Simple Security Monitor Circuit

Here is security monitor circuit. A remote listening circuit. The area to be monitored is connected via a cable and allows remote audio listening. You can use this in your garden and listen for any unusual sounds, or maybe just wildlife noises. If you have a car parked in a remote location, the microphone will also pick up any sounds of activity in this area. The cable may be visible or hidden, screened cable is not necessary, and you can use bell wire or speaker cable if desired. Starting from the right-hand side, the power supply. I have used 12V as a standard power supply voltage, or a 12V car battery may be used. The circuit is in two halves, a remote microphone preamp, and an audio amplifier based around the National Semiconductor LM386 audio amplifier. At the opposite end, C7 a 10u capacitor brings home the amplified audio to the listening location. The signal is first further amplifier by an x10 voltage gain amplified using the TL071. C8, a 22p capacitor again rolls off some high-frequency response above 100kHz. The schematic drawing is like circuit below :

They are necessary as long wires may pick up a little radio interference. After amplification by the op-amp, the audio is finally passed to the LM386 audio amplifier. R14 acts as volume control. R13 and C12 prevent possible instability in the LM386 and are recommended by the manufacturer. Sound output is around 1 watt into an 8-ohm loudspeaker, distortion about 0.2%. If preferred headphones could be used, although I’d recommend a series resistor of the same value impedance as the headphones.

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