24
\$\begingroup\$

I mean the simple analog headset pluggable into the jack of a phone. Not USB, not bluetooth, not fancy proprietary plugs with extra connectors - just a generic stereo+mic jack.

enter image description here

The four "bands" on the jack plug are GND, right earphone, left earphone and microphone. And there's nothing to cover the buttons - usually "Volume up/down" + "Media key" for receiving the call.

How do these buttons communicate being pressed to the phone?

\$\endgroup\$
  • 3
    \$\begingroup\$ Short answer: The microphone input isn't just used for the microphone. \$\endgroup\$ – Connor Wolf Jun 25 '16 at 20:52
35
\$\begingroup\$

Each switch bridges the high-impedance microphone with a low resistance, allowing internal circuitry to sense the buttons. Here's a helpful image:

enter image description here

The MIC+ line has a bias voltage (to supply the mic), and by adding some additional circuitry to the mic preamp, it's easy to differentiate those resistor values.

This is the most common scheme for "on-headphone" controls. Additionally, it's very easy to implement in the headphones, allowing for cheap headphones and requires only a little bit more circuitry in the phone.

\$\endgroup\$
  • \$\begingroup\$ Thanks. Google was particularly unhelpful, totally flooded with the software side, how to read the button, what event is generated etc. \$\endgroup\$ – SF. Jun 25 '16 at 21:04
  • 1
    \$\begingroup\$ @SF: Only if you don't know the correct keywords. "headset button specification" gets you source.android.com/devices/accessories/headset/… \$\endgroup\$ – Matti Virkkunen Jun 25 '16 at 23:00
  • 1
    \$\begingroup\$ Oh, and in that case the schematics might be slightly misguiding - at least as far as Android headsets go. The values are: 0 ohm for the "Play/Pause/Hook" "media button", 240 ohm Vol+, 470 ohm Vol-, 135 ohm optional "Voice Assist". All with 1% tolerance. Mic >1000ohm. \$\endgroup\$ – SF. Jun 26 '16 at 0:48
  • 2
    \$\begingroup\$ Also, makes me wonder: if I plug a standard (non-phone) earphones, with no microphone or any controls - these would appear as the "Play" button constantly pressed? (the MIC connector touching the same GND area of the jack as GND). \$\endgroup\$ – SF. Jun 26 '16 at 0:54
  • 4
    \$\begingroup\$ Minor issue with your diagram. It looks like the right speaker output is shorted to ground. \$\endgroup\$ – kasperd Jun 26 '16 at 9:59
21
\$\begingroup\$

There are two basic types.

  1. Use a variable resistance between mic and Ground to signal different actions. The simplest being shorting mic to ground for Answer/Hangup/Take Picture. Ex: Android's Wired Audio Headset Specification v1.1:

enter image description here

  1. In or Out of band signaling. These use a microcontroller to insert a coded signal on the mic. Apple started this with one of their (now older) iPod Shuffle, and I believe thats what the current iPhones and MacBooks use.

enter image description here

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.