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Police Siren Using 555Timer

The Exact Working of Police siren made up of two 555 timer.

Exact Working of Police siren made up of two 555 timer.

Before reading you must know that 555 timer  police siren is based on the principle of astable multivibrator , here two astable multibirator is used


Exact Working of Police siren made up of two 555 timer.

Basic Astable circuit using 555 timer .


In Police Siren circuit, There are two Astable multivibrator made by  555 timer , The output pin 3 of first astable multivibrator is connected to control pin of second 555 timer ,

The first astable circuit has Frequency = 1.45/(Ra+ 2Rb) * C  i.e  F =  0.7107 Hz ; but we are using a diode to short Rb therefore F =  0.21 Hz

For second  Astable multivirbrator  F=  0.00589 KHz = 589 Hz ( which is huge)

working of 555 timer police siren

This Graph is made by changing some values of c and R  so that It becomes very clear that what exactly happening in the circuit  , look at the graph there is square pulse having Blue Color generated by first astable circuit . It is fed to 'Control Pin’  of the Second 555 timer .  Pin 5 allows direct access to the 2/3 voltage-divider point. This is the reference level for the upper comparator. 
By applying a voltage to this pin 5 , it is possible to vary the timing of the chip independently of the RC network.

Now as you can see when Blue square pulse is high  it increases the control voltage of second 555 timer In other words  the upper comparator inverting (- ) voltage is raised to a higher level than initial level so the capacitor(C3) need to be charged up to a higher extent at which it changes upper comparator output  and capacitor takes more time to reach to that extent  therefore  the spikes generation Frequency is lowered . This phenomenon can be seen in above graph the position where blue pulse is high there is low density of  red spikes.

Now take case where blue pulse is low in this case opposite happens , the control voltage is lowered hence the capacitor(C3) will reach to that extent quickly so it change the ‘R’ and ‘S’ value quickly and this happens again and again  as a result more spikes is generated  so frequency is higher in this case.


So we can say The police siren sound like dee-dah , for dee we use one frequency and for dah we use another frequeny  .

working of 555 timer based police siren

This graph is taken by adding a electrolytic capacitor along with 64 ohm speaker in series with original values  given in police siren circuit . 

 “Spike word is not suitable  actually red spikes is also square pulse with very small constant voltage level due to high frequency of pulse “ 

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