Introduction: Police Siren

When I was a child, hearing police sirens always gave me an action intense mood and made me want to join the police to hunt down law violators. Since I have been working on 555 timers, I decided to fulfill my childhood dream and create my own intense adrenalin providing Siren. My project is a police siren circuit which includes two 555 timers to produce a turbulent wailing sound. These two 555 timers will be used as a low-frequency oscillator controlling the second 555.

Step 1: List of Hardwares

  • Double 555 timers
  • Solderless breadboard
  • Jumper wire
  • Speaker with the internal resistance of 8 ohm
  • Single 68K, double 10K, and single 1K resistors
  • 15V power supply
  • Double 100n capacitors along with 10u polarized capacitors

Step 2: Building the Siren Circuit

Procedure 1: Creating a low frequency oscillator

As I am building a siren, the main task is to successfully produce up-down wailing sound. First of all, I am gathering all my components and creating a template that I can follow up with. Connect the first 555 as the center of the initial circuit and connect the ground pin to the ground on the breadboard. Next, connect 1K, 68K, a wire coming out of the threshold pin, a wire coming out of the trigger pin, and a polarized 10u capacitor in a series like shown in the schematic. After linking another 100n capacitor to the 555, the first procedure of building the circuit is basically done.

I just created a low-frequency oscillator that is able to produce a square wave of voltage at the output pin. Capacitor charges and discharges behind the input voltage and systematically causes the output voltage to drop and rise instantly. This is created by the trigger and threshold waveforms (both shapes of the waves are attached above). The negative pulse on the trigger sets the internal flip-flop when the voltage drops below 1/3Vcc causing the output to switch from low to a high state. On the other hand, the threshold pin sets the internal flip-flop when the voltage exceeds 3/2 Vcc causing it to switch from high to low state. Thus. these two keep the voltage bouncing in between 1/3Vcc and 2/3Vcc. This makes the trigger and threshold waveforms to oscillate the way they do in the circuit.

Procedure 2:

After building a low-frequency oscillator, the next step is to complete the siren circuit. Connect the first 555 timer's output pin to the voltage control pin of the second 555 timer. Next, connect two 10K resistors, and 100n capacitor along with the wires coming out of the threshold and trigger pins in series. It is time to add the speaker which will generate siren to our circuit. Connect the speaker to the output pin of the second 555 timer with a 10u capacitor in between. Lastly, connect the rest of the circuit to the 15V power supply and grounds. By the way, I used an 8-ohm resistor to simulate the speaker in my LTSpice simulation.

The output coming from the low-frequency oscillator controls the voltage control pin of the second 555 timer. The voltage shift in the voltage control makes the frequency of the second oscillator to rise and fall.
This is how the siren is making its sound from the circuit.

Step 3: Further Development Ideas

Since I created a police siren sound, it would be complete and actually usable to some extend if I add LED red and blue shifting lights in it. I will be experimenting with this idea in my next project.