Posted on Apr 20, 2012

The circuit uses two gates of a 7400 IC cross-connected to form an astable multivibrator driven by the 1-pulse per second output of the digital clock IC. The hee-haw circuit has a low frequency astable modulator added to make a self-contained European-type siren. Tone and rate can be varied as desired by changing capacitor values.

Click here to download the full size of the above Circuit.

If the tone is too harsh, a simple R-C filter will remove the harmonic content— the multivibrator output is almost a square wave. With the resistor values shown, no startup problems occur; but if the 2.2 k or 2.7 k resistors are changed too much, latch-up can be a problem.

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