Posted on Feb 7, 2014

An adaptable siren generator circuit with a multitude of uses is shown. It is based around a 556 twin-timer chip, IC1. An audio tone is created by one timer section and is directly coupled to the driver transistor, TR1. The other half of the timer is used to modulate the frequency of the audio tone using the CONTROL VOLTAGE pin terminal of the audio-oscillator section, pin 11.

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(NE556 TIP41A)  If capacitor C2 is omitted from the circuit, a twin-tone alarm generator will be created. However, with C2 in place as shown, a "wailing" tone is produced. Capacitor C1 governs the rate of tone change and LED D1 flashes for extra effect. A waterproof Mylar-coned 8-Ω loudspeaker was used for LS1, and the volume can be adjusted for different impedances by altering the value of resistor R5. The circuit operates from approximately a 9-V rail.

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