Posted on Apr 19, 2012

A pair of timer IC`s are the heart of a circuit that simulates the warbling bee-haw of a British police siren. One of the 555 timers, U2, is wired as an astable multivibrator operating at about 900 Hz. The other, Ul, operates at approximately 1 Hz. Its output at pin 3 is a square wave with a 50% duty cycle-on and off cycles of about 0.5 second each. The output of Ul is applied to pin 5, the control-voltage terminal of U2.

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The frequency of the 555 timer IC is relatively independent of supply voltage, but can be varied over a fairly wide range by applying a variable voltage between pin 5 and ground. When Ul "s output becomes high, U2 operates at about 800 kHz. That switching between two frequencies produces the warbling hee-haw signal.

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