""Hi-Fi"" Siren

7-54 — elektor july/august 1981 "1rli-fi" siren The title "siren" above a circuit diagram usually implies a circuit for a two-tone or multi-tone "horn" which sounds continuously and can be used for an alarm of one sort or another. However, the siren described here is not particularly suitable for that application, but no doubt inventive readers will find sound. The current, and therefore the noise, continues to increase gradually due to the falling voltage across C4. At a certain moment this voltage reaches the negative trigger threshold of N4, so that the flipflop is reset and the output of N4 goes high. At this n particular moment the voltage on the emitter of T5 is at a minimum and the noise level is at a maximum. As the output of N4 has now gone low, the fre- quency of the oscillator constructed around N2 is lowered somewhat, thereby simulating the Doppler effect. Capacitor C4 will then slowly recharge up to the supply voltage level, leading to a gradual . very slowly at fir...