Various siren circuit is already published in this website. Here is a simple circuit of siren build around single chip CD4011 (IC1). Siren circuit published in this website is so simple utilization readily available component The four independent 2-input NAND gates of IC1 two each gates configured as low frequency and high frequency oscillator.
Diode D2 conduct when output from low frequency oscillator is high and this conduction diode D2 enable high frequency oscillator. The value of capacitor C3 can be changed to 8 µF in order to vary the oscillator speed. Resistor R1 and capacitor C1 is used as tone changer components. Sudden decaying of the output when diode D1 ceases to conduct is prevented by capacitor C2. Transistor T1 is used as speaker driver transistor.
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 150 KΩ
R2 = 500 KΩ
C1 = 0.015 µF
C2 = 0.22 µF
C3 = 0.5 µF
C4 = 10 pF
IC1 = CD4011
T1 = BEL187
D1 = 1N4148
D2 = 1N4001
LS1 = 8Ω speaker
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