I am trying to trigger an iPhone's camera with my own development board. I figured it out volume up/down signal is trigging the iPhone's camera, but I managed it at an Android device with touching an 220R or 600R resistor from mic pin to gnd pin. iPhone didn't even recognize my board as a headphone.
A 10K resistor from mic pin to gnd pin gives me the recognition as a headphone for iPhone but touching an 220R or 600R resistor from mic pin to gnd pin does nothing.
I noticed there is a 2.4V between mic and gnd pins and when volume up/down event happens this voltage drops to 2.2V.