Electronic siren using LM389 IC

This electronic siren circuit is designed using LM389 chip. With transistors T1 and T2 was built astabil multivibrator whose frequency can be adjusted with potentiometer P1 between 1 and 7 Hz. Multivibrator produces a square wave that is available to the collector of T2 (pin 11 of IC1) and the control transistor T3. This transistor is connected to the emitter or muting the amplifier input. Now muting input is connected on the frequency rate of multivibrator with supply voltage and thus attenuates at the same rate, the output signal of the amplifier.