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Multivibrator discovers its own space in many applications, as they are among the most widely used circuits. The application can be either a home or industrial application or communication etc. The multivibrator can also work on this type of application, either as a digital oscillator or flip-flop or a pulse generator circuit or as a timer.
There are mainly three types of multivibrators. They are :
1. Astable Multivibrator: Actually it has no stable state. It has two almost constant states that change quickly from one to another and back to the same state. Therefore, it is mainly alters from high to low and low to high devoid of any input input input after pre set time.
2. Monostable multivibrator: Of two states, one is stable state and another is quasi state. When the trigger input is given, it changes from a stable state of quasi state. And it automatically switches to a stable state of quasi state after a predetermined time.
3. Bistable Multivibrator: Both the state is stable in it. Two different input triggers are used to change the status between low and high.
IC555 Pin Diagram:
![[Image: IC555-Pin-Diagram.jpg]](http://www.electronicshub.org/wp-content/uploads/2014/01/IC555-Pin-Diagram.jpg)
A lot of electronic circuits using IC of the NE555 timer have already been posted here and this is just another one. Here is the circuit diagram of a police siren based on the NE55 timer IC. The circuit uses two ICs of NE555 timers and each of them is wired as astable multivibrators. The circuit can feed anything between 6 to 15V DC and is quite strong. By connecting an additional power amplifier to the output, you can further increase the loudness.
IC1 is wired as a slow astable multivibrator operating at a duty cycle of about 20Hz @ 50% and IC2 is wired as a fast astable multivibrator operating at about 600Hz. The output of the first astable mutivibrador is connected to the control voltage input (pin5) of IC2. This causes the output of IC2 to be modulated by the output frequency of IC1, giving a siren effect. In simple words, the output frequency of IC2 is controlled by the output of IC1.
Circuit diagram
![[Image: police-siren-using-NE555.png]](http://www.circuitstoday.com/wp-content/uploads/2010/02/police-siren-using-NE555.png)
It can be understood in the following video:
