How to Make a Police Siren Circuit?

In this article, we create a police siren circuit using 555 timer integrated. This circuit smoothly rotates the output sound between two different tones/frequencies, similar to the sound emitted from police cars.

You can also reinforce this circuit with the previous project of the 555 timer project series, the police lights circuit: The Police Strobe LED Circuit.

Required Materials

  • 2 x 555 Timer Integrated
  • 8Ω Buzzer/Speaker
  • Resistors: 100K, 33K, 2 x 1K, 2 x 10K
  • Capacitors: 1000uF, 100uF, 10uF, 100nF
  • Circuit board/Breadboard
  • Jumper/Cables
  • (5-9) V Power Supply

Circuit Diagram

police siren

Working principle

In the electronic piano circuit we have established in one of the previous projects, we can produce different output tones using different resistance combinations. To better understand how this circuit works, you can check out this tutorial: Electric Piano Circuit.

Here we used a similar arrangement that uses the unstable (astable) 555 timer integrated mode to produce a single tone. But in order to mimic the sound of a police siren, we have to travel back and forth between 2 tons or frequency.

And this transition between the two tones was made possible by using a second 555 timer integrated in unstable mode and connecting its output to the control pin of the first 555 timer integrated. Now we get 2 different frequencies from the second 555 timer integrated, depending on the output status of the first 555 timer integrated.

Due to the 1000 uF capacitor, the voltage in the control pin gradually increases and decreases instead of a sharp rise and fall. As a result, we get a smooth output that swings between 2 tons, similar to a police siren.