In the event of a sudden blackout at night, this circuit switches on automatically to provide sufficient light for around 30 seconds (extendible), which is enough to switch on an emergency lamp or light up a candle.
The circuit is simple, low-cost and maintenance-free as it uses no battery, bulky transformer or incandescent bulb. It operates off AC voltages ranging from 100V to 300V RMS, which is particularly useful for rural areas where fluctuations in voltage are excessive. The circuit uses a white LED as the light source and an electrolytic capacitor for backup power, which makes it a ‘fit and forget’ circuit requiring no frequent replacement of the battery or bulb.
Capacitors C1 and C2, resistors R1 and R2, and diodes D1 through D4 provide DC power for charging capacitor C4 when AC mains is available. Zener diodes ZD1 and ZD2 regulate the DC supply to 24 volts and capacitor C3 removes the ripples. Transistors T1 and T2 are used to switch on the white LED (LED1) when mains fails. Capacitor C4 is used as a storage capacitor in place of a battery.
When mains is available, capacitor C4 charges to 24 volts through diode D5. Transistor T1 is biased through resistor R3 to hold the base of transistor T2 to ground, switching off the white LED. If AC power fails, transistor T1 will lose its bias and transistor T2 will get biased through resistor R4 to switch on the white LED (LED1). LED1 will glow for a duration depending on the value of capacitor C4 used and then dim gradually.
The time duration for which the white LED glows can be increased by replacing C4 with a higher-value capacitor or connecting another 4700µF, 35V capacitor in parallel with C4 (see the table).
Care must be taken while testing and installing the circuit as it contains live voltage in the power supply section. LED reflectors/holders can be used to focus the light. It is best fitted with unused fan/tube holders at the roof. The mains supply for the circuit should remain switched on all the time.