Build Your Own Electronic Siren Circuit Diagram

Description:

An electronic siren fabricated using individual electronic components.

Circuit diagram

Notes:

The generated sound mimics the characteristic ascent and descent of a standard American police siren. Initially, when the circuit is activated, the 10µF capacitors are depleted of charge, and both transistors remain inactive. Upon pressing the push button switch, the 10µF capacitor begins to accumulate charge through the 22kΩ resistor. This voltage is then applied to the base of the BC108B, leading to its gradual activation. Releasing the switch causes the capacitor to discharge via the 100kΩ and 47kΩ base resistors, and the transistor transitions back to an inactive state. Alterations in voltage affect the siren’s frequency. The oscillator’s operation proves somewhat challenging to fully comprehend. As the BC108B transistor engages, its collector voltage decreases, consequently activating the 2N3702 transistor swiftly (less than 1µs). Simultaneously, the 22nF capacitor swiftly absorbs charge as it’s connected between the collector of the 2N3702 and the base of the BC108B, quickly reaching nearly the full supply voltage. The charging current for this capacitor is then significantly reduced, and therefore the collector-emitter voltage of the 2N3072 is enhanced; the collector potential diminishes. This voltage variation is transmitted through the 22nF capacitor to the base of the BC108B, slightly elevating it from saturation. As this occurs, its collector voltage increases, ultimately turning off the 2N3072 transistor more completely. This process continues until both transistors are in an inactive condition. The 22nF capacitor subsequently discharges through the 100kΩ, 22kΩ resistor, the closed push button switch, the 9V battery, the speaker, and the 56Ω resistor. The discharge time takes approximately 5-6 milliseconds. Immediately upon the 22nF capacitor being depleted, the BC108B transistor will reactivate, and the cycle repeats. The variance in voltage at the collector of the BC108B (resulting from the charging 10µF capacitor) alters the siren’s tone. As the 10µF capacitor accumulates charge, the siren's tone rises, while its depletion produces a fall in tone. A 64Ω loudspeaker may be used instead of the 8Ω and 56Ω resistor, and component values may be modified to generate diverse audio effects.