AUDIO_ALARM

Published:2009/6/14 21:59:00 Author:May | From:SeekIC

In the circuit, UI amplifies the audio picked up by the condenser microphone. Resistor RI limits current, while R2 and R3 center the output of the amplifier to 1/2 B+ to allow a singleended sup-ply to be used. Diodes Dl and D2 rectify the output of U1, 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 1/2 B+, which is set by R11.As long as the noise level is low enough to keep the voltage at pin 3lower than the voltage at pin 2, the output of U2 stays low (approximately 1V). That is enough to bias Q1 partially on. A voltage divider, formed by R8/RIO and Q1 (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 Q1 fully on and drives Q2 into saturation. The piezo buzzer then sounds until the power is cut off.



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