Room Monitor

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

From the right hand side, T1 is the mains transformer. The primary matches your local electric supply, secondary rating 12V, 1 amp. The bridge rectifiers are rated 50V peak at 1 amp or can be made from four 1N4001 rectifiers. C3 smoothes 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 wideband oscillation 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 is excessive.)

Input Circuit
The power supply and amplifier are connected by cable to a remote location (the room you want to monitor for activity). The input circuit is built around an electret mic insert and amplifier and is connected by three wires. Alarm or telephone cable can be used, if the cable has more than three cores, just leave the unused wires floating.

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, see this page in the practical section for more detail.

Gain is controlled by the 10k preset, once set for a particular room environment listening volume is controlled at the remote location using the 10k potentiometer. The preamp power supply is decoupled by the 1k and 200u capacitor, the first stage is run at a low collector current to ensure a high signal to noise ratio. The second stage is an emitter follower which ensures a low output impedance for driving long cables. As the output impedance is low, hum and noise pickup are minimum and therefore screened cable is not required.