Thanks all, much appreciated. Lot of learning ahead re FFT, Goetzel and ADC in general. I might invest in that or, frankly more likely, decide that the relatively trivial application doesn’t justify the effort!
For anyone curious, its only a ‘fun and learning’ project. When I or my wife tap in the burglar alarm code before exiting the house I want to deliver a reminder about something. I’m unwilling to access the commercial alarm’s PCB so thought of recognising the ‘beeps’ described. (An earlier test using instead a capacitive touch switch failed, as the alarm case is itself earthed.)
I’ll see if a less ambitious detector would cut the mustard instead. (“In the last 5 seconds have there been at least two AC signals greater than 100ms in length and over 400mv in amplitude throughout, with a gap of over 50ms?”)
Might even lapse into some legacy (solder!) practical electronics and try a CR filter after the preamp, plus further amplification (the signal to A0 is currently only around 500mv pk-to-pk), and then a rectifier before the result to D6 instead of A0.
Terry