Posted on Feb 7, 2014

This circuit allows audio monitoring of a remote location and so doubles as a room monitor or baby alarm. It canbe run from a 12 Volt battery or mains power supply as shown below. Interconnect uses 3 wires, so multi core cablecan be used; alarm or telephone cable is suitable just leave any extra wires free. From the right hand side, T1 is the main

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s transformer. The primary matches your local electric supply, secondaryrating 12V, 1 amp. The bridge rectifiers are rated 50V peak at 1 amp or can be made from four 1N4001 rectifiers. C3 smooths the supply and C4 decouples any high frequency noise. The audio amplifier is built around a single LM380. This IC will deliver up to 2 watts into an 8 ohm speaker. C1 provides extra supply decoupling from mains hum, whilst the zobel network formed by R2 and C5 prevent a widebandoscillation at 10MHz. (Authors note: I have once experienced the 10MHz wideband oscillation. This was on an LM380 amplifier without the zobel network; when oscillation occurs, the audio is distorted and current consumption isexcessive. ) The power supply and amplifier are connected by cable to a remote location (the room you want to monitor foractivity). The input circuit is built around an electret mic insert and amplifier and is connected by threewires. Alarm or telephone cable can be used, if the cable has more than three cores, just leave the unused wiresfloating. The electret mic is amplified by a two stage, direct coupled preamp comprising two BC549C transistors. Electret mics have two terminals, older types may have three terminals. Gain is controlled by the 10k preset, once set for a particular room environment listening volume is controlled atthe remote location using the 10k potentiometer. The preamp power supply is decoupled by the 1k and 200u...

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