Here are a few things to consider before beginning this project. You need to ask yourself if you're up to the challenge. If you have a lot of electronics/soldering/circuit design experience then this is probably pretty simple of a project; however, if you're anything like me I didn't realize how much work I would be doing. Is there any other way to go about achieving the same results? In retrospect, I kind of wish I had just spent the same amount of money to buy an automotive LED tail light off of Ebay and then modified it to fit in my motorcycle's tail light housing.
There are some decent sources out there and I'm certainly not the first person to try and do a build like this but my reasoning behind writing up an instructable is to compile a few of the things that I learned and maybe help make the process easier for someone else who is trying to achieve the same thing.
Step 1: What Do You Need?
LEDs - $15-$30
Prototyping Board or Perfboard - $5 RadioShack
Solder - Hopefully you already have some.
Jumper Wire - Scavenge if you don't have any readily available. (braided Cat5 would be awesome)
2 X LM317 Voltage Regulator - $1.50 at RadioShack
Various Resistors - A few buck$
Miscellaneous parts including, jbweld, paint, and heatsinks.
Step 2: LEDs
Step 3: Running the LEDs
I had luck with getting the company to send me a ton of free samples for many of the chips in there. (Dont abuse) I also considered using a product called schmartboard that claims that it makes surface mount soldering doable. I didnt want to invest in something that ended up not working so I cant say for sure if this option is good or bad.
Step 4: Voltage Regulator
What voltage do you want your array to run at? Use this, http://led.linear1.org/led.wiz it will help you visualize too if you are like me and dont have a very extensive background in electronics, series / parrallel circuits, etc.
The LM317 will drop approximately 2V so you need to keep that in mind and make sure your overall voltage will be high enough. Heres my example. I the output voltage to my tail light on my motorcycle and got the following values. When the engine was off each line, tail and brake, output approximately 10V while running would reach upwards of 13V. 10V 2V(LM317) left 8V to power my LEDs. The specs on the LEDs were a forward voltage of 1.9 2.3max. I decided that putting 4 in series would be cutting it too close so I opted for 3 in series so that my array would run at 6V. That gives plenty of juice to run everything and it is easy enough to set the LM317 to output 6V.
Step 5: Wire 'em Up!
It would be a lot easier to line these things up so that you could use less wiring and more bridging across the anodes and cathodes of the LEDs themselves. It is probably a lot easier to troubleshoot too if you have a short somewhere. If you do decide to get fancy, just be creative and try not to get a headache.
Step 6: The Math
This site has some information about how to wire these including a table. http://www.reuk.co.uk/LM317-Voltage-Calculator.htm There are other calculators out there that will calculate the values based on Resistor1 and Resistor2 values. In a nutshell, you follow the wiring diagram on that page or on the back of your Radio Shack package if you bought it from there. You need 2 different resistors to set the output current to whatever you want. Obviously your output is going to be lower than your input 2V for the voltage drop of the LM317 itself.
Step 7: The Circuit
It is located about half way down the page but the whole topic would probably be beneficial to read. I left out the diodes which I assume were protection for backwards current into the LM317 but I thought that might be unlikely since the whole array is made of diodes. Who knows though, if you decide to put those in make sure you up your voltage to account for their drop.
(Update) The voltage regulator that provides voltage for the brake lights was getting fairly hot. I heatsinked both of them with an aluminum heatsink and that seems to have helped distribute the heat a lot but my guess is that the voltage regulator is getting 2 X 12V when the brake light is applied. Since the brake light wont ever be applied for longer than a few minutes at a time and it is now heatsinked I think it will be fine. Maybe someone more knowledgeable can explain why that one regulator would be getting so hot. Maybe thats what the diodes are for :)
Step 8: Final Thoughts
One concern I have is durability and waterproofing. My understanding is that tail light assemblies are normally vented to the elements to provide moisture to get out. That means that moisture can get in. I was going to spray the back with a plasti-dip spray to seal it up but I want to use it for a while to make sure that I dont need to get back at the solder points.