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
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.