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.

LED powered by microphone circuit

  • \$\begingroup\$ What type of Microphone are you using? if it is electret type, then you need to pull it up not done as you have it with 1K resistor. Be sure to check the data sheet for the pull voltage and the resistor values. You should be able to use the 10V you already have with a proper resistor value. \$\endgroup\$
    – Suirnder
    Apr 11, 2013 at 13:47
  • \$\begingroup\$ Sorry, i messed up my diagram. I'll fix it. The microphone is an electret type and the pull up resistor is conencted to 10V. \$\endgroup\$ Apr 11, 2013 at 13:53
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    \$\begingroup\$ You should replace Lm741 with LM386,LM386 is an audio amplifier. \$\endgroup\$
    – Suirnder
    Apr 11, 2013 at 14:07
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    \$\begingroup\$ @Suirnder an LM386 will NOT work in this circuit. It's an audio power amplifier, not an op amp. The inputs are pulled down by 50k resistors internally, and the output automatically biases to 1/2 the supply voltage. \$\endgroup\$
    – Bitrex
    Apr 11, 2013 at 14:29
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    \$\begingroup\$ @DanJosephPorcioncula listen to the answers but listen to all of them Dan. I (and @Bitrex) have already pointed out that it's not simple to just plug-in the LM386 and expect to get what you want. If you read the spec sheet of the 386 it says it automatically biases its output at midrail and as your rails ar 10V and 0V midrail means 5V. \$\endgroup\$
    – Andy aka
    Apr 11, 2013 at 16:24

1 Answer 1


You have the 10k feedback resistor on the op-amp connected to ground instead of to -Vin

You are also going to have problems using a 741 op-amp - it won't cope with inputs close to its negative supply. It basically won't work - you'll need a mid-rail point and reference input resistors to the mid-rail.

Another problem is the 741's output voltage - according to the spec it won't properly swing-down to 0V (the most negative supply rail). You might just get about 1V from it but nothing lower. This will keep one of the LEDs on.

Assuming you create a mid-rail suitable for the 741 you'll have other problems driving the LM324 circuits. Ideally you should convert the peaks produced from the amplifier into a more consistent dc voltage that tracks the peaks. There are circuits that can do this but you'll need to solve the above first.

You haven't got de-coupler capacitors on the op-amp supplies - good practice (to avoid circuit oscillations and other problems) is to use at least 10nF.

It's got nothing to do with matching your microphone so I'd also suggest you get a new instructor who knows something about op-amps.

  • \$\begingroup\$ It works as a feedback resistor though I forgot to fix the diagram. \$\endgroup\$ Apr 11, 2013 at 13:52
  • \$\begingroup\$ Hm... Would changing the op-amp from a 741 to a LM386 be suitable? And the mid-rail, is it 5V? So I need to change the values that define the reference voltages of the Comparator. And for converting the peaks, I could use a peak detector (diode in series with a parallel RC) right? \$\endgroup\$ Apr 11, 2013 at 15:48
  • \$\begingroup\$ @DanJosephPorcioncula - don't use the LM386, use an op-amp that has rail-to-rail inputs and outputs or, at the very least, an op-amp that can have inputs down to 0V and an output that can swing down to 0V. It still means your input op-amp will be doing crazy things - it'll be acting as a rectifer with gain but it might just work... then you'll need a diode and RC to smooth the transients in the audio and give the leds a chance to glow. \$\endgroup\$
    – Andy aka
    Apr 11, 2013 at 16:28
  • \$\begingroup\$ I'll try to find a suitable op-amp then. Thanks. \$\endgroup\$ Apr 11, 2013 at 16:32
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    \$\begingroup\$ Try an AD8031 - available surface mount or DIP and not too expensive. If you're happy with the help please consider "accepting" the answer - apparently you press a symbol near the up-down arrows near the top of my answer!!! Let us know how you get on too. I'll also point you to this electronics.stackexchange.com/questions/61995/… - it's a very similar job to yours but done slightly differently \$\endgroup\$
    – Andy aka
    Apr 11, 2013 at 17:50

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