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  • Sun. Nov 28th, 2021

How does iPhone headphone remote circuit work?

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ByRemotepro

Jan 26, 2020

I was unable to detect any resistance value by testing the pins and pressing the + – button as I was able to do with the center button. When I set my multimeter to the diode setting, I got back a voltage reading. I have no idea what this means as I have never used this feature of my multimeter before but when I pressed either + – button I got back a consistent voltage result.

Interesting. I don’t have the kit here to verify what I’m thinking, but it sounds quite simple… Option 1:

  • Center Button – simple ballast resistance
  • + Button – single Forward Biased Diode
  • – Button – Forward Biased Diode in combination w/ ballast resistance

This is a simple design, which would require only three buttons, one passive and one semiconductor – all extremely cheap.

To detect the difference, the phone would look regularly (25 times / second or so) at the resistance and voltage drop between pins 3 and 4. If the center button is pressed, resistance will increase, but there will be negligable voltage drop. If the + button is pressed, there will be a negligable change in resistance, but the applied voltage (from the handset) will drop by a detectable amount. Finally, if the – button is pressed, both resistance and voltage drop would change.

Option 2 would be to use a slightly different combination:

  • Center Button – simple ballast resistance
  • + Button – Reverse Biased Zener Diode breakdown at say 0.15V
  • – Button – Reverse Biased Zener Diode breakdown at say 0.35V

If the center button were presed, the handset would detect a signal at the 0.05V step, through the ballast resistor. If the + button were pressed, the signal would be detected at the 0.15V step through the first Zener as it reached its breakdown voltage. Finally if the – button were pressed, the signal would be detected at the 0.35V step through the second Zener. No button pressed leaves the circuit ‘high impedence’ (not quite open circuit, as the microphone is still connected).

Both of these scenarios would require that the system stop ‘polling’ for button presses when the microphone were in use, such as when using voice control, or when on a call – ar at the very least, significantly reduce the polling period. Any polling while the microphone was in use would cause distortion to the received audio.

Of course, there are other possibilities, and Apple’s engineers are smarter than me – but these are my thoughts based on your description so far.

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