555 timer ic

Police Siren Circuit using NE555 Timer

In this project, I will guide you through the construction of a basic Police Siren Circuit using the NE555 Timer IC. I will also provide an explanation of how the circuit operates and the fundamental principle behind the project.

The underlying principle of this project is based on a concept known as a Multivibrator. Therefore, before delving into the project details, let’s first establish what a Multivibrator is.


  • Multivibrator
    • Astable Multivibrator
    • Monostable multivibrator
    • Bistable Multivibrator
  • IC555 Pin Diagram
    • Pin Functionality of IC555
    • Internal Block Diagram of IC555
  • Circuit Diagram of Police Siren Circuit using NE555 Timer
    • Components Required
  • Working


Multivibrators are an extensively employed circuit with a broad spectrum of applications. These applications can range from basic household uses to intricate industrial or communication applications. Multivibrators serve various roles, including functioning as oscillators, digital flip-flops, pulse generators, timers, or signal generators, among other functions.

Multivibrators are categorized into three distinct types. Indeed, they are.

Astable Multivibrator

It doesn’t possess a stable state; instead, it operates with two quasi-stable states that quickly transition from one to the other and then back to the initial state. Its transitions primarily oscillate between high and low states, independent of any input triggers, following a predetermined time interval.

Monostable multivibrator

Among its two states, one is stable while the other is quasi-stable. Upon receiving a trigger input, it transitions from the stable state to the quasi-stable state and then autonomously reverts to the stable state after a predetermined duration.

Bistable Multivibrator

Both states are stable in this configuration, and it necessitates the use of two distinct input triggers to transition from a low to a high state.

For further information on the 555 timer, refer to the article “555 Timer as Multivibrator.”

Transistors are capable of constructing any of the three multivibrator types mentioned earlier. Nevertheless, there exists a readily accessible IC type that can serve as an astable, monostable, or bistable multivibrator: the IC555.

IC555 Pin Diagram

Pin Functionality of IC555

1 Gnd Input Provides ground
2 Trigger Input Trigger comparator input pin. Negative trigger (< 1/3 Vcc) is given in monostable operation
3 Output Output Its output pin
4 Reset Input Internal flip flop reset pin. Necessity be high to enable output
5 Control Input Control voltage input to manage charging discharging of external capacitor
6 Threshold Input Threshold comparator input pin. Positive trigger (>2/3 Vcc) is given in bistable operation
7 Discharge Input Discharge pin. Gives discharge path to external capacitor
8 Vcc Input For +Ve biasing voltage. Between 4.5 V to 16 V

The IC 555 stands out as an incredibly versatile integrated circuit, with its diverse capabilities making it suitable for a broad spectrum of applications. It’s available in an 8-pin DIP or SOP package, boasting a direct current output capacity of 200 mA. This integrated circuit is classified as a mixed-signal chip due to its combination of analog and digital components. Its utility spans across various functions, including timing generation, clock waveform production, square wave oscillation, and many more.

Internal Block Diagram of IC555

As depicted in the above diagram, the IC 555 consists of two comparators, one of which is an RS flip flop, while the other comprises discrete components like transistors, resistors, and various other elements. A voltage divider, utilizing resistor R, divides the biasing voltage into three segments. The non-inverting terminal of the trigger receives 1/3 Vcc, and the inverting terminal of the threshold comparator receives 2/3 Vcc. Both comparator outputs are directed to the R and S input terminals of the flip flop. Although the IC’s genuine output is found at output terminal Q, the actual output is Q’ (Q bar). When the external capacitor accumulates a sufficiently high voltage, it is permitted to discharge via the discharging transistor.

When a negative trigger input is provided at the trigger input pin and 1/3 Vcc is greater, the trigger comparator yields a high output. The flip flop subsequently reverts to its original state, causing the chip’s output, which emanates from Q’, to go high.

Upon applying a positive trigger of 2/3 Vcc to the threshold input pin, the flip flop enters the set state since the output from the threshold comparator is high. As Q’ rises, the chip’s output diminishes. During transistor discharge, an external capacitor receives the discharging path. The high input resets maintain the flip flop in an enabled state. In its low state, the flip flop deactivates and yields a low signal at the output. No other threshold outcome is comparable to the outputs of the trigger comparator.

Circuit Diagram of Police Siren Circuit using NE555 Timer

Components 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


The noise generated by this setup resembles that of a police siren. The 555 timer IC is a versatile integrated circuit employed in crafting timers, pulse generators, and oscillators. It serves as a timer, oscillator, and flip-flop component. In this particular endeavor, I employed two 555 Timer ICs, both serving as Astable Multivibrators.

Police Siren Circuit using NE555 Timer

Initially, the 555 timer is configured as a low-frequency oscillator to control the voltage at pin 5 of the second 555 IC, which serves as a control pin. The modulation of the voltage on pin 5 causes the frequency of the second oscillator to fluctuate.

The police siren circuit described here utilizes the NE555 timer IC. To construct this circuit, two NE555 integrated circuits are employed. Both timer ICs are configured as astable multivibrators in this design. However, they operate at distinct frequencies.

IC1 functions as a slow astable multivibrator, producing a 20Hz frequency with a 50% duty cycle. In contrast, IC2 operates as a fast astable multivibrator, generating a frequency of 600Hz. IC1’s output is then connected to the control pin (pin 5) of IC2. This arrangement modulates the output frequency of IC2 in relation to the output frequency of IC1. The circuit is powered by a DC supply voltage ranging from 6 to 15 volts.

To adjust the siren’s frequency range, you can replace resistors R2 and R4 with appropriate potentiometers. Connecting a power amplifier to the output can enhance the sound’s pitch. By strategically placing flashing LEDs, you can create a realistic police siren effect. A Perfboard is suitable for constructing the circuit, and if desired, you can replace the two NE555 ICs with a single NE556.


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