Following the request described a couple of months ago, I haven't stopped thinking of the cellphone remote control. So, an ATtiny2313 will be the processing unit and to ADC the incoming sound command I will use a tone decoder.
Why not build a software frequency counter? Because it would keep the 2313 running all the time (that's about 99,999% of the time needed) and it would use internal timers that I need to validate the incoming command sequence.
A simple tone decoder is the good old NE567 that I located inside a PSTN answering machine from 90's. The local shop doesn't sell them anymore.
Some more details. The ring tone will be bitonal, so that any ambient noise gets cancelled when the remote cellphone rings. High tone will be used as a carrier, low tone as a signal to the microcontroller. Low tone will be chosen away from flies and mosquitoes sound :-) The ON 001000100010 decoded sequence will have a lower frequency than the OFF command.
Let's warm up the iron!