power a 5V source from an android headphone
There are solutions with a lower-cost BOM and more parts (10 cents or so is possible), but a very easy approach would be to use an LM567 tone detector chip. 



Obviously you'd use a lower frequency than 100kHz in the above example, maybe 1kHz, and you'd want the bandwidth to be fairly wide, maybe 10%, to account for component tolerances. 

This would have the advantage that it would not easily be triggered by random audio at the input.