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.
If the tone is too harsh, a simple R-C filter will remove the harmonic contentâ€” 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.