LED Emergency Light Project

LED Light Project Inspired by a Power Outage

By Phil Kane

This project was inspired by a power outage we experienced at home one summer. I spent the first few minutes fumbling around in the dark trying to find the flashlights I had stored (somewhere) for just this event. Then after waiting around for another 20 minutes, we realized it might be a while before the power returned. We fumbled around with flashlights trying to find the candles we had stored (somewhere) for just this purpose. Finally, we found the candles and matches (fortunately they were together) and settled in for the duration.

Looking for any excuse to begin another project, I decided what we needed was an emergency light that – in the event of another power failure – would turn on automatically. I wanted something that would be relatively quick and inexpensive to build, preferably with parts that I had on-hand. Since we tend to congregate in the living room, it had to fit with the decor (sort of).

The Emergency Light Solution

I came up with a simple solution (Fig. 1) that also doubles as a "decorative" living room ornament. It just required a DC adapter, a battery, a few electronic components, some yellow LEDs and a candle warmer.

Part List

Component Name Part Description Mfr. Part No.
12VDC Adapter, 750mA, 2.5mm AMS8-1200750SU
C1 470µF Electrolytic Capacitor 35V R470/35-VP-R
C2, C3 10µF Tantalum Capacitor TM10
D1 50V, 1A Silicon Rectifier Diode 1N4001
D2 7.5V Zener Diode 1W Iz = 20mA
IC1 3.3V Low Dropout Voltage Regulator TO-220, 3-pin LM1117T-3.3/LD1117V33
LED1, LED2, LED3, LED4 Ultra Yellow Water Clear T1-3/4 LED RL50-PY543
Q1 2n3906 PNP Transistor 2N3906
R1 160Ω 1/2 Watt Resistor CF1/2W151JRC
R2 1kΩ Resistor 1/4 Watt CF1/4W102JRC
R3, R4, R5, R6 56Ω Resistor 1/4 Watt CF1/4W560JRC


Circuit Operation
Fig. 1

Circuit Operation

The 12 volt DC adapter is the main power state sensor. PNP Transistor Q1 acts as an on/off switch for battery BAT1. When main power is on, the output from the adapter will cause the base of Q1 to be high relative to the emitter. As a result, Q1 will be off. Zener diode D2 ensures the voltage to the base of Q1 does not rise above 7.5V. If main power fails, the base of Q1 will be pulled low by resistor R2. This turns Q1 on and powers the 3.3V regulator IC1. The voltage regulator provides constant current to the LEDs until the battery voltage reaches IC1s dropout voltage. Switch S1 (optional) disables the lighting system when not needed.

ConstructionFig. 2

Construction

I built the entire circuit on a piece of perf board (Fig. 2) using point-to-point wiring. To make the system compact I used AA batteries in a 4-cell holder. The whole assembly was placed in the candle warmer (Fig. 3).

EnhancementsFig. 3

Enhancements

There are a number of enhancements you can make. You can replace the linear LDO regulator with a more efficient switch-mode regulator. Also, it might be preferable to use white LEDs. They operate at a higher forward voltage (3.3V), so you will need to use a regulator with a higher output voltage. You might also consider adding a circuit that will automatically turn-off the LEDs when a specified level of ambient light is reached. I intend to incorporate all of these enhancements in the next version of my own emergency light.

Emergency LightCompleted Emergency Light

For almost two decades, Phil Kane has been a technical writer in the software industry and occasionally authored articles for electronics enthusiast magazines. He has a bachelor's in Electronics Engineering Technology with a minor in Computer Science. Phil has had a life-long interest in science, electronics and space exploration. He enjoys designing and building electronic gadgets, and would very much like to see at least one of those gadgets on its way to the moon or Mars one day.
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