Hi all!!!
I use this very stable circuit for any gadget. Is for convert the ugly sound or minielevation of audio in nice "bips". This is for ALL use you want.
The 555 works as comparator. In abscence of any signal (this is, all the stages off) connect this circuit. Whit the preset, adjust in the more sensitivity point —this is, near the continuous bips— and retire a very little. If sounds at the moment of the connection, adjust it in the more good level point. The 22 k resistor and the 10 uF cap are for short timming of bips.
The piezosounder is the type that include internal oscillator.
I have built many of this with 100 variations, but this is one of the betters.
Of course, with the preset you control the sensibility you want.