I would like to use a car microphone for bluetooth communication.

The mic is a BMW one: 84316938762.

The wiring diagram (TIS) marks the mic's two connectors for the pin 1 and 2: TIS (X18507).

However measuring with oscilloscope on these pins, there is pretty much nothing. (Measuring in AC mode, 5-10mV range).

How can I use this microphone? (If I need a pre-amp, please post some schematics, or a specific recommendation).

The BT module which I'm using is BC127, there is not too much info about the microphone's requirements.

  • \$\begingroup\$ Ask asked, this question is too road to be answered here. \$\endgroup\$ – DKNguyen Aug 6 '19 at 22:27
  • \$\begingroup\$ The link to the microphone is just a search link. You should link to an actual part datasheet. \$\endgroup\$ – JYelton Aug 6 '19 at 22:28

This microphone is most likely an Electret mic. This requires a DC bias for the FET amplifier inside the mic capsule.

Use a quiet (noise-free) DC power supply in the range of 3-5 Vdc with a 2200 Ohm resistor in series from the supply to the mic (+) line. The DC supply ground is connected to the mic ground.

Use a coupling capacitor to pass the mic signal to your preamp (or scope). The value of the capacitor depends on the input impedance of your preamp.

Use a scope to observe the signal. This will give you an idea of how much gain you need in your preamp.

A typical electret mic has an output around -50 dBu to -40 dBu when held close to your mouth and speaking at a nominal level. Obviously, level is much lower if your are attempting to pick up a voice that is several feet away.

  • \$\begingroup\$ Thank you! How is the 2.2K calculated? Is there some guidelines or formula? I don't have any datasheet for this microphone unfortunately. \$\endgroup\$ – Daniel Aug 7 '19 at 7:30
  • \$\begingroup\$ Bias resistor value is not critical. Pretty much any value from 1k to 10k will work, depending on the bias supply voltage. 2200 Ohms is a reasonable compromise with a 3V to 5V supply. \$\endgroup\$ – Dwayne Reid Aug 7 '19 at 9:14

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