This is the electronic siren schematic, built using three ICs: CD4011 NAND gate logic, CD4066 Bilateral Switch and CD4046 Micropower Phase-Locked Loop (PLL). Itis quite simple electronic siren design, low cost and easy to assemble.
With this circuit we can create a changing tone siren. The oscillator IC1A-B consists of two NAND gates logic, oscillating at very low frequency. The oscillation leads the IC2, which is an electronic bilateral switch, which opens and closes in the rate of oscillation. When the switch is closed then the voltage charging the capacitor C3 through the resistor R2. When the switch opens the IC2 C3 discharge through the R3.
The voltage which is charged by C3 regulates the VCO IC3 which is a PLL, so that we have at the output of a changing sound. You can experiment with the values ??of resistors and capacitors to change the sound of the siren. The R1, C1 time control function, the R3, C3 control the change of sound and the R4, C2 control the frequency of sound produced.