On this problem, I will current you the best way to design a simple Police Siren Circuit using NE555 Timer IC. I’ll even share with you methods the circuit works and what is the underlying concept of the problem.

The concept behind the working of this problem is Multivibrator. So, sooner than going into the small print of the problem, enable us to understand the thought of a Multivibrator.


Multivibrator are current in plenty of functions as they’re among the many many most broadly used circuits. The making use of could also be one thing from a simple household software program or a fancy industrial or communication software program. The multivibrator may additionally work in functions like an oscillator, digital flip flop, pulse generator circuit, timer or it might be even utilized in generator circuit and far further.

There are primarily three types of Multivibrators. They’re

Astable Multivibrator

It has actually no safe state. It has two quasi safe states which rapidly alters from one to a unique and as soon as extra to the an identical state. So it primarily alters from extreme to low and from low to extreme no matter any enter set off enter after a pre settled time.

Monostable multivibrator

Out of two states, one is safe state and one different one is quasi safe state. When a set off enter is given, it switches from a safe state to quasi state and it robotically switches once more to the safe state after a pre-decided time.

Bistable Multivibrator

Every the states are safe in it. To alter the state between excessive and low two utterly completely different enter set off is used.

Be taught the submit: 555 Timer as Multivibrator to know further about 555 timer.

All the above three types of multivibrators could also be merely constructed with the help of transistors. Nevertheless there’s one type of IC which is properly obtainable that may be utilized as Astable, Monostable or Bistable multivibrator and it’s known as IC555.

IC555 Pin Diagram

IC555 Pin Diagram

IC555 Pin Diagram – ElectronicsHub.Org

Pin Efficiency of IC555

Pin No.
Pin Title
Provides flooring
Set off
Set off comparator enter pin. Unfavorable set off (
Its output pin
Internal flip flop reset pin. Necessity be extreme to permit output
Administration voltage enter to deal with charging discharging of exterior capacitor
Threshold comparator enter pin. Optimistic set off (>2/Three Vcc) is given in bistable operation
Discharge pin. Affords discharge path to exterior capacitor
For +Ve biasing voltage. Between 4.5 V to 16 V

IC 555 is one among most versatile chips and as a consequence of its multi-functionality, it might be utilized in almost all varieties of functions. It is a DIP or SOP bundle kind of chip having Eight pins with direct current drive output of 200 mA. This IC is made up of analog along with digital elements that’s why it’s known as blended signal chips. The ultimate software program of the IC is to generate timing, clock waveform, sq. wave oscillator and plenty of further.

Internal Block Diagram of IC555

Internal Block Diagram of IC 555

Internal Block Diagram of IC 555 – ElectronicsHub.Org

As you presumably can see from the decide above image, IC 555 contains of two comparators one known as RS flip flop one different one is the combination of the some discrete elements viz transistor, resistor and further. The voltage for biasing is parted amongst three elements by voltage divider having associated price of resistor R by the use of these non-inverting voltage get 1/Three Vcc of the set off whereas the inverting terminal get 2/Three Vcc of threshold comparator. R and S enter terminal of the flip flop receives the every comparator output. The precise output of the IC is Q’(Q bar) output whereas output terminal Q. The discharging transistor that provides discharging methodology to exterior capacitor on the time it reaches extreme.

On the time 1/Three Vcc > unfavourable set off is given on the enter pin set off,  the set off comparator switch to extreme output and the flip flop attain to distinctive sate and the chip output which is output from Q’ strikes to extreme.

Now when 2/Three Vcc < optimistic set off is given to threshold enter pin, the flip flop is in set state as a result of the output from the sting comparator is extreme. The chip output strikes to low as a result of the output Q will flip to extreme. The outside capacitor that receives the discharging path in the midst of the time of transistor discharges. The extreme enter resets keep the flip flop to permit. On the a part of low state flip flop put out of movement and receives low on the output. No completely different outcomes of threshold along with set off comparator outputs.

Circuit Diagram of Police Siren Circuit using NE555 Timer

Police Siren Circuit using NE555 Timer Circuit Diagram Updated

Parts Required

  • 555 Timer IC x 2
  • Resistors – 1KΩ, 10KΩ x 3, 68KΩ
  • Capacitors – 100nF x 2, 10µF x 2
  • Mini Speaker
  • 9V Battery
  • Breadboard
  • Connecting Wires


This circuit produces a sound identical to the police siren. The 555 timer IC is an built-in circuit utilized in a variety of timer, pulse know-how and oscillator functions. The 555 may be utilized to produce time delays, as an oscillator, and as a flip-flop issue. I’ve used two 555 Timer ICs on this problem and every these 555 ICs act as Astable Multivibrators.

Police Siren Circuit using NE555 Timer Image 1

At first 555 is coupled like a low-frequency oscillator so to command the voltage on the second 555 IC at pin 5, which is a administration pin. The shifting of the voltage on pin5 is the premise of the second oscillator frequency to face up and down.

Police siren circuit, which is outlined proper right here, is based on the NE555 timer IC. The circuit is constructed with the help of two NE555 IC. Every the timer ICs on this circuit are configured as astable multivibrator. Although, every the ICs inside the circuit work at two utterly completely different frequencies.

The IC1 is an astable multivibrator of sluggish frequency and is engaged on a frequency of 20HZ with 50% of duty cycle whereas the IC2 is a fast astable multivibrator with a frequency of 600Hz. The output of the IC1 is then outfitted to IC2 at its administration pin (pin5). Using this affiliation, the output frequency of IC2 shall be modulated with respect to the output frequency of IC1. The circuit works on the DC present between a selection of 6V  to 15V.

In order to fluctuate the frequency fluctuate of the siren, you presumably can fluctuate with the values of R2 and R4 resistors by altering them with acceptable potentiometers. The pitch of the sound could also be enlarged with the attachment of the ability amplifier on the output. The right impression of police siren could also be produced by connecting flashing LEDs on the suitable place. The circuit could also be made on a Perfboard. NE556 may be utilized inside the place of two NE555.



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