This is a siren sound generator circuit. Which is less expensive and can create simple. The IC1 is using CD4011 (Digital Nand Gate) and the IC2 is CD4046.
Operation of the circuit, IC1 is a clock frequency 1 Hz.(Frequency adjusted by the value of R1 and C1.) Then it pluse IC1 regtifier through diode D1 to charge slowly with the C2 control of R3.
Then the C2 signal sends to IC2 at pin 9 (Control voltage input). For IC2, which acts VCO (Voltage Conrolled Oscillator).The frequency of the output voltage control. The frequency is changed by voltage and out to the speakers. The VR1 is the resistance adjustment and C3 affect the mean frequency of the circuit.If you turn off the switch S2 is not a siren sound, but it is another voice. And use of CMOS circuits to make very low watts of power, but also be useful in combination with other controls.
Relate circuits link (sometime you may use it)
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