The mic output is amplified by the two
transistors, the output is fed down the cable via the 220u capacitor.
Circuito original em:
www.mitedu.freeserve.co.uk/Circuits.
Notes:
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.
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 recoveredby 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.