This circuit produces a warble sound and may be used for alarm sirens or for producing unique testing tones.
The circuit is based on two 555 timers. The first one, the U1, operates as an astable multivibrator at about 10Hz. Its rectangular output is filtered by the R3, R4 and C3 filter, to produce a triangular wave which modulates an 1KHz oscillator based on the second 555 in astable mode.
The exact frequency of the second oscillator or the tone deviation from the first oscillator can be easily modified to produce almost any type of warble sound. The output may be amplified by an audio amplifier to any required power level.