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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?

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    \$\begingroup\$ Short answer: The microphone input isn't just used for the microphone. \$\endgroup\$
    – Connor Wolf
    Jun 25, 2016 at 20:52

2 Answers 2

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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.

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    \$\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, 2016 at 23:00
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    \$\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, 2016 at 0:48
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    \$\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, 2016 at 0:54
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    \$\begingroup\$ @SF the phone detects that and disables the MIC and button functions. And some phones, like the iPhone 3 (irrc) had issues so you had to get a TRRS to TRS adapters to prevent the short causing issues. \$\endgroup\$
    – Passerby
    Jun 26, 2016 at 1:38
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    \$\begingroup\$ @SF. no, its much more tolerant in practice. Even android's page shows 70 ohm or less A, 110 - 180 ohm B, 210 - 290 ohm C, 360 - 680 ohm D. \$\endgroup\$
    – Passerby
    Jun 26, 2016 at 1:51
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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

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  • \$\begingroup\$ How is that MCU powered in Apple earphones? \$\endgroup\$
    – FZs
    Sep 19, 2022 at 21:14
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    \$\begingroup\$ @FZs there is a bias voltage on trrs microphone pins, or even a dc bias applied to the regular audio pins. electronics.stackexchange.com/a/27733/17178 apple specifically doesn't provide details publicly so it's guesswork at times. \$\endgroup\$
    – Passerby
    Sep 20, 2022 at 1:16

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