Unexpected temporary blackouts often plunge people into complete darkness. Whether it is a hospital, theater or home, this situation requires an automatic emergency lighting system. The preferred emergency lighting source is white LED, which is becoming popular as a compact and efficient alternative to incandescent lamps.
D1 and D2 realize the rectification of AC voltage, and D3 limits the obtained DC voltage to about 5V. When the AC power disappears, the voltage on the M1 gate (usually 5V) drops to 0 and turns on M1, which sends the power from the battery to the photodetector section.
R2 is a cadmium sulfide photoresist, which changes from kiloohm level to megaohm level in the variation range of brightness from day to night. R1 is used to adjust the brightness threshold. When the light is dim, U2 outputs a logic high level and supplies power to the U3 based timer section. U3 turns on M2 and M3, and M2 and M3 turn on U1 and led.
At the same time, the circuit charges C1. When the voltage on C1 reaches VCC / 2, the timer terminates and turns off m2, and then turns off the led to save battery energy. If C1 is 100 µ F, the LED will remain for about 10 minutes after blackout. If you don’t want to limit the time, you can turn on the led by closing S1.
Under normal power supply, the leakage current of the battery is about 1 µ a. A typical AA battery has a capacity of about 2000mAh and a standby time of more than 200 years when the leakage current is 1 µ a. During the power failure and the timer has been terminated, the standby current is about 7 µ a for the R1-R2 value shown in the figure. Even calculated according to this leakage rate, the standby life is more than 3 times of the 10-year storage life of a typical battery. The current pulled from the AC power supply is about 6 µ a, or the current is less than 1MW at 120V AC voltage, while the current pulled from the battery is about 100mA (when the LED is on).
Source; International LED network