This 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. The frequency 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 gradually increases.
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 decreases. The presets
VR1 and VR2 should be adjusted for optimum performance.
