Emergency Light |
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Introduction
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, so 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 in with the decor (sort of).
The 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.
Figure 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 7.5 volts. 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.
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)
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.Part List
12VDC adapter, 200mA, 2.5mm
C1 470µF Electrolytic Capacitor 35V
C2, C3 10µF Tantalum Capacitor
D1 - 1N4001
D2 - 7.5V Zener Diode 1W Iz = 20mA
IC1 - 3.3V low drop out Voltage Regulator TO-220, 3-pin
LED1, LED2, LED3, LED4 - Yellow LED (16,000 mcd, 2.1V, 20mA)
Q1 - 2n3906 PNP Transistor
R1 - 160Ω 1/2 watt resistor
R2 - 1KΩ resistor 1/4 watt
R3, R4, R5, R6 – 56Ω resistor 1/4 watt
Phil Kane has been a technical writer in the software industry for more than 10 years. He has also occasionally authored articles for electronics enthusiast magazines.