I made a nice interface for using my HTC Desire S for HF Digimodes as it's piece of junk as a phone...
The schematics come in two versions, one is of extreme simplicity but the second has to be fine tuned to work with the phone.
As you may know, the Android phone is made to be also a music player so it has a stereo output. This is the key for making the PTT circuit. One channel is for modulation and the second one is for detecting and for PTT circuit. I used a double detector to have a sufficient voltage to trigger the 2n2222 PTT transistor, in Open Collector circuit. If you transceiver use a negative voltage for PTT, the schematic is simple to transform.
The second version use the same circuit for audio In/Out but have a small preamplifier for the audio before the detection is made. This is suitable for low volume Android devices and uses the approximative 3 V from the microphone. The input of the Android device is not using a condenser microphone, therefore we can use that DC.
The preamplifier is made with a 2N2222 transistor and it may be necessary to tweak the positive polarisation for a better result.
As a response to a comment from Catalin, YO3FVR (TNX!) , I must admit that, indeed, there is a error in the HTC connector!
From ring to tip, the sections are:
-Microphone / Control / AUDIO IN
-GROUND
-AUDIO OUT Right
-AUDIO OUT Left
So, I think this answer to your question!
Many thanks for notice me about this!
