From what we've read, it seems that there are many LED conversions out there. That actually means that people are converting their lightbulbs to LEDs by tinkering around with them a bit. Sounds like a daunting project, but it's really interesting, as LEDs are much brighter and generate almost no heat.
First of all, LED lights are slowly seeping into the mass market. It's not unusual to see high-end light fixtures use LED. From our research, it seems that most people still use incandescent or highly effective fluorescent bulbs. If I had to choose between all of those options, I'd choose LEDs. They're just better to work with. They provide nice diffuse bright lighting that can also be focused. So read on to find out more.
Claudiopolis explains that he's tried all sorts of LED-conversions. His latest solution is simple and efficient. You'll need quite a bit of patience in order to make this work, but consider the fact that you'll get countless hours of low consumption light, it's worth it. His tutorial covers how to convert a regular halogen bulb to an LED bulb, while maintaining full 12V light bulb usability. The result of this build can be used indoors or outdoors.
You first start to empty the bulb with a screwdriver. Once most of the cement is taken out, use a hammer and deftly take out the halogen bulb. You then use a hole-puncher to make evenly distributed holes in a thin sheet of aluminum, that's cut to fit the reflector of your halogen bulb. Next you'll need to 
After each LED is placed, use compound glue around each LED so that they firmly stay in place. You'll use a nailclipper to cut the legs of the LEDs so that they fit together. Next it's time to solder the LEDs together and to solder in the resistors using the scheme that you established previously. Afterward, you'll insert your new LED bulb into the halogen bulb reflector using some compound glue. That's it!
Have you built this and seen how "LEDs are much brighter" because I've tried led bulbs and they aren't there yet in terms of being brighter than cfl or incandescent. Also "generate almost no heat" is incorrect. It depends on the led and circuitry used. I have plenty of Cree and SSC led flashlights that beg to differ regarding the generating almost no heat comment...
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Can you even buy resistors at radio shack anymore?
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The latest generation of LEDs (from Cree or Luxeon) are more efficient than CFLs and produce good light, but they are not cheap, and cooling can be an issue. I would not buy a pre-fabbed "LED bulb", at least not the ones that I have seen advertised, because they are just not large enough. The best LEDs are about as efficient as good fluorescent lights, so if the power fed to the bulb is not about the same as an adequate CFL, it probably isn't big enough.
Undercabinet lights seem to be the killer application for LEDs. I rigged up undercabinet lights using an LED-specific regulator, and 9 CREE XR-E P4 Neutral White LEDs, with the LEDs epoxied to pieces of aluminum flat stock. The "Cool White" LEDs have a higher lumen rating, but the light is too blue.
It's important to keep the lights from getting too hot -- it reduces their lifetime. One light per chunk of aluminum, with the thinnest possible layer of epoxy holding it in place, is probably ok.
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MR16 lamps with LEDs are available on eBay already made and prices are coming down. Assembling one is possible but gathering the parts and punching aluminum are not readily available to most. Or not worth the time involved.
But there are worse problems with lamps made of discrete arrays of T1-3/4 or T1 LEDs. I've tried various things with these small LEDs over the last 10 years and have noted the following:
1. Color--For most people the "white" LEDs are far too blue and color rendering under them is often poor due to weak red output. Typical color temp is 6000K, about like daylight fluorescent, which most don't like either.
2. Color consistency between small LEDs tends to vary noticeably as well. This seems to have gotten better of late but is still a problem.
3. Output depreciation and color shifting: Some white LEDs hold their color and output surprisingly well, while others will turn even more bluish or have noticeable drop-off in output within just a few months. The fact they may continue to glow for years is of little use if the output drops over 50% in a few weeks or months. Unfortunately, you can't tell how well a sample of LEDs will perform over time without testing them. There is no way to know in advance. I've had good and poor LEDs from the same supplier. It is disappointing to build a bunch of discrete LEDs into any project only to find they're worthless after having reached 1 or 2% of their vaunted 100,000 hour life.
4. "Warm white" discrete LEDs exist, but most available are not warm white at all. They are simply the standard 6000K whites with the phosphor tinted yellow. The red is still weak, and there is output loss from the beginning because of the extra color filter. The only halfway decent warm white discrete LEDs I've seen were oddly enough in some Christmas light sets. For some reason those are closer to what a true warm white is. A yellow tinted standard white is not close at all, but that is what most small discrete LEDs will be.
5. The "power" LEDs, both multi chip discrete built into 8 or 10mm housings and the Cree and Luxeon types are better in nearly every way than the inexpensive small discretes--more output, better color, and better color consistency from one to the next, but the tradeoff is that they are more costly, and most MUST be heatsinked effectively. The multichip discretes are not as expensive but if run at rated power can burn up, and are difficult to heat sink. The Cree and Luxeon chips are the best currently available, but the Crees alone run around $8 per emitter on eBay currently. Both have warm whites available that are pretty good but there is the usual 20% or so light penalty and red content is still well below halogen. Crees and Luxeons must be heatsinked or they will quickly burn up.
As others have said, the best light output per watt that can be expected is similar to CFL. But an emitter or array of emitters built with power LEDs or a large array of the small discretes will certainly cost a lot more than a CFL and if built with power LEDs or Cree/Luxeon emitters will have serious thermal issues to be settled.
Right now, the most effective LED substitutes for conventional lighting I've seen are the MR16 that contain 3 Cree XR-E emitters, run around 4 watts, and replace a 20 watt halogen. Color is good and available in warm and cool white. Optics are also fairly good. Cost is typically around $30 but they can be found for less. They are fairly tolerant of the nominal 12 volt supply, either AC or DC. I believe they are rated for 25,000 hours, so they would outlast several halogens and the high initial cost is balanced by life and energy savings. Higher powered lamps may be available or soon will be but expect to pay a lot more for them. And expect poorer optics. The only way to make brighter lights with any sort of LED source is to use lots more of them and this affects cost, optics, and thermal management considerably.
The largest "emitter" I've played with so far is a 12 watt disc assembled by Seoul Semiconductor of 4 Cree emitters on the same disc. It has output similar to a 60 watt incandescent or 13 watt CFL. But the chip alone cost $33, and it came with no lens nor heat sink. I had to put it on a fairly large heat sink and am waiting to find some sort of lens to make it useful. It is nice and bright and the color is good, but it's a pretty costly substitute for either a 60 watt incandescent or 13 watt CFL.
For certain types of lighting, LEDs are an acceptable substitute if you can live with the limited power, color issues, and the optics. For general illumination, LEDs have a way to go yet, and will probably be costly for years to come, at least in comparison to CFLs of higher light output.
I have seen on eBay power emitter arrays of as much as 100 watts, so they exist today. But they are very costly and I imagine use a large heatsink and fan to keep them running safely. I have no idea of their optics or their color.
Small discrete arrays are fine for flashlights or nightlights, but I would not recommend them for anything much larger or where color is important. And I sure wouldn't recommend building a bunch of them into anything until you're certain you've gotten a batch of good ones. I've had too many fade out or change color on me within too short a time. The only advantages the small discretes have is that they are low cost and since the chips are not adjacent to each other, thermal management is not usually a issue.
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