Emergency Lamps is one of those circuits which are fairly commercialized and are also popular in the DIY section. Keeping aside the feature of finished product and simple use, DIY circuits provide flexibility measures to circuit designing.
DIY LED Circuit
A typical commercialized LED Circuit
Here is a High efficiency Emergency Lamp using High bright White LEDs. It automatically turns on when power fails and turns off when power resumes. It is powered by a 6 volt 4.5 Ah rechargeable battery which can power the LEDs for a long period.
The battery charger section comprises a 0-9 volt 500 mA step-down transformer, a full wave bridge rectifier (D1 through D4), a 470 uF (C1) smoothing capacitor and 7808 voltage regulator IC. IC1 provides regulated 8 volt DC for charging the battery.
LED driver circuit uses a medium power PNP transistor BD 140 which acts as an automatic switch. When the mains power is available, battery gets charging voltage and current through D5 and R1. At the same time, unregulated voltage from the bridge rectifier passes through R2 to the base of T1 to inhibit its conduction. So both the LEDs remain off. When the power fails, T1 forward biases and the LEDs turn ON. If the brightness of LEDs is low, use 22 ohms 1w resistor as R3 and R4.