Connecting a Teensy 3.6 to mic in on an Android phone?

[feklee]

I already asked this on Electrical Engineering Stack Exchange, but so far without an answer: How to connect a line-out to mic on Android plug? (CTIA)

We have set up a sound installation in the Hong Kong jungle: Twitch live stream

The output of a Teensy 3.6 is connected to the input of a Xiaomi Mi 4X Android phone using a 3.5mm plug with four rings, wired according to CTIA standard. This does work. However, there are two issues:

• The phone still picks up ambient sound, meaning it doesn't turn off the internal mic. This issue does not happen with a regular headset. When a regular headset is connected, the internal mic is disabled.
• The signal picked up by the phone from the Teensy is very weak, although if I connect my headphones to the Teensy it's actually very loud.

Any idea how to wire a 3.5mm plug so that it is recognized as a headset by Android?

 

[feklee]

I measured the resistance between the sleeve and ring 2 on my Bose QC25 headphones: 5.6 kΩ

This led me to adding a 5.5 kΩ resistor to my plug:



There were some other random audio issues, but they magically went away. Possibly there was also a short somewhere in my circuit, part of which I resoldered. Anyhow, now it works!

Notes:

• Rings 3 and 4 are intentionally left unconnected. I don’t need headphone output.
• With another headset that I tried, the phone, a Xiaomi 4X, did not detect it properly. That headset did work on another phone. There seems something odd about the Xiaomi 4X.

In case you’re curious:


Live stream on Twitch (few more hours, until battery is dead)