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Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 3444
Maxim > Design Support > Technical Documents > Application Notes > Display Drivers > APP 3444
Keywords: Emergency lighting, white LEDs, light detection, photoresistor, comparators
APPLICATION NOTE 3444
Light Is On When Power Is Out
Feb 16, 2005
Abstract: A loss of AC power during darkness (sensed by a photoresistor) turns on an array of battery-
powered white LEDs. To conserve power, a timer turns off the LEDs after a particular period of time (for
example, 10 minutes).
Power outages can occur when least expected, 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
with loss of power. The preferred emergency light source is the white LED, which is gaining popularity as
a compact, power-efficient alternative to the incandescent light bulb (Figure 1).
(Note: Use safety precautions when working with the high-voltage AC line.)
Figure 1. When AC power is lost, this emergency-lighting system automatically provides 10 minutes of
illumination from a string of white LEDs.
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