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Simple Siren
By: Thomas Scarborough,
Cape Town, South Africa
EPE June 04
THE circuit shown in Fig.4 provides a smooth, piercing, wailing siren with a minimum of components. Not only this, but three spare gates of hex inverter IC1 remain, which means that a true cacophony could be created by running two sirens off the same i.c. Gate IC1a is configured as a slow oscillator which repeatedly charges and discharges capacitor C1. The charge on C1 is used to control the conductance of power MOSFET TR1, which in turn modifies the frequency of the audio oscillator formed around IC1b. IC1c serves as a buffer. The period of the siren is determined by C1 and resistor R1, and its frequency by C2 and R2.
Fig.4. Circuit diagram for a Simple Timer.
Ideally, C2 and R2 will be selected to find the resonant frequency of the piezo sounder for maximum volume. If a piezo tweeter is used, the Simple Siren will produce an impressive volume. An inductive sounder (e.g. a speaker) may be used if a capacitor (e.g. 100F) is wired in series with it.