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Following on from this post I wired up a way to record audio on an iphone (the important design points quoted here):

The iPhone as well as other mobiles will apply a DC voltage about 1.5-2.5V to the microphone. The objective of this is twofold: it serves the iPhone as a way to measure the external DC impedance of the microphone and also powers the preamplifier built into many miniature electret microphones.

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Thx to this thread i found the solution in the missing impedance on the line signal path. When plugging in the headphone jack, I could see the Iphone was providing 2.8 V for about 1 s to detect the impedance of the microphone. I added a 2.1 kOhm resistor between mic+ and ground and now the Iphone switches to external input and does not switch back.

As you can see from the image, I had a go at this and had initial success. Using the resistor to trigger the iphone to prompt "Headset or Speakers". The variable resistor was to adjust for a range of input levels. I was able to record audio on my iPhone successfully on several occassions. I didn't use the device for about 4 months, and now it doesn't work. I get the prompt "Headset or Speakers" but the iphone then just records on its own internal microphone regardless. I haven't been able to figure it out. The lavellier microphone works correctly as expected (shown in image)

Lightning Audio Recording iphone lighting audio recording

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    \$\begingroup\$ If it have been a firmware upgrade since last time it worked, Apple might have changed the way sound is handled (it's not uncommon). \$\endgroup\$
    – MiNiMe
    Commented Oct 17, 2023 at 15:08

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I thought it was a result of update but it was my mistakey memory in the end --- I was trying to input to the iphone on its output lines. I used the microphone lightning adapter and now can successfully record from external audio sources.

Wiring corrected

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  • \$\begingroup\$ mmm - Hi, Thanks for coming back with an answer to your question (I think you're saying it was user error?). In order to effectively mark the topic as solved, please consider "áccepting" your choice of the best answer i.e. click the "tick mark" next to an answer to make it green - either your answer (which can be áccepted after 2023-10-19 @ 14.50 UTC i.e. 48 hrs after the question was asked) or another answer, if one is written (which can be accepted any time). That shows you don't need more help & consider the problem solved. TY \$\endgroup\$
    – SamGibson
    Commented Oct 17, 2023 at 18:40
  • \$\begingroup\$ My mistake was I was trying to feed audio output of my source to audio output of the iPhone. (Hence why it wasn't recording) To summarise how to record audio onto your iPhone through lightning connector, you need the following things: 1. a 2K2 resistor across the audio input. 2. You need a TRRS to lightning microphone cable. (Black cable plugged into iPhone) 2. You need need a convertor cable to plug the audio out to come out on the top part of the TRRS connector. (Cable with orange and green ends) \$\endgroup\$
    – mmm
    Commented Oct 19, 2023 at 14:18
  • \$\begingroup\$ Glossary: TS connector - mono, unbalanced signals. TRS connector - stereo signals TRRS connector - stereo signals + microphone signal T= tip, R= ring, S= sleeve \$\endgroup\$
    – mmm
    Commented Oct 19, 2023 at 14:19

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