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Thread: Problem with microphone powering up a comparator circuit

  1. #1
    Junior Member
    Join Date
    Aug 2016

    Problem with microphone powering up a comparator circuit

    The circuit below is supposed to turn on a LED when the microphone input reaches these levels:

    1V, 3V, 5V, 7V.

    What this circuit does in real life is when we turn on the power, all 4 LEDs light up.

    When we remove the mic amplifier, it doesn't power up the LEDs since it only produces mV.

    When we changed the amplifier gain, it lights up 2-3 LEDs but speaking to the microphone doesn't turn on the remainng LEDs.

    Our instructor suggested that the impedance of the microphone and our circuit does not match. Is that the reason or can any other fix be done?

    The OP-Amps inside the indigo box is part of a LM324 IC while the outside Op-Amp is a 741 IC.
    Any suggestions will be much appreciated!
    Click image for larger version. 

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  2. #2
    There is ,at least, one error in the diagram: the 10K resistor should go to the "-" op amp input instead of ground. As wired there is no feedback in the op amp. The offset voltage is multiplied by the open gain and appears at the output, probably near the positive rail.

  3. #3
    Senior Member
    Join Date
    Nov 2015
    Cold hollow VT
    Have you looked at an LM3914 or LM3915 bar graph driver chip from TI (National semi) instead of the LM324? It will save you alot of discrete parts and perhaps give a bit more range to what you are trying to accomplish.

  4. #4
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    I believe the 741 opamp does not work with signals close to its negative power rail, which is GND in this design. I'm pretty sure you either need a negative power supply, or a different design that biases all the signals within the allowed common mode input range of all the opamps.

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