This circuit is a siren with manual modulation that, instead of exciting a speaker, excites high-performance piezoelectric buzzers or even piezoelectric tweeters (without the internal transformer). The use of a transformer allows to obtain a high voltage signal capable of providing much higher performance for the indicated transducers. The transformer has a primary winding of 110/220 V and a secondary winding of 6 + 6 V, with currents from 100 to 250 mA. The effect adjustment is obtained in the 100 k ohms trimpot. When we press S1, capacitor C1 charges, which causes the oscillator to start operating with the production of an increasingly high-pitched sound. When we release S1 the capacitor discharges and the sound become more and more serious until it stops completely. Capacitors C1 and C2 are electrolytic for voltages slightly higher than those used in power, and C3 and can be either polyester or ceramic p. The value of C3 can be changed in the range from 22 nF to 220 nF, to find the one that provides a range of variations coinciding with that of the transducer used has maximum performance.
