This is a republication of an post I wrote way back when iPhones still had headphone jacks. It's sad for phones to lose such a cool and easy to access analog input method. All of this should still work with the headphone dongle though.
I am trying to hook a rowing machine up to the iPhone (which I will write more about later). I intend to use the headphone jack to read the signal produced by the rower.
The iPhone has a TRRS jack which combines a (stereo) line out with a mic in. In this image, you can see how the two are mapped on the plug.
I bought a cable that splits the TRRS in the two plugs pictured above. I found these cables extremely hard to find. I got this one from Headset Buddy, but I'm not satisfied with the quality. One of the two I ordered doesn't work and they feel quite fragile in general. I couldn't find any others though.
Initially I tried plugging a line out directly into the mic in, which doesn't work because the iPhone needs to 'sense' there is a microphone attached. That problem is solved with a simple circuit.
The 1 kOhm resistor let's the iPhone sense there is a mic attached. You can experiment with that value a bit, but too little or too much resistance will prevent the detection. The capacitor is there to take care of the DC noise. You should experiment by removing it - it is not essential.
Now, if you turn your volume down to a minimum, you should be able to record your line out. (Simply play a song from you computer and use your favorite app to record it.)
In a future post, I will show how to apply this practically.