Intro: Joule Thief in a Floss Case
Yes, yet another joule thief instructable. Everybody knows how to make one of these, so I won't be explaining the circuit in too much detail. Most of the instructables I've seen deal mostly with that. They usually just stop there. For me, a project isn't finished until I've put it in an enclosure, so that's what this is about. I will; however, put in my 2 cents worth concerning the electronics. When I first tried to build a joule thief, I failed miserably. Recently, I discovered that the problem was with the inductor. For the first try, I just grabbed the smallest toroid I could find, which happened to be a yellow one I pulled out of a power supply. Then I just wound it with whatever wire I had lying around. Everybody had their recipe - 12 turns of this, 20 turns of that. Nothing worked. The problem is that not all toroids are created equal. They're even color coded. It turns out the only ones that work well are the black metallic ferrite ones. I made another one out of magnet wire and a real ferrite toroid that still didn't work. When I finally made an L/C meter (which can measure inductance) I decided to test those inductors I made. The little yellow one turned out to be around 27 microhenrys per side. The reading on the ferrite one was over 900 microhenrys per side. After researching the subject, I found out that 300 microhenrys per winding is the optimal amount. The yellow toroid could never go that high, no matter how much wire I used, but after removing some turns from the ferrite one, the circuit finally worked. In this instructable I'm using a small, bead type, ferrite toroid with about 14 turns (per winding = 28 total) of .4mm diameter magnet wire that I pulled out of an old CRT computer monitor. The important thing is that it has an inductance of just under 300 microhenrys per side, and boy does it work well. With a practically dead battery (just under 1 volt) The LED is much brighter than my store-bought, key chain light using its expensive 3v button cell. I'm using the cheapest and simplest joule thief circuit with no diodes and no capacitors. I used the oldest, beat up, transistor I could find. It's even missing most of the middle leg, but that doesn't matter. The rest of the parts are just whatever I had lying around and some dead batteries. And now, finally, to the instructable ;-)
Step 1: The Case
I saved my empty dental floss case with this project in mind. It turns out that a AAA battery just fits inside. It's also made out of durable, pliable material. The first thing I did was to pull out the spool and take off the floss cutter. I was thinking about using that to connect one side of the battery since it fit perfectly in a corner slot on the inside. Unfortunately, I couldn't really make it work. It turns out that every millimeter counts here. I decided to use a spring that I salvaged from a broken battery holder and a screw. It would have been better if it had been a flush type instead of the raised one I ended up with, but that's all I could find. After I soldered a solid core wire to the spring, I held the battery up to see where the positive side would go and I drilled a hole there. There was absolutely no space left over. The first screw I used was made out of a metal that just wouldn't stick to solder, no matter how much I filed and sanded. I stuck a bit of solder to the end of the one I ended up using while it was not screwed into the plastic. I didn't want to melt anything. I soldered the red stranded wire onto that as quickly as possible after it was screwed in. The reason I used solid core for the negative was to give it a bit of rigidity so the spring wouldn't pop out. I'm using no hot glue for this project. With the battery mounted, it was time for the joule thief.
Step 2: Mount the Electronics, LED and Switch
My joule thief fits into a dental floss case with plenty of space left over. I guess the only limitations are battery size and the toroid. My joule thief actually overdrives the LED a bit if the battery is around 1v, so I'm expecting to have to replace the LED sometime in the future. I drilled a 5mm hole for the LED in the lower right corner of the case (if you're looking at it from the front). It's a fairly tight fit and the rest of the electronics hold it in place well enough. Again, there's no real leeway for errors there, so go slowly. I ended up drilling just a tiny bit too high, but it still fits fairly tight. By the way, I clipped off a few protruding plastic parts from the inside to make room for the battery. The last thing I did was to mount the switch. I didn't want to have to remove and replace the battery to turn the thing on and off. I found a tiny switch in my junk box that I salvaged from an old radio or something. I drilled two 1mm holes and mounted it on the outside. I carefully and quickly soldered the positive wire to the switch on the inside. This holds the switch in nice and snug. My switch is momentary, which means I must hold it down to keep the light on. You may want to use a locking switch if you don't want to do that. I guess that just about raps it up. The joule thief also works great with those cells that you can rip out of an old 9v battery. That's what I'm using in it right now.