2-way radio headset microphone....?

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Im trying to make my own headset for a motorcycle helmet so that it will work with a motorola 2-way radio among other things.

Im a little confused about the microphone wiring. The motorola radio has a 2.5mm stereo jack with the speaker signal on the tip, microphone input on the ring, and ground on the sleeve. I bought a cheap hands free set for the radio that had a mono earbud and a mic with push to talk button, made by motorola. I took it apart and there is a board with a capacitor and resistor in a circuit with what appears to be an electret mic capsule.

How does this work with only a ground and one other connection to the radio? is it actually a dynamic mic if so what is the cap and resistor for? How can i tell?

The mic signal ring on the radio connector is outputting like 3.5VDC. I did hook up a dynamic mic element to the mic ring from the radio in series with a NO pushbotton to ground as a PTT, and when the PB was depressed it worked fine. I would like to use an electret mic since noise cancelling ones are cheap and readily available.

THANKS
 
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