Security Monitor

A remote listening circuit. The area to be monitored is connected via
a cable and allows remote audio listening.

Security Monitor

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 od activity
in this area. The cable may be visible or hidden, screened cable is not necessary and you can use bellwire
or speaker cable if desired.

Circuit Description:
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.

The remote preamp uses an ECM microphone to monitor sound. A direct coupled 2 stage amplifier built around Q1
and Q2 amplify the weak microphone signal. Preset resistor R2 acts as a gain control, and C1 provides some
high frequency roll off to the overall audio response. Q1 is run at a low collector current for a high signal
to noise ratio, whilst Q2 collector is biased to around half the supply voltage for maximum dynamic range. The
power supply for this preamp is fed via R10 and R6 from the 12V supply. C4 ensures that the preamp power supply
is decoupled and no ac voltages are present on the power lines. The amplified audio output from Q2 collector is
fed onto the supply lines via C6 a 220u capacitor. The output impedance of Q2 is low, hence the relatively high
value of C6. C6 also has a second purpose of letting the output audio signals pass, whilst blocking the dc voltage
of the power supply.

At the opposite end, C7 a 10u capacitor, brings home the amplified audio to the listening location. The signal
is first further amplifier by a x10 voltage gain amplified using the TL071. C8, a 22p capacitor again rolls off
some high frequency response above 100kHz. This is 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. Audio
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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