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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.