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An LED conversion makes a flashlight more stout and radically improves battery and bulb life.

\fter using up countless flashlight J A \ batteries and replacing way

T\ 3too many bulbs, I decided that something had to be done. I read about the flashlight conversion in HP84, and decided to do some research into LEDs myself.

I was using Maglite brand Mini Maglite flashlights with two AA batteries in stock configuration. The standard bulbs are not shock resistant, which meant frequent replacements at about US$4 each. And they used a lot of energy, which led to short battery life.

Selection & Testing

My first problem was to identify and then locate a suitable white LED. After removing the miniature bulbs from all of the Maglite-type flashlights that I owned, I carefully checked their size against the LEDs available through Radio Shack. I soon discovered that a 3 mm white LED would be ideal, but none could be found that had enough light output for me. I opted for the somewhat larger 5 mm LED. It is Radio Shack #273320, and is rated at 3.6 V, 20 mA at 1,100 mcd output.

I devised a crude light meter using my Micronta 22-175 multimeter connected to an old photo resistor. The resistor changes resistance with changes in light intensity—the higher the light intensity, the lower the resistance in the photo resistor. I measured the resistance of the photo resistor using the various bulbs and voltages to compare factory bulb output with the LED bulbs. In this way, I could crudely quantify changes in light output.

I determined that the best value with a new conventional bulb and two fresh AA batteries was about 130 ohms on my light meter. To determine if the LED could just replace the standard lamp, I trimmed the leads off to about 3/8 inch (10 mm) long, and carefully inserted them into the lamp holder. Using the same batteries, the LED produced a value of 192 ohms, and it was not very bright.

I decided to see if it could handle 4.5 V. I turned my variable power supply up until the no-load output was 4.7 V, and placed my new LED's terminals across the supply lugs. It worked! After being left on continuously for more than an hour, the output value on my light meter had not changed—it was about 98 ohms.

The LED registered fewer ohms on the photo resistor, indicating a higher light output, than in the original bulb configuration, but the light output was range sensitive to a higher degree. It dropped off more rapidly for the LED versions with increasing distance from the photo resistor. I discovered, after sacrificing a couple of LEDs, that the voltage threshold was about 5.5 VDC. Above that voltage, the LED died quickly.

I am admittedly pushing the voltage higher than other LED applications. But in my tests, the LEDs lasted for more than an hour at 5.5 VDC, so operating at 4.5 VDC should not cause any significant problems. The high voltage condition only occurs with fresh batteries. After

Mini Maglite Exploded View
An exploded view: barrel padding, flashlight, LED, bulb holder, base, batteries, screw, and spring.

some use, the voltage begins to drop into a more normal range for the LED.

My LED now has hundreds of hours on it with no signs of output reduction or color shift, so I would guestimate bulb life of at least 500 hours, and possibly as many as 20,000 hours (an 80% reduction in life expectancy compared to an operating voltage of 3.5 VDC).

The real issue for me is reliability. The stock lamps in these lights last only tens of hours, while my LED conversion has already lasted hundreds of hours. So even if it were to fail "prematurely" compared to other LEDs operating at lower voltages, it would still have outlived the factory counterpart by an order of magnitude.

Assembly & Batteries

When I attempted to screw the lens assembly back onto the body of the flashlight, I quickly discovered that I had overlooked a very important item—the hole in the reflector was way too small. I decided to enlarge the diameter of the existing hole using a drill bit. I needed to make the hole large enough not only to pass the narrow part of the LED, but also the larger collar that is at its base. After some trial and error on a small sample of 1.5 mm styrene plastic sheet, I determined that a 1/4 inch (6 mm) hole would work.

After the hole in the reflector was enlarged to 1/4 inch, and the reflector replaced on the light body, it was time for the battery question. It just so happens that common 1.5 V calculator batteries (N cells) are slightly smaller in diameter, but significantly shorter than the standard AA batteries my flashlight was designed for. This allowed me to use three, 1.5 V batteries to achieve 4.5 VDC total.

I separated the clear plastic product covering from the LED packaging, and used the paper backer to form a tube to place in the barrel of the flashlight. This made up the difference in diameter between the smaller calculator batteries and the flashlight barrel designed for larger AA batteries. I cut the tube short so it would not make contact with the threaded end of the flashlight body when installed.

I carefully inserted the three new batteries into the flashlight and discovered that I was about 3/4 inch (19 mm) short of the grounding spring in the end cap. I quickly determined that a small wood screw would be an excellent "low buck" solution, and it was. Later it became apparent that a single 3/s inch (10 mm) diameter spring about 2 inches (5 cm) long would be a better solution, and that is what I use now.

A closer look at the LED, bulb holder, and base.

A closer look at the LED, bulb holder, and base.

Maglite Parts List
LED Flashlight Conversion Parts List



Cost (US$)

1 LED, 5 mm

Radio Shack #276-320


3 N cells, 1.5 V

Radio Shack #23-023

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DIY Battery Repair

DIY Battery Repair

You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.

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  • luciana
    How to insert battery into mini maglite?
    9 years ago

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