Emergency lights are very crucial in some situations, the traditional emergency light uses fluorescent tube as light source hence it provides light upto maximum 2 hours to 3 hours. When we use white LEDs instead of fluorescent tubes it will give light upto 7 hours approximately.
Construction and Working
This circuit constructed with step down transformer (9-0-9V) and step downed AC voltage is rectified by the 1 Amps bridge rectifier (you can also use bridge rectifier module), the LED connected with resistor R1 gives status about output DC voltage from bridge rectifier. By using positive regulator IC 7808 the DC voltage is regulated and this regulated supply fed into light sensing circuit.
The light sensing circuit detects the light by using normal 5mm LDR, this LDR provides low resistance when feels light and gives high resistance to the supply flow when feels dark condition sensing level of LDR can be varied by VR1 10KΩ variable resistor .
During the day light time LDR allows supply through it to the Q1 transistor base hence the Q1 transistor turn ON and connect MOSFET gate terminal to the ground so the LED array disconnected from DC bias ground supply. When dark condition arrives the LDR blocks current flow through it so the Q1 transistor base receives minimum voltage that is not turn ON the transistor. Hence the Q1 transistor remains in OFF condition there is no current flow between collector and emitter that is emitter open. No voltage is received at gate terminal of MOSFET hence the current flow occurs between drain and source terminals, so that LED array glows, the battery (6 volt, 4.5Ah) is connected across the bias when the power failure occurs this battery provides power supply to the circuit.