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February 21, 2012

Retrofitting Old Flashlights With LED's

Flashlight LED Conversion

Flashlight LED Conversion


I have a pair of 14.4 volt flashlights that were part of rechargeable toolsets that go with these NiCd battery packs. Each flashlight had a subminiature bayonet-base incandescent bulb, for which the filament had died after several years of use. When they were working, they were great for lighting for automotive and other repairs, since they could be aimed easily and sat solidly on top of the battery pack base. The obvious thing to do was to modernize them with LED’s, but I had no immediate source of replacement LED-based bulbs for that voltage and socket type. All I could quickly find were LED “bulbs” to fit the more common automotive 1156 socket size. However, I was able to get some unmounted, white, high-brightness 3.2-3.6V LED’s, so I decided to use those to retrofit into the bases of two old bulbs from my flashlights. Four of the LED’s in series per bulb brought me close to the nominal battery voltage.

MagLite and Duracell flashlights, converted to LED

MagLite and Duracell flashlights, converted to LED


The Canadian supplier I used for the LED’s was vendor “DIPMicro” on ebay, and the LED’s were about $0.20 apiece in a pack of 20. To mount the LED’s, I soldered four in series for each 14.4V flashlight, anode to cathode (the cathode is the side with the flat at the LED base), with about 1.5 cm between LED’s, and leaving one anode and one cathode at the end of each string for mounting to the base. Use an alligator clip or similar spring-loaded heat sink near the base of the lead of each LED during soldering to prevent thermal damage to the LED’s. See the end of the post for what I did for the MagLite and the Duracell lamp.
White LED's, pre-soldering

White LED's, pre-soldering


4-LED string, soldered

4-LED string, soldered


To get a bayonet base, I broke away the bulb glass from an old bulb using a pair of pliers (wear safety glasses when doing this in case of flying glass). I soldered the open anode of the 4-LED string to the bulb centre conductor, and the cathode to the bulb base. The V-shaped notch in the bulb base per the photo provided a nice path for getting the cathode lead past the circular hole in the flashlight mirror.
Edit March 5, 2012: One should add a series resistor to protect the LED’s in a usage such as this one. Please see the comments from user ‘Pennhaven’ below and links within for more details. Thanks, Pennhaven, for the great information.
Old bulb, pre-conversion

Old bulb, pre-conversion


I used insulation stripped from old wire to insulate the LED string anode and cathode leads from the base. After assembly, I bent the LED string so as to have all 4 LED’s essentially facing forward just in front of the reflector. The end result was a flashlight of comparable brightness to the original, but with longer battery and lamp life. The new version is also less susceptible to bulb damage from mechanical shock.
The Converted Flashlight

The Converted Flashlight


Had I needed more intensity, I could have added another string of 4 LED’s, paralleled with the first string, but that would have been tough to cram into the available space.
Edit 2012 03 28: since I had some LED’s left over, I also converted a few smaller old bulb flashlights to use LED’s: a ‘MagLite’ that uses 2 AAA cells and a Duracell flashlight that uses 2 AA’s. In the case of the MagLight, I found I could just trim the LED leads and get it to fit in the existing pin sockets. I did have to drill out the hole in the reflector to get the LED to fit through it, but the plastic there drills easily. Works great, no soldering required, and has focussing too. In the case of the Duracell lamp, I found I could cram in 3 LED’s in parallel and got quite a bright light. In that case, I soldered the 3 LED’s onto an old bulb socket much like I had done above with the 14.4V lamp.
MagLite and Duracell flashlights, converted to LED

MagLite and Duracell flashlights, converted to LED


MagLite and Duracell flashlights, converted to LED

MagLite and Duracell flashlights, converted to LED


Due to the 3.2V nominal voltage for those 2 flashlights, which is well below the max rating of the LED’s, I found I had no need of series resistors.

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