This circuit produces a sound similar to the police siren. The two 555 act as an astable multivibrator. The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element.In the beginning 555 is coupled like a low-frequency oscillator so to command the voltage at the second 555 IC at pin 5 which is a control pin.The shifting of the voltage on pin5 is the root of the second oscillator frequency to get up and down.
Police siren circuit which is explained here is worked on NE555 timer IC. The circuit is build with the help of two NE555 IC(i.e. IC1 andIC2) which is the basic block of this circuit. Both the timer IC in these circuit is connected like a astable multivibrator. Although both the IC in the circuit work at two dissimilar frequency.IC1 is a astable multivibrator of slow frequency work on frequency of 20HZ and having 50% of duty cycle while IC2 is a fast astable multivibrator works on frequency of 600Hz. IC1 output is then serve to IC2 at the control pin(oin5). Through the help of this arrangement IC2 output frequency will be modulated with the IC1output frequency. The circuit works on the DC supply between a range of 6V to 15V.
The frequency range of the siren can vary with the exchange of R2 and R4 resistor with potentiometer. The pitch of sound can be enlarged with the attachment of the power amplifier at the output point. The accurate effect of police siren can be produced by connecting flashing LEDs at the correct place. The circuit can be made on a Perfboard. NE556 can be used in the place of two NE555.