A white light LED power outage emergency light

Unexpected power outages often plunge people into complete darkness. Whether it is a hospital, a theater or a home, this situation requires an emergency lighting system that can be activated automatically. The preferred emergency lighting source is white light LED, which is gaining popularity as a compact and highly efficient alternative to incandescent lamps (see figure).

D1 and D2 rectify the AC voltage, and D3 limits the resulting DC voltage to about 5 V. When the AC power disappears, the voltage on the gate of M1 (usually 5 V) drops to 0 and turns on M1, which sends power from the battery to the photodetector section.

R2 is a cadmium sulfide photoresistor that varies from kilo-ohms to mega-ohms over the range of light levels from day to night. R1 is used to adjust the light threshold. When the light is dim, U2 outputs a logic high and powers the timer section based on U3. U3 turns on M2 and M3, which turns on U1 and the LED.

At the same time, the circuit charges C1. When the voltage on C1 reaches V _CC /2, the timer expires and M2 is turned off, thereby turning off the LED to save battery energy. If C1 is 100µF, the LED will remain on for about 10 minutes after a power outage in the dark. If you do not want a time limit, you can turn on the LED by closing S1.

The battery leakage current is about 1µA when powered normally. A typical AA battery has a capacity of about 2000mA-hours, which gives a standby time of over 200 years at 1µA leakage current. During operation with power off and the timer expired, the standby current is about 7µA for the R1-R2 values ​​shown, and even at this leakage rate the standby life is more than three times the typical battery shelf life of 10 years. The current drawn from the AC source is about 6µA, or less than 1mW at 120V AC, and the current drawn from the battery is about 100mA (when the LED is on).