Audio Perimeter Monitor

This circuit is intended for audio surveillance of an unattended area, examples being a back garden or open space. It can be used to listen for wildlife or just as an extra pair of ears. 

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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 two 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.