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Yelping siren circuit

2017-08-15 21:07  
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A siren is a loud noise making device. Most modern ones are civil defense or air raid sirens, tornado sirens, nuclear test sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types: pneumaticand electronic.Unijunction transistors Ql and Q2 are both connected as relaxation-type, sawtooth oscillators. Transistor Ql 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 independently, with its frequency determined mainly by the rc time constant. With the values shown, Ql 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 Ql directly across capacitor C2. That coupling causes the frequency of the tone generator to increase, along with the rise in sawtooth voltage from Ql. The tone generator`s frequency drops to its lower design value when Cl discharges and produces the falling edge of the sawtooth.