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.