Police Siren circuit using 555 Timer is the one we are about to see here. This circuit uses two NE 555 ICs and each of them wired as astable multivibrators. And a speaker is used in order to produce the siren sound.
WORKING OF POLICE SIREN CIRCUIt:
In this the IC 555 runs in free mode so that it can operate as an oscillator.While working as an oscillator the IC 555 can produce a series of clock pulses.The first IC1 is designed in such a way so that it will be operating around almost 20 Hz.This can be achieved by using diode in between the pins 6 & 7 of IC1.The action of diode is to allow current only in its forward direction so this makes the current to pass through only in one direction so that the pulse generated will be reduced to half.Then this generated pulse will be sent to the IC2 which acts as the control voltage for it.
The IC2 is wired as an fast astable multivibrator operating around 600Hz.Since the output of the IC1 was given as control voltage to the IC2 the output of IC2 will be modulated by it.Hence this modulating action is responsible for the siren effect.Then the output can be fed into a speaker thus it produces the siren sound.The circuit can be powered from anything between 6 to 15 DC voltage.By connecting this output to power amplifiers we can further increase the loudness of this siren.