The Siren power sweep circuit using 555 is used to produce variable levels of siren sound.
In the circuit, we have two 555IC’s which functions as astable multivibrator. But, both of the IC’s frequency is set to be as of different frequencies. The output of astable multivibrator is high for sometime, then becomes low, again high and low, and it continues.
When the circuit receives a voltage, the IC1 starts to function as a Astable Multivibrator from the resistors connected to pin 7, and capacitor connected to pin 2. SO, the IC 555 starts to deliver clock pulses with a particular frequency for a particular time. This clock pulses are given to the IC2, which then gets triggered. It remains triggered for the high pulse duration of a IC1 astable multivibrator and goes to OFF state when there is a low output from IC1. When IC2 gets triggered, it also starts to act as a astable multivibrator. The output of IC2 is given to the Loudspeaker which sounds like a siren. According to the pulse received from the IC2, the siren gives high sound for high output from IC2, and sounds low for low output from IC2. So, there will be an increase and decrease in sounds of siren. So, its called the sweep circuit.