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2N3702 Electronic siren

2017-08-16 08:51  
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An electronic siren made from discrete components. A push button switch is used to create the sound, which rises in pitch when the switch is pressed, and falls in pitch when the switch is released.This is a compact electronic siren circuit based on three transistors.This circuit is suitable for in corporating with other alarm or siren projects such as burglar alarms, automatic factory sirens etc or a simple push to on alarm.
The wailing sound of a siren is generated by a VFO  consisting of Q1 and Q2. Capacitor C2 provides the feedback for the oscillator. The frequency of the oscillator is varied by the voltage applied to the base of Q1 through R3. When switch S1 is closed, capacitor Cl charges, thus increasing the oscillator frequency. When S1 is released, capacitor C1 discharges, and the oscillator frequency decreases.Capacitor C3limits the maximum oscillator frequency. The average battery current drain is about 15 mA.Oscillator action is as follows. As the BC108B transistor switches on its collector voltage falls and so the  2N3702  transistor is switched on. This happens very quickly ( less than 1us). The 22n capacitor will charge very quickly as well.