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This is the schematic. I've tried to build it and it doesn't work. Can someone please tell me where I've got it wrong?

The resistor value I used is 470 ohms and the Vsource is 9 volts.

The electret microphone is placed at the open terminals.

It is supposed to make the LED turn on and off to the beat of the sound input coming into the electret microphone. Instead, the LED remains turned on.

enter image description here

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  • \$\begingroup\$ What's it supposed to do? Where did you get the circuit? Is it your own idea? \$\endgroup\$
    – JRE
    Commented Jul 10, 2022 at 8:27
  • \$\begingroup\$ @JRE it's supposed to make the LED turn on and off to the beat of the sound input coming into the electret microphone \$\endgroup\$
    – user312602
    Commented Jul 10, 2022 at 8:29
  • \$\begingroup\$ @tobalt already edited it, thanks for the suggestion. Instead of doing what I expect it to do, the led just remains turned on implying that there's no signal coming from the electret mic. \$\endgroup\$
    – user312602
    Commented Jul 10, 2022 at 8:39
  • \$\begingroup\$ Based on what knowledge or data, why do you think it should work? Why you used the parts you did? Why is the resistor 470 ohms or voltage 9V, for example. I think the LED should get damaged. \$\endgroup\$
    – Justme
    Commented Jul 10, 2022 at 8:39
  • \$\begingroup\$ @Justme I honestly don't know, I got the schem somwhere on the net. Thats why I was hoping someone could point out to me here how electret mics are supposed to work. \$\endgroup\$
    – user312602
    Commented Jul 10, 2022 at 8:40

2 Answers 2

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An electret microphone normally develops about a volt or so across its terminals when biased with an excitation resistor. Your excitation resistor is a little bit low in value (470 Ω) and, this may produce possibly a couple of volts DC across the electret microphone should it not have been connected to your transistor. And here's the problem.

The base emitter region of a BJT normally will be taking current when anything as much as 0.5 volts to 0.8 volts is present. In your situation it will be conducting DC current and this will swamp any small AC voltage that might have been developed due to sound. In effect this means that your BJT is permanently activated and lighting your LED (no matter how much sound might be being produced).

In other words, your circuit design is flawed and you need to rethink it.

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  • \$\begingroup\$ Finally an answer, thank you. What value of resistor do you recommend that I should use? \$\endgroup\$
    – user312602
    Commented Jul 10, 2022 at 8:46
  • \$\begingroup\$ It's not as simple as that; the BJT needs an emitter resistor (possible around 100 ohms) and you need to bias the transistor separately with two resistors (potential divider) to just start to set current flowing through the collector. Enough current so that the LED is barely lit. Then you need to connect the base to the elecret mic via a capacitor so that it's working DC voltage does not affect the base's DC working voltage. Then you should start to see the LED get brighter when higher volume sounds are present. However, making it work with the beat of a sound is much more tricker. \$\endgroup\$
    – Andy aka
    Commented Jul 10, 2022 at 8:50
  • \$\begingroup\$ Of course you might get lucky with raising the 470 ohm resistor by 2 or 3 orders of magnitude but that would be a flaky design and highly susceptible to temperature variations and the electret microphone actually used. \$\endgroup\$
    – Andy aka
    Commented Jul 10, 2022 at 8:54
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Electret mics need typically external operating voltage because they contain a fet to be used as the first amplifying stage. In addition they need a resistor and capacitor to complete the fet preamp.

enter image description here (a Wikimedia Commons image)

These extra parts are the common ones, but other ways to connect it are possible and used. V+ is typically 1 to 5 volts, the resistor is typically 2 to 10 kilo-ohms and the capacitor is typically 1 to 10 uF.

But this gives only few millivolts of AC voltage when one speaks to the mic say at 3 inches distance. You need much more complex circuit than one transistor to make a LED blink from that voltage. An on operational amplifier based amplifier circuit is the easiest one for good results. Another possibility is say 2...3 stage transistor amplifier.

To design them you should start from the needed current through the led and the available few millivolt AC voltage at the output.

Copy and build circuits from the internet only when you can verify they work. The verification should be based on understanding the circuit or on seeing it works. Seeing only a video "it works" is often worth nothing, because there's so much useless crap published.

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  • \$\begingroup\$ So a fet produces AC voltage but outputs a DC voltage because of the fet? Did I get this understanding right? \$\endgroup\$
    – user312602
    Commented Jul 11, 2022 at 0:19
  • \$\begingroup\$ The fet produces changing DC current, the current is about 0,1...0,5 mA , but swings few microamperes up and down due the sound pressure variations. The DC voltage over the resistor varies along current variations. The capacitor lets only the AC component (=those said voltage variations) of the voltage to reach the output. \$\endgroup\$
    – user136077
    Commented Jul 11, 2022 at 0:48

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