Wow. I think that I will dive in here. I also think about the possibility of replacing every single light in my car with LEDs. Considering my very limited budget, I knd of hesitated on replacing my reverse lights with a LED replacement simply because I realized that my reverse lights will only be on for 5-10 seconds at a time once or twice a day. That is it. For what I see, it will not be easy to get the LEDs to produce that many lumens that cheaply. I also worry about thermal management a bit if you do have to stay in reverse for a while (when backing up a trailer or when you are backing up to line yourself up up with a trailer). It seems like currently, LEDs lack a bit when you need large amounts of lumens without any need for heatsinking. Cheap bulbs excel here. I am always optimistic with modding things with LED, and I always enjoy obstacles, but maybe it is best to invest the same time and money into a very nice set of LED tail lights. This is my opinion, but it may save you the frustration.
I suppose you can throw a bunch of high-power LEDs together and wire them with a resistor with minimal heatsinking if desired. It would not be that difficult. You can do it cheaply, too. The life span of the LEDs will be shortened, but you shouldn't kill them. You may not even notice any shortened life span or dimming just because the lights will not even be on for long enough. I would say the heck with any fancy drivers or excessive heatsinking. Just find a way to give the emitters enough current to give you satisfactory current, and get the light you need. Really get a chance to use them and use the LED's inherent long life qualities. Those of us with flashlights will never have to worry about dimming since the LEDs are not even on for long.
Anyways, if you are really sure about this, you can go many routes. There is the simplest and/or cheapest route, or there is the safest and/or expensive route. I am not saying that the cheapest route will be prone to failure, it is that it will be lacking any type of current regulation and possibly voltage surge protection.
First of all, lets see what we have to standards we need to meet. I heard somewhere that those standard 1156 bulbs used for the reverse lights (singe contact) are 25W bulbs that produce maybe 300lm. They have superbright auxiliary reverse lights that use 55W bulbs, but I bet you would be happy to get atleast the same or more brightness than stock, right? If allowed, I am sure you want more brightness.
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The simplest/cheapest route: Get four Edison Opto K-series LEDs available at DX for ~$4.34 each. Wire them in series and hook them up to a resistor, or even a LM317 set up in current limiting mode. You can mount the four emitters close enough and use a small enough round heatsink that you fit this into the taillight enclosure with little trouble. You may not need secondary optics if your tail light has a basic reflector for the reverse lights. If you really want optics, there are very few that are small enough that can fit through that hole. If you plan on cutting that hole out bigger, then any reflector or optic with legs would work. This way you just glue them down and they will not go anywhere. The heatsink can be a chink of copper, aluminum, or even a trimmed down CPU or GPU heatsink. You will have to use your imagination here. Any heatsink should be better than none at all, even if you plan on running them to heck. You will have to figure out which resistor value you need. I like going here:
http://www.metku.net/index.html?path=mods/ledcalc/index_eng
Go to the LEDs in series area. Type in the supply voltage (lets say 14.5V since the car should be running in reverse hopefully), the forward voltage of the LED, then the current in "mA". You will have four in series, so enter the number of LEDs. Presto! Finding the Vf of the LED can be tricky. It depends entirely on the drive currents. From the datasheet, the LED will need around ~3.2V each at 350mA and it will need ~3.6V at 1000mA. I say draw a quick graph and plot those two points. Draw a line between them and pick a current level. There you go. The LED will provide around 80 lumens each at 350mA and maybe around 160lm at 1000mA. This a rough, but conservative guess. Powering each at 350mA will give you ~320lm total (comparable to the old bulb). Power them at 500mA each and you may get 400lm total. Power them at 700mA each and you may get 500lm total. Finally, power them at 1000mA each and you may get 640lm. Consider the total wattage, of course, as well as the heat generated. Things will get pretty hot in there, but do not forget that LEDs handle more heat than you think. Just because you may almost burn yourself on the heatsink does not entirely mean that your LEDs are in danger.
You do not have to use the Edison K-series LEDs. Lumiled K2s are very rugged, but far less efficient (less lumens). Cree XR-Es and Seoul P4s are nice, but they can be expensive. Edison Opto K-series use the same die as the XR-E, but it just uses a cheaper packaging. The Edison is also just like the Luxeon 1W and 3W emitters, so cheap optics and reflectors will work with them. Rebels are pretty cheap, and they can be relatively efficient (the 0070 and 0090 parts come to mind). However, they can be a bit more difficult to solder and mount. You may be up to the challenge though. They do not have an entire arsenal of optics, either, but you may not even need optics. What do you think? If I made reverse lights, I would go this route. If the LEDs would blow for some odd reason, or I get rear-ended, I would not cry that I lost a bunch of money from the reverse lights. I may point at the mangled LEDs and laugh, actually. I am kidding.
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Now, to the opposite end: safe while using a larger or less limited budget. You can use any emitter that the heart desires. Use the Q-binned Cree XR-Es are super bright, and they have nice optics built on to them to give you a wide 70 degree radiation pattern. The tail light reflector may not reflect any light due to this narrow beam, however. That should not matter. Seouls are bright, too. Rebels come to mind if you do not mind soldering the small parts. The 0100 Rebels are real mean competitors to the Q-binned XR-Es. You can squeeze many Rebels into a small area, too. Any older, less efficient LED seems plain wasteful.
Drivers for automotive applications can be hard to come by. The 12V system is odd because the voltage can vary from 12V to 14.5V. You have to make sure that the Vf of the LEDs are low enough so the total voltage is not so hight that it certain drivers will not work with it. A buck/boost driver would work, but I do not know if you can get any that are of heavier duty for this application. A buck driver would work fine. There are several out there that can handle up to 1000mA. Since your car is running, then most likely, the Vf will be lower than the car voltage. You can search around for the right driver. The Downboy and SOB comes to mind. These are found at the Sandwich Shoppe. The PowerPuck and BuckPuck will work and can be found at LEDSupply.com for a decent price. Heck, if you want, you can still go the resistor route.
For heatsinking, you have to find the drive levels of the LEDs and find the total wattage and go from there. For around 15W during short bursts, you can get away with something the size of a Modamag D sized tri or quad LED heatsink, for example (a weird example actually). A small heatsink from a computer's graphics card or old Processor may work if not too small. Again, your imagination would have to run wild here.
As for optics, anything may work. Most tail lamp assemblies have the faceted lens that spreads light out in an oval shape or will give a horizontal bar shaped beam, so you do not need an expensive optic that gives a perfect beam. Any plastic reflector that throws a beam in one general direction will work. For the XR-E LED, the newly released Fraen multi-faceted reflector may work. The same with Polymer Optics' XR-E optics. I guess that if those facets on the lens really spreads the beam enough, you could just let the LED go naked. The XR-E already has a more narrow beam than other LEDs.
Finally, the most important component: voltage spike protection. In some cars, you can see some nasty voltage spikes from electric motors kicking on and off (solonoids for your compressor pump for your A/C, electric radiator fans, and your heating/cooling systems' blower motor). Spikes may occur from the ignition system, as well as the alternator. All this noise and the high voltage spikes reek havoc on your poor LEDs and the slightly delicate driver circuitry. The ICs and transistors can be destroyed, as well as your LED die. The correct Zener diode may help suppress these spikes. Better yet, use a Avalanch Diode (AVS diode may be the exact name?) or some sort of voltage suppressing diode. These diodes will clamp during the voltage spikes and either dissipate these spikes as heat, or allow a nearby resistor or fuse to take the hit instead. Also, a nearby capacitor will help smooth out the noisy power, allowing the driver circuits to run better. If you need some help selecting a diode, just ask. I had to get some for an automotive LED project and they can be difficult to explain better.
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I would say that whether you choose the cheap or expensive way, it does not hurt to get a protection diode to save your LEDs from failure. They do not cost too much (<$6). So far, what do you think? Did I scare you any, or give you more confidence? You are not walking into a super-easy mod. Do not get discouraged, though. The cheap mod is always an easy way of doing things. If anything would go wrong when testing or experimenting the LEDs, then you will not be as heartbroken as when you used an nicer XR-E and a nice driver circuit. I would say get a few LED emitters, a driver, and some metal for a heatsink and go at it. You will learn so much! For resistors, you can afford to buy a bunch of different values so you can switch them out during testing in case you are sending too much current to the LEDs or not enough.
Holy cow! I cannot help but to leave long posts, but this is ridiculous!!! I hope you had the patience to read atleast most of it. Sorry! If any of it did not make any sense, or is not clear enough, please tell me so I can correct it.
Well, I hope that you will find this useful. If you decide to not do this, then hopefully aomeone else may find this useful. This same approach can be used in any other LED lighting on your car (tail/brake lights, etc). Please give us more information and specifics about your car, your exact desires (brightness, budget, and whether you want to cut any plastic from your tail light enclosures). From there, we can be of more help. Otherwise, we are trying to make extremely vague instructions that are too long because we have to include every type of LED and other variable. Sigh. I need to stop typing and give it a rest....
-Tony
