Electrically Operated Siren by 2N3702 and BC108B

Electrically Operated Siren by 2N3702 and BC108B

Circuit Project Description

The circuit was designed with just one circuit element wherein the tone of the siren varies with the actions of a push button switch.

Oscillator – an electronic circuit that produces a time varying or repetitive electronic signal, sine wave or square wave,  without an external input signal using positive feedback Circuit Explanation

As the circuit starts to function by switching it on, it will allow the capacitor to discharge the stored supply while deactivating the transistors. Apparently, it will charge again as the push button switch closes the circuit allowing the voltage to run across the 22K ohm resistor which will be directed to the base of BC108B transistor. This transistor is capable of driving a siren and is readily available with a safety factor of 1.5. It gradually turns on when voltage is applied then gradually turns off as the capacitor discharges.

The frequency of the siren alters as it experiences a change in voltage. This is where oscillation occurs. The second transistor used was 2N3702 which was designed for use as general purpose amplifier and switch requiring collector current 300mA. Between the two transistors lies the capacitor that gets full supply voltage very fast. The collector emitter voltage of 2N3702 increases as the capacitor discharges. When the collector potential drops, the 22nF capacitor absorbs the change and affects the base of BC108B triggering the saturation gradually.The rise of the BC108B voltage turns off the 2N3702. This process continues until both transistors are off. The discharge of the capacitor happens within 6 secs which immediately turns on the BC108B transistor.

The siren’s tone changes due to the difference in the collector voltage of BC108B, due to the discharging (low tone) and charging (high tone) of the 10uF capacitor. To produce a different sound effect of the output, the values of the loudspeaker can be modified.


Electronic sirens are commonly used in public alert systems for providing sound to cities and municipalities, to the plants and locations with potentially dangerous materials, open spaces, sport arenas and halls, and for large warning systems for informing the public in the case of emergency situations. In addition to the uses, these sirens are commonly used on public and government vehicles such as police car, ambulance, fire trucks, etc…


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