YELPING_SIREN

Published:2009/7/10 1:41:00 Author:May | From:SeekIC

Unijunction transistors Q1 and Q2 are both connected as relaxation-type, sawtooth oscillators. Transistor Q1 is the low-frequency control oscillator and Q2 is the tone generator. Sawtooth waveforms are produced at the emitter terminals. Without R4 connecting the two emitters, each oscillator operates inde-pendently, with its frequency determined mainly by the re time constant. With the values shown, Q1 operates from 1. to 1.5 Hz and Q2 operates from 400 to 500 Hz. When R4 is connected between the two emitters, it couples the low-frequency sawtooth from Q1 directly across capacitor C2. That coupling causes the frequency of the tone generator to increase, along with the rise in sawtooth voltage from Q1.The tone generator's frequency drops to its lower design value when C1 discharges and produces the fall-ing edge of the sawtooth.



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