It's REALLY important normally to not connect anything DIRECTLY to the computer. These LED based lights are designed to work on THREE dry cells (AA Batteries) instead of the more usual TWO. This means that they run on 4.5v which is very close to the 5v that the computer uses. They already have a “Current Limiting Resistor” which is very close to the correct value, so I think I can get away with it, I have not calculated anything! It's a really quick and dirty prototype that works, I haven't had any problems...
Step 1: Prepare Your Tools and Clear Your Space
Well let's assume you have at least some idea of how to use the tools in the picture. It's not rocket science but hot things will burn you and sharp things will cut you. Be a grown up, take responsibility for your own actions, and take suitable safety precautions.
Step 2: Open the Box - Carefully...
Well ok, you can rip the cardboard off and throw that away, it's the little plastic Battery Box that's important. It's really easy to open up, there's only two screws holding the cover on. It's worth noting that I didn't include a screwdriver in the list of tools, be sure that it fits the screw correctly, damaged screws are difficult to get in or out of anything and if the box is damaged too then you may have to resort to some very messy alternatives.
LED's are really efficient at what they do, (turning electricity into light,) but if the power is not limited SOME HOW, they can behave like a dead short! That would almost certainly damage my computer so I really don't want to do that. The resistor acts like a kink in a hose, it slows the flow down a bit. In bigger applications you would almost certainly need a more complicated power supply and probably more complicated control circuitry too, but for this little job, one resistor should be ok.
I am basically going to add a piece of wire to the circuit so that the whole string of lights can be powered from an alternative source, (in this case; the computer), but I still want that little resistor.
Step 3: Pull Out the Wires - GENTLY!
The wire used here is a bit brittle so care needs to be taken, but it's fairly easy to pull the battery contacts out, and then extract the whole thing from the plastic case. Set the case aside for the moment, it will need some minor modification later to accommodate the new wire.
Step 4: De-Solder the Existing Wires
I've just found it a lot easier than trying to stick the new wire to the existing joint.
Making sure that they are the right LENGTH. There's a bit of leeway, but in this product one leg has to be a LOT shorter than the other, and there's no room to hide oodles of wire. This is the long leg being soldered here, but the process is basically the same.
Make sure you have enough bare wire to make a secure joint.
Twist the wires securely together, touch the soldering iron to the wire and then the wire to the solder, at least that's what I do and it works. I'm sure there's some electronics geek who can tell me I'm doing it wrong!
Ordinarily I'd use pliers to hold the wires and avoid the risk of burned finger tips but it all got a bit awkward for taking pictures. Take care folks. Burned fingers HURT!
Step 6: "Tin" the Tag
In general this was fairly easy, but wouldn't you know it, when I came to take the pictures I had the DEVIL of a job getting the solder to stick to one of the tags. A quick scrub with a wire brush solved the problem but it was a pain, and it left a rather untidy joint.
THe idea is to leave a nice neat blob of solder on the tag that the wires will stick to in the next step.
Step 7: Join Them Up!
All that's required now is to bring the two pieces together and apply that hot iron for a moment to re-melt the solder and fuse both parts together.
If there was anything more complicated in the circuit than a resistor (which is fairly impervious to heat anyway) I would suggest holding the wires with something like a metal clamp or a pair of pliers, not only would this avoid burning your fingers but it would offer a degree of protection to the components from heat conducting allong the wires.
Step 8: The Case for Surgery
I chose to make an extra hole in the case for the additional wire. The location I chose was not the most practical, but by balancing the aesthetic symmetry I think it looks neater, it does make for an interesting challenge cramming the wires back in though.
Since the existing hole is merely a slot in the edge of the case, I carefully made another hole next to it with a file.
Step 9: Pack It All Back in the Box
It's a tight fit, but fortunately there is some spare space in there. I've put the new slot in a spot which does impinge on the switch, slightly, and if it was a REAL problem I could move the hole to a more convenient location. As it is I think it looks neat and it's not TOO arduous to squeeze it all in.
Step 10: Shut the Box.
Theres a bit more "stuff" in there now so it needs a little pressure to get the screws to engage properly, and of course there's a risk of trapping a little bit of wire in the gap. Because of the amount of force required I found it easier to slip the switch over the edge of the work surface, just to avoid crushing it, and it allowed a better grip.