警察汽笛

This circuit produces a sound similar to the police siren.
It makes use of two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC.
The first IC (left) is wired to work around 1Hz. The 47uF capacitor is charged and discharged periodically and the voltage across it gradually increases and decreases periodically.
This varying voltage modulates the frequency of the 2nd IC. This process repeats and what you hear is the sound remarkably similar to the police siren.

Two presets VR1 and VR2 are provided to vary the siren period of repetition and the tone of the siren.
By varying VR1 you can set how fast the siren changes from high freq. to low freq.
VR2 sets the siren frequency. Adjust VR1 and VR2 to suit your taste.