How to Let the Phone With Android to Control Servos Via Audio Channel

If in program on Android phone it's hard to get access to serial port or, as it was in my case - I couldn't find a connector for my phone, it's possible to let the Android phone generate necessary logical signal via audio channel. To test it I made the phone to control two servos (via stereo output channel).
Because I did not want to damage my phone during development I connected to it first via bluetooth headset. When schema was completed and tested I successfully tried it directly via audio jack.