This alarm siren circuit produce warbling sound, you can use it in your toys or in security alarm. The circuit use two 555 IC oscillator. In this alarm siren circuit, the first oscillator is employed to produce audio frequency. The second oscillator is employed to produce a modulating signal. This modulating signal make this alarm siren generates warbling sound. Here is the schematic diagram of the circuit:
You can try to change the capacitor C1 to change the warbling frequency. Using the component shown in the schematic diagram, this alarm siren gives 6 seconds warbling period. You can lower the C1 capacitor value to get higher warbling frequency, for example, you can try some values between 10 uF to 47 uF to get the suitable effect. This alarm siren circuit works with 5-15V power supply, but note that the 555 IC would suffer a significant self-heating when you use 12 volts or higher, and this may cause a shorter lifetime for the 555 IC chip. ERRATA: Q1 should be 2N3906, NOT 2N3904 (thanks to Nils Nelson for building and testing this circuit).