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Microphone / Radio 101

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I'm trying to get the audio out on a computer to go into the microphone in on a walkie talkie. I popped the microphone off the circuit board on the walkie talkie and soldered two wires in place of the microphone; which had two prongs going into the circuit board.

I then spliced an audio cable with a stereo jack on the end into the two wires and I plugged the stereo jack into the audio out on my desktop computer and it transmits whatever is playing just fine when I push the push to talk on the radio.

I then took the same wire job over to my tiny gumstix computer and when I plug into the audio out on the gumstix the darn radio transmits without pushing the push to talk (No, VOX mode is not set to on :)
What gives? There is something simple about microphone wiring I just don't get. I did notice on the gumstix computer I get a low voltage and maybe 80milliamps when I plug into the audio out.

I've also noticed some microphone wires (just using one from my desktop headset) have three wires in them and some have one shielded wire and one wire (ground I suppose) wrapped around it.

I *thought* a microphone was just a DC circuit.

Help? Anyone? Anyone?


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Usually the third wire is for power. The power is for the mic amplifier built into the mic. Most of the time there is just a ground wire and the mic wire. That mic wire carries the power for the amp and the mic audio signal on the same wire as well. It is possible that the single wire on the walkie has a 3rd function: PTT. When the radio senses a load on the mic line, it assumes you wants to talk (PTT) and keys the radio.
Try adding a capacitor (0.1 - 1uF) in series with the mic wire to see if the auto PTT goes away.
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The walkie talkie probably has an electret mic which requires a small bias current to operate the internal JugFET buffer amplifier.

It's normally just a resistor connected to the positive rail, 30mA sounds too high, it's more likely to be 3mA maximum.

Electret microphone - Wikipedia, the free encyclopedia

Just connect a resistor (2k2 to 10k, try 4k7 first) where the mic should go and AC couple it to the computer via a 4.7µF capacitor.
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