Linking Android phone to motorcycle helmet comm set
Well, I figured it out. First, I discovered that the helmet microphone is an electret and I reverse-biased it. Oops. I discovered this by plotting a U/I curve of the microphone; I noticed the diode-like curve so I reversed polarity and also measured two other headsets.



(Horizontal is milliVolt, vertical is micro-Ampères)

Still, the helmet_rev curve is way higher and even with the correct polarity PTT didn't work because the voltage over the mic was still low (0.74 V). So I plugged in the second helmet set that I have, and that worked immediately.

Conclusion: the microphone in the helmet is broken. Not sure how since it still captures sound, but in any case it is out of spec for this application. Could I have damaged it by connecting it in reverse, even at such low voltages?