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I'm looking at a teardown of Sennheiser Momentum wireless earbuds. It is a rigid-flex design that folds into the casing: sennheiser1

My question is about the flex that sticks out to the right in the picture above. Here is another picture with the part I’m wondering about circled in red: sennheiser2

When folded into the casing it would end up facing out toward the plastic.

The Earin M-2 wireless earbuds also have something similar: earin

What is it and what does it do?

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  • \$\begingroup\$ Maybe it's an antenna? \$\endgroup\$
    – Andy aka
    Apr 3 '19 at 13:02
  • \$\begingroup\$ Antenna is my guess too, given that it seems to have a 'guard trace' in the form of a ground plane around the trace leading to it. \$\endgroup\$
    – Hearth
    Apr 3 '19 at 13:57
  • \$\begingroup\$ I was thinking it could be a proximity sensor? To sense if it is inserted into an ear. Not the actual volume etc touch controls, just proximity \$\endgroup\$
    – Filippa
    Apr 3 '19 at 14:41
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This looks like a pad for capacitive touch sensing. This could be used in a wireless earbud for sensing if the earbuds are in your ear or for sensing finger taps for actions like pausing music.

A monitoring device like a microcontroller will charge the pad and measure the time it takes for the voltage to decay, which is a function of the capacity. The capacitance of the pad will change when a finger is nearby

Capacitive touch sensing illustration Image source: https://www.embedded.com/waking-up-a-capacitive-touch-sensing-device-with-an-mcu-peripheral/

You can see a tutorial about some capacitive touch sensor modules here: https://learn.adafruit.com/adafruit-capacitive-touch-sensor-breakouts

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