How should a mobile phone headphone audio output (such as the headset jack on top or bottom of an iPhone) be interfaced to a microcontroller digital input? (3.3V controller. Assume no audio frequency capable A/D is available.)
Assume a modulation scheme such as FSK or similar at appropriate frequencies.
If you make sure to put out a single clean frequency at a time, then a zero crossing detector is all you need. Audio can always be AC coupled, so a comparator or opamp acting as a comparator is all you need. One input gets filtered 1/2 supply voltage. The other gets that as DC bias but also gets the AC coupled audio. This will make a square wave at the audio frequency. A micro can measure this frequency and interpret its meaning in the context of a higher level protocol.
Note that this won't work for multiple frequencies sent at the same time, like DTMF for example.
