Figure 1.  Circuit Diagram for a Siren using 555 Timer IC's

This is a warbler siren circuit that uses two LM555 timer IC's.  A single LM556 IC, which contains two 555 timers inside it, may also be used for this circuit.

Both the 555 timer IC's in this circuit are configured as astable multivibrators, i.e., they both generate a continuous signal of given frequency. 

U2 is configured to have a much higher frequency than U1.  U2 provides the main siren sound to the speaker, while U1 modulates the output of U2 to give it a warbling effect.  What U1 does is to interrupt U2 periodically, causing its otherwise continuous sound output to warble.

The pitch and speed of the siren may be adjusted by varying the values of R5 and R2.  To increase the volume of the siren, an amplifier may be inserted between U2 output and the speaker.

See Also:  The 555 as an Astable Multivibrator

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