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I have what I believe to be an electret microphone on a 3.5mm jack. This kind: microphone

I also have a socket that provides access to the three lines: adapter

Finally, I have several microcontrollers and a heap of miscellaneous resistors, capacitors, etc. My intent is to have the microcontroller read sound from the microphone. HOWEVER, I'm having trouble finding information on how to do this. I keep finding explanations of how to wire a bare 2-pin electret microphone - which is not what I have. Or, it's not clear that's what I have - perhaps inside the mic housing there's a tiny transistor and/or resistor etc, or maybe it's just the electret piece. I've found like 5 images all showing how you should wire a bare electret, and none depicting how a consumer microphone IS wired.

How is this device to be used? What voltages and currents are provided to it, and what comes out representing signal? Is left used for power and right used for signal? Are they both wired together? Is one entirely unused? I assume I need to provide some amount of voltage to one or more lines? On which of the three lines do I listen for a signal, and how? Do I apply some combination of resistors and capacitors? Is it a sub-millivolt signal, or more like 500mV?

One of my assumptions is that at the jack, these microphones all operate the same, since they all get plugged into the same computer. It COULD be that the computer has some kind of autodetection mechanism which like, switches on different circuitry when you connect different mics, though.

Can anybody provide some guidance, here?

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  • \$\begingroup\$ you are going to have hum pick with exposed unbalanced pair so use coax or STP wire. \$\endgroup\$
    – D.A.S.
    Commented Feb 11, 2021 at 5:49
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 I mean, my sample rate is only like 8ksps; hum will hardly be the most prominent defect, hahaha, but thanks \$\endgroup\$
    – Erhannis
    Commented Feb 11, 2021 at 6:15
  • \$\begingroup\$ Telephones use to have 3kHz BW with >60 dB SNR and 8kHz sample rate. I guess this generation is too used to compression and fading noise with huge THD on mobile phones. But 50 Hz hum will be very annoying with a buzz from SMPS \$\endgroup\$
    – D.A.S.
    Commented Feb 11, 2021 at 6:20
  • \$\begingroup\$ Shrug; 8ksps was a very rough estimate. It's enough for me to understand speech fine, but the details are fuzzy. It'll probably do for my purposes, anyway. \$\endgroup\$
    – Erhannis
    Commented Feb 11, 2021 at 6:24

1 Answer 1

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Generally, electret mics with a 3.5mm TRS (tip-ring-sleeve) male connectors have a connection like the following:

  • Tip and Ring are shorted and connected to Mic+.
  • Sleeve is connected to Mic-.

So if you use the adapters shown, L and R will be Mic+ and GROUND will be Mic- (i.e. GND).

An electret microphone capsule has a FET inside, so there should be a load resistance through a supply to get the sound output:

enter image description here

Image Source: Radioworld.com

Example: PCs provide 3~5VDC through a ~2k resistor. And most 6mm electret mic capsules are happy with 3V through 2k.

Remember that these microphones have an output in milliVolts range, and you may not be able to read the sound output with an MCU. So you'll need an amplifier. There are tons of simple electret mic amplifier schematics on the Internet. The ones with opamps are the simplest. But it's possible to build an amplifier with BJTs/FETs as well.

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  • \$\begingroup\$ Just an addition for the OP: You can visit this post to see an example circuit, near the very top, that supports an electret microphone requiring an external supply. It's voltage gain is about what's needed to achieve "line level" output. So it may be helpful \$\endgroup\$
    – jonk
    Commented Feb 11, 2021 at 5:52
  • \$\begingroup\$ The resistor also controls the DC output and AC Gain which you can increase by using 5V or 12V with 5k to 8k oHms with the idea the DC should be near or above the mid supply DC. \$\endgroup\$
    – D.A.S.
    Commented Feb 11, 2021 at 6:01
  • \$\begingroup\$ So, the stuff inside the dashed box is part of my mic, and the stuff outside it is how I'd connect it? (Potentially plus an amp. Any estimates of how many millivolts the mic would output for, say, someone speaking directly into it?) \$\endgroup\$
    – Erhannis
    Commented Feb 11, 2021 at 6:19
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    \$\begingroup\$ @Erhannis exactly. The output depends on the sensitivity of the mic, supply voltage and external resistor. But you can expect less than 10mV for normal speech, and around 100mV for shouting/yelling. \$\endgroup\$
    – Rohat Kılıç
    Commented Feb 11, 2021 at 6:28
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    \$\begingroup\$ It works, thanks! I needed to attach the output to a "middle" voltage via a large resistor, so the signal stayed inside the ADC's range, and then scale up the signal in code by a factor of like 20, and the quality is garbage, but it's fine for at least some of the tests I was going to do. Thanks! \$\endgroup\$
    – Erhannis
    Commented Feb 11, 2021 at 7:42

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