Here the circuit diagram of electronic siren based NE555. This circuit produces a sound like factory siren. It applies a 555 timer IC which is utilized as an astable multivibrator of a center frequency of about 300Hz. The frequency is controlled by the pin 5 of the IC. When the power supply is switched ON, the capacitor charges slowly and this alters the voltage at pin 5 of the IC therefore the frequenct gradually increases.

When the capacitor is totally 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 needs to be adjusted for optimum capabilities.

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