ac equivalent circuit?
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