Posted on May 3, 2012

The wailing sound of a siren is generated by a VFO consisting of Q1 and Q2. Capacitor C2 provides the feedback for the oscillator. The frequency of the oscillator is varied by the voltage applied to the base of Q1 through R3. When switch S1 is closed, capacitor Cl charges, thus increasing the oscillator frequency. When S1 is released, capacitor {;1 discharges, and the oscillator frequency decreases.

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Capacitor C3limits the maximum oscillator frequency. The average battery current drain is about 15 mA.

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