A pair of timer IC's are the heart of a circuit that simulates the warbling hee-haw of a British police siren. One of the 555 timers, U2, is wired as an astable multivibrator operating at about 900Hz. The other, U1, 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 seconds each. The output of U1 is applied to pin 5, the control-voltage terminal of U2. 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 U1's output becomes high, U2 operates at about 800kHz. That switching between two frequencies produces the warbling hee-haw signal.
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