Power outages often occur unexpectedly, leaving you in total darkness. Whether in a hospital, a theater, or your home, such occasions call for an emergency lighting system that turns on automatically. The preferred emergency light source is the white LED, which is gaining popularity as a compact, power-efficient alternative to the incandescent light bulb (see the figure).

D1 and D2 rectify the ac line voltage, and D3 limits the resulting dc voltage to about 5 V. When ac power is lost, the M1 gate voltage (typically 5 V) goes to zero and turns on M1, which routes power from the battery to the light-detector section.

R2 is a cadmium-sulfide photoresistor that experiences resistance changes from kilohms to megohms when the light intensity changes from daylight to darkness. R1 adjusts the light-level threshold. During darkness, U2's output is logic-high, which powers the timer section based on U3. U3 turns on M2 and M3, which turns on U1 and the LEDs.

At the same time, C1 is charging. When its voltage reaches V_CC/2, the timer expires and shuts off M2, saving battery energy by turning off the LEDs. For C1 = 100 µF, the LEDs remain on for approximately 10 minutes following a power failure during darkness. (Closing S1 turns on the LEDs regardless of the timer interval.)

Leakage current from the battery is about 1 µA when power is available. The capacity of a typical AA-size battery is about 2000 mA-hours, so the standby time at 1 µA is more than 200 years. Once the timer expires during a power outage, standby current for the R1-R2 values shown is about 7 µA. Even at that rate, the standby life is about three times the 10-year shelf life of a typical battery. Current draw from the ac line is about 6 µA, or less than 1 mW at 120 V ac, and current draw from the battery (with LEDs on) is approximately 100 mA.

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