Audio Alarm Circuit

In the circuit, U1 amplifies the audio picked up by the condenser microphone. Resistor Rl limits current, while R2 and R3 center the output of the amplifier to %B+ to allow a single-ended supply to be used. Diodes D1 and D2 rectify the output of Ul, and C3 filters the resulting pulsing dc. Thus, a dc voltage that is proportional to the ambient sound level is produced.That voltage is presented to the noninverting input of U2. The inverting input is provided with a reference voltage of between 0 and lAB+, which is set by Rll.As long as the noise level is low enough to keep the voltage at pin 3 lower than the voltage at pin 2, the output of U2 stays low (approximately 1 V).
Audio Alarm Circuit - schematic

That is enough to bias Ql partially on. A voltage divider, formed by R8/R10 and Ql (when it`s partially on), prevents Q2 from turning on.When the noise level is high enough to bring the voltage at pin 3 higher than the voltage at pin 2, the output of U2 goes high. That turns Ql fully on and drives Q2 into saturation. The piezo buzzer then sounds until the power is cut off.

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