If you've been thinking about building an LED tail light for your motorcycle - or any other vehicle - then you need to read this guide. Why? If you're trying to figure out how to do it than you probably have already figured out that LEDs are super bright, turn on and off way faster than incandescent bulbs, and are generally awesome.

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?

Project Cost = Approximately $50. This will depend a lot on if you have any electronics/soldering supplies on hand, if youre willing to order parts online or if you are content forking out more money to buy from RadioShack.

Parts List:
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

You can kind of pick and choose what type of LEDs you want to get. I got mine off of ebay because they were in my price range and I didnt have to buy too many. After researching a lot I decided that the best solution would be to get the 4-post LEDs. These are the same ones used in automotive applications. I believe the real ones are made by Phillips / Lumileds. You are searching for the term Superflux LEDs. The ones I got were kind of a knock-off but they have awesome viewing angle which works good for tail lights. Make sure that wherever you get your LEDs that you also get all the specs for them. That is the most important part as you will be designing your array and circuits based around their specifications.

Step 3: Running the LEDs

Before you proceed any further Ill have to give a little bit of a disclaimer. The more you start to figure out about LEDs the more you realize that most people arent really running them how they should. LEDs are very sensitive to current and the voltage doesnt matter as much as long as they hit their forward voltage drop so that they will light up. I really wanted to go with regulated current on this project but everything I could come up with was just too expensive or beyond my capabilities. I was able to get some free samples from a company called maxim for some really awesome current regulator chips but they were all surface mount chips and beyond my capabilities. If you think surface mount chips are really cool then you should look at this brochure.
http://www.maxim-ic.com/solutions/hb_led_drivers/hb_led_brochure.pdf
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

In the end I went the cheap route and decided to regulate using the LM317t voltage regulators that I found at Radio Shack. Here are the things that you must keep in mind when figuring out how to run your LEDs.

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!

Now that you have a plan about how to run your LEDs, you can wire them up. I had this bright idea that I wanted a cool pattern. My array contained 48 LEDs. That equated to 16 series of 3 LEDs all running in parrallel. Once again use http://led.linear1.org/led.wiz as it is really easy to figure this out. Since I wanted to have a cool pattern, my wiring was pretty intense. For the most part I was able to bridge between the posts of my 3 groups with some solder and then for the LEDs that were wired in series but too far apart to just use a solder bridge, I put in a jumper wire. This was pretty simple but then when I had to run 16 different + lines and 16 lines, the whole thing started to get pretty messy.

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

Once everything is wired up, then 90% of your work is done. You can test out your array using batteries but just make sure that you under run them instead of hooking them directly to 12V and burning them out, that would be bad. The next step is integrating your voltage regulator LM317s.

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

You can view the following circuit that I basically copied for my setup using 2 different LM317s, 1 for the tail light and 1 for the brake. The brake bypasses a resistor to give a higher output current and make it brighter. http://badweatherbikers.com/cgibin/discus/discus.cgi?pg=prev&topic=32777&page=379550

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

That about wraps it up. I used hot glue to secure the board in the tail light, I added two downward narrow viewing angle superbright white LEDs to light up the license plate as the old bulb did that by itself. To secure the heatsinks to the LM317s I used JB weld mixed with some arctic silver thermal grease at about 2:1. I spraypainted the front side of the board a reflective silver color because it was showing up through the tail light. Now it looks more like a light source and not the board inside.

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.