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Two relays are normally used for switching the emergency light but this Circuit does not use any relay or transformer. Switching action is performed in this circuit by two transistors Tl and T2 which are used here as electronic relays, Capacitor Cl has been used as a series dropper and the diode Dl rectifier this supply. Capacitor C2 filters there rectified voltages and the zener Diode D3 keeps the supply stable at 7.5 volts, Transistor Tl is of PNP type and it does not con­duct when positive supply is present at its base. Transistor T2 does not work when T1 is non-conducting. In this way both of these transistors remain non-conducting when the mains AC Line is present. In this stage the battery is kept charging through the diode D2. When the AC supply fails then negative supply of the battery reaches the base of the transistor Tl through the resistance R3(1K) and this transistor switches ON. Transistor T2 also starts conducting and the bulb connected to its collector lights up. Now, if the mains AC supply is restored then the bulb indicates the presence of AC main supply. It is necessary to use a heat sink with the transistor T2. The bulb used in the circuit is 6V/350mA and the two on/off switches work together.


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