As hurricane Sandy was nearing the New York area, I gathered up all of our flashlights including the cheap giveaway ones like this Mets flashlight. I made sure they all had batteries, but one flashlight had a bad bulb even though I had plenty of C batteries to use in it. I thought of a way to make my own LED replacement bulb using the base of the burnt out bulb, some LEDs I salvaged from some yard lights and a corner of a PC board.
Step 1: Yard Light
When we bought our house, there were a bunch of these lights all over the yard. Only one of them still worked, so I tried to fix the rest of them. It seemed like they wouldn't charge, and I guessed it was because the plastic over the solar cells had become hazy and light wouldn't shine through anymore. I tried sanding the plastic, but it turned out to be a thin coat of resin that the cells were set in. A little rubbing with some fine sandpaper went right down to the cells and they were useless. The haziness never cleared up, and the cells were catching on the sandpaper. Luckily, I was able to salvage the LEDs and a few of the rechargeable batteries. The rest of the parts went to the recycling center.
Step 2: Burnt-Out Bulb
I had plenty of flashlights, but this one had a burnt-out bulb. I had plenty of batteries, but that wouldn't make this light work. I figured I could use the base of the bulb and make my own LED replacement. Originally, I thought I would use the 2 prongs that electrify the filament but as it turns out, the solder doesn't like to stick to these. I broke the glass gently with some pliers, keeping the filament leads intact, but eventually I pulled the leads out of the base because they were too delicate.
I used the continuity setting on my multimeter while I was still planning to use the filament leads, this isn't really necessary.
Step 3: LED Board
I took the bright white LEDs that I recovered from the yard lights and I soldered them to a PC board in a triangle pattern. I soldered them in parallel since they are already a bit underpowered with only the 3v coming from two C batteries. Since they are underpowered, a single bulb wouldn't put out much light at all. I used the 3 LEDs to increase the light output with the low voltage.
I like to use this UTP CAT 5 cable because it's incredibly cheap. I have a big spool of it and it's great for low power electrical work. I use it a lot in breadboards and with my Arduino boards because it only has a single 24 gauge wire strand.
Once the 3 positive leads were soldered to the PC board, I cut some short wires and soldered each of the negative leads to the next one. I then soldered longer leads to the centers of the positive and negative leads.
The bulb is supposed to fit into the dome from the bottom, but my LED board won't fit so I had to solder the leads from the LED board to the light bulb base through the cone of the flashlight. Solder the negative lead to the side of the bulb and the positive lead to the tip of the bulb. The flashlight has a washer collar that fits over the bulb and makes contact with the switch. I had to make sure this would still make good contact with my modified bulb since the wires and the solder widened the base by a little.
Step 4: Success!
The flashlight works again, and though its a little dim for regular use, I think it has a few improvements over the original. First, an LED is more resilient than a bulb. You could now drop this flashlight and not worry about breaking the bulb, but the case may be a different story. Second, LEDs are much more efficient than an incandescent bulb. This flashlight will now last for much longer and in an emergency it makes an ideal flashlight for extended use.