circuit produces a sound similar to a factory siren.
It makes use of a 555 timer Ic used as an astable multivibrator
of a center frequency of about 300Hz.
is controlled by the pin 5 of the IC. When the supply
is switched ON, the capacitor charges slowly and this
alters the voltage at pin 5 of the IC hence the frequenct
After the capacitor is fully
charged, the frequency no longer increases. Now when
the push button siren control switch is held depressed,
the capacitor discharges and the siren frequency also
The presets VR1 and VR2 should be adjusted
for optimum performance.