Using a single cable such as speaker wire or doorbell cable,
this circuit can be remotely positioned, for example, at the
bottom of a garden or garage, and used to detect all sound in
that area. The cable can be buried in a hosepipe or duct and is
concealed out of sight. The mic is an ordinary dynamic mic
insert and should be housed in a waterproof enclosure with the
rest of the circuit.
The mic output is amplified by the tw1o transistors, the
output is fed down the cable via the 220u capacitor. Here, it has
a dual purpose of preventing the DC supply from upsetting the
bias of the circuit, and also allowing the smaller ac audio
output to pass down the line. At the power supply, the audio is
recovered by the 10k preset and 220u capacitor. It is used
to feed a small audio amplifier (such as the 2watt design) shown
earlier on this site.