This circuit produces a sound similar to a factory siren. Uses 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 IC. When supply is turned on, the capacitor charges slowly and this alters the voltage on pin 5 of IC so the frequenct increases gradually. After the capacitor is fully charged, the frequency no longer increases. Now when the switch button siren control is pressed, the capacitor discharges and the siren frequency also decreases. The presets VR1 and VR2 should be adjusted for optimum performance.