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siren circuit schematic

Simple Siren Circuit

This circuit generates a tone that sounds very similar to a siren. The generator part of the circuit is made of the combination of PNP and NPN transistors. Toghether, the two transistors build up a free runing multivibrator. If the C2 capacitor was connected to the positive line of the power supply, it would have worked as a constant frequency oscillator.

However, we dont want a constant frequency oscillator. We want a siren. So to generate an up and down going signal tone, the resistor R2 is fed from an RC circuit. When the switch S1 is pressed, the capacitor C1 charges via R1 slowly until it reaches the maximum voltage level of 4 volts. This increasing voltage results to a decreasing time constant at the R2/C2 junction. This furthermore results to an increasing frequency of the multivibrator.

After the switch S1 is released, the capacitor C1 discharges slowly resulting to a decreasing frequency cycle. Through the combination of the two time constants a sawtooth waveform is generated.
The signal heared from the speaker will be an increasing or decreasing tone depending on whether the switch S1 is pressed or released.

Siren circuit diagram

siren circuit schematic

Siren PCB layout

siren pcb

6 Comments

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  • dinesh92x

    I think there is one mistake ,2nd paragraph ,3 rd line “the capacitor C1 charges via R1 slowly ” it should be “the capacitor C3 charges via R1 slowly ”
    Check!!
    And my question is which side c2 will charge.?
    and why did you say ” This increasing voltage results to a decreasing time constant at the R2/C2 junction”” time constant is fix for R2C2
    Please help.
    Waiting on my E mail or here!!

  • Capacitor

    Plz can hi extend this voltage to 5v regulator power supply

  • roh1290

    hi Ugur Akgoz,
    this circuit is good and i’m looking forward for police siren and ambulance siren pcb layout and circuit diagram plz if u have pl send it to my mail id @ psbr.rohith@gmail.com

    plz help me frnds……….

  • keips

    hi Ugur Akgoz, how can i extend the voltage of this simple siren in to 12 volts? pls reply and send it to my email address,, thanks

  • Ugur Akgoz

    Hi to all.
    This circuit is very very similar to my first project i have made ever at school. It is very recommend to beginners of hobby electronic, the sound is very realistic and the builder has garanteed hours of fun. The relation between resistor, capacitors and frequency is easily learnable and understandable. Similar PNP and NPN transistor will do it also. Its easy to extend the circuit to a 12V / 5 W car siren.
    I recommen this circuit to all.
    (Thanks to this wonderfull website)
    Greetings to all from Ankara

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