Ken Yang was working at Maxim when he submitted this Design Brief. (He's since moved to the power group at a network equipment company.) The circuit turns on emergency lighting (in the form of a string of white LEDs) and turns them off again after about 10 minutes. The objective is to give personnel time to safely exit the building when a power outage occurs and then to shut off the emergency light to conserve battery power.
Yet unlike the emergency lights most of us are used to, with their incandescent floodlamps and big gel-cell batteries, this circuit runs on just two alkaline AAs and Ken's design. Okay, there are some building-code issues here too. But as a starting point for a marketable design, a lot of readers thought Ken was on the right track.
Here's the Design Brief:EMERGENCY LIGHT TURNS WHITE LEDs ON WHEN POWER GOES OUT 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 VCC/2, the timer expires and shuts off M2, saving battery energy by turning off the LEDs. For C1 = 100µ F, 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 A when power is available. The capacity of a typical AA-size battery is about 2000 mA-hours, so the standby time at 1 A is more than 200 years. Once the timer expires during a power outage, standby current for the R1-R2 values shown is about 7 A. 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 A, or less than 1 mW at 120 V ac, and current draw from the battery (with LEDs on) is approximately 100 mA.