I was fairly happy with the way my LED taillight worked out, but I had some niggling issues with it. It did exactly what I'd designed it to do, and had a good light output, but like all prototypes it was three time the size it needed to be. The large 10mm LEDs I'd chosen for their viewing angle rubbed against the inside of the lens which required a few of them to be replaced with smaller 5mm ones for greater clearance, and I was less than impressed with having such a large number (twentyfour) of LEDs all squished together and supported by nothing more than the structural strength of their legs. Don't get me wrong - it works just fine and is still in use today on a friend's bike, but I felt that I could do better.
Cost was also an issue, as I didn't want the expense to blow out or it simply wouldn't be worth doing (especially considering that it already cost something like twentyfive times the price of a standard replacement bulb to build). While browsing some electronics sites on the web I cam across a mention of SuperFlux LEDs. These impressive little beasts come in a low profile, 8mm square clear plastic package with four legs each, with the legs forming pairs so that one pair is the cathode and the other the anode. They draw significantly more power than a standard LED but still much less than a traditional bulb, and require the extra legs as heatsinks. Their total light output is greater than the current brightest traditional LED how ever it's spread over a greater area, with viewing angles of 70-120 degrees as compared to 5-25degree viewing angles on traditional LEDs. What this all means is that you can use less SuperFlux LEDs to illuminate the same area with a wider viewing angle and a comparable percieved brightness. Having four legs per package provides a much more stable mounting and the lower profile makes for a much smaller & neater overall installation. When you compare prices for SuperFlux vs traditional LEDs the new ones are about 3-5 times the price, but when you account for the fewer required pieces the total cost is about the same. SuperFlux LEDs come in all the regular colours, as well as a red-orange designed especially for vehicle rear lights. You can find more information on these beasts at www.lumileds.com.
My original design for version 2 using these new LEDs called for a simple 3 x 3 grid of red SuperFlux LEDs and 3 traditional 5mm white LEDs for the number plate, with all the red pieces lit dimly for taillight and at full brightness for brake light, and the white pieces at full brightness at all times. This was simply an update of the design for version 1, having a resistor in line with the output of the taillight voltage regulator to lower it's output. This worked nicely until I plugged in my trailer, which showed up a flaw in my design. Unlike traditional LEDs these ones have what could be described as a "firing voltage", below which they simply won't turn on. (To be correct traditional LEDs have the same thing, but it's a much lower voltage). Plugging in the trailer's lights put enough of an extra load on the circuit that the voltage dropped to the point where the SuperFlux LEDs turned off, leaving only the number plate LEDs lit until the brake was applied. A redesign was called for which resulted in version 2.1 using six SuperFlux LEDs in a 2 x 3 grid lit at full brightness at all times as a tail light, and a further twelve placed around them to illuminate at full brightness on application of the brakes (as well as the three white 5mm LEDs for the number plate). This removes the resistor from the output of the regulator allowing a much greater loss in voltage before the LEDs are affected (to the point where the bike probably wouldn't run in fact). This design has plusses and minuses over the original one; on the downside the central array of LEDs produces a smaller (but much brighter) "hotspot" of light in the centre of the lens with a much larger and brighter spot for braking. The rear light's still much brighter than the standard bulb and very visible but doesn't have the more even spread of light in either versions 1.0 or 2.0. It also means there's less possible variation in "shape" of the light - complex designs become more difficult using veroboard as there's two sets of wiring required, one for each array. On the up-side it provides an even greater "failsafe"; in all three designs if one LED fails (an extremely rare event) you lose a "string" of three LEDs, however version 2.1 provides a greater number of "strings" which offers less impact on the overall effectivness of the light.
They say the proof of the pudding is in the eating, so here's some pics of the finished product (I suggest you don't try eating it though...).
Tail light Version 2.1, in taillight & brakelight modes.
As you can see there's a significant and very noticable difference between the two modes. As usual when photographing very bright lights the camera doesn't do it justice as the degree of light (especially in brake & tail light mode) overwhelmed the camera, but suffice to say that the apparent light output in both modes is easilly twice that of a standard bulb, and with the lens removed with cause pain if you view it directly. When viewed in real life the individual dots aren't quite as noticable and the lens is better illuminated, and the pinkish hue over the numberplate doesn't happen in real life. Hoorah for digital artifacts! The circuit could easilly be modified to take more LEDs but since you want at least 3 times the output for brake & tail lights as tail lights alone the required space becomes problematic.
Here's some pics of the actual board in all three modes, with a pic of version 1 for comparison. You'll notice I spraypainted the whole thing white before soldering the LEDs in place - this is just for appearances. I would have used silver but black, white, and a sort of muddy green were all I had on hand at the time.
Version 1 vs Version 2
Tail light mode & brake light mode
As you can see, the new board's much neater and more robust. The narrower design also allows for a much more simply created board which means less fiddling about cutting slots to fit the board over the lens mounting posts. Obviously there's more space that could be saved but at this point I don't see any reason to, and it leaves free space for later modifications and additions (like the brake light flasher circuit I'm currently working on!) I also reduced the numberplate LEDs from five to three, as it was overkill to have so many, and it allowed me to simplify the design by using a single resistor for all three LEDs in series instead of one resister per LED with five LEDs & resistors in parallel.
I'm not going to any great lengths describing the construction of the light, since for the most part I'd just be repeating what I wrote in the first version. I recommend you read that article in detail first, then apply the principles to this one.
Firstly shape the board. The easiest way would be to use the picture above, counting holes to measure the board and fitting to the light body on your bike. (I'm assuming you HAVE a bike.. if not, what're you doing reading this?) The fiddliest part's getting the holes right for the 4 mounting posts. I used a power drill with a small bit for the two central holes, then a rat's tail file to enlarge them, checking periodically against the bike. Use the same file for the outer two notches.
Once it fits (try it with the lens in place too!) sit your SuperFlux LEDs in place on the board and mark their outlines with a pencil. Remove the LEDs from the board and transfer those outlines to the rear of the board to give yourself an orientation for cuting tracks. Use the board layout picture below to figure out which tracks you need to cut. It's easiest if you think of it as being six individual strings of LEDs rather than one bunch of eighteen. When you're sure you've got the tracks right solder your resistors, wire links, regulators and connector block into place. Note that the right hand regulator (brake lights) is mounted up-side-down. This is the time to paint the board & unlit components if you so desire.
Finally mount the Superflux and regular LEDs noting the orientation of the chamfered corner of the SuperFlux ones and the flat side of the regular ones. Get yourself a 12 volt power supply or battery (there's one in your bike!) and test the thing to make sure it works correctly. If so, simply fit it to the bike (using an appropriate flat piece of plastic behind it to insulate it from the chromed light body), cutting off the original socket & stealing the rubber grommets from it for mounting the board if you haven't already done so. Test it, and when you're happy I recommend sealing the lens in place with a little silicon to be sure the weather's not going to encroach, since you're unlikely to need to open it again in a hurry. It's also a very good idea to spray a little clear laquer over the back of the board to help weather proof it so the copper tracks don't corrode- you can buy proper PCB laquer which you can solder through, but any similar sealant will suffice as long as you're sure it's working first.
If it's not working the first thing to check is the orientation of the LED's. Remember that one backward LED will stop the entire string from lighting. Superflux LED's have a shaved off - if that corner's in the top left the uppermost pair of legs go to the negative. Traditional LED's have a flat side - the leg on that side goes to the negative. If that doesn't work then your problem's probably wiring, tracks, or regulator. Failing that a burned-out LED will also stop the string from operating, however unless you've connected an LED to voltage without it's resistor it's unlikely to be blown. You can test LED's with an ohm-meter - in working condition they'll show an open circuit when connected one way, and a closed circuit the other.
PCB Payout, viewed from the front. Confusing aint it? Unused tracks not show for simplicity.
I've got some more ideas for this device - as I mentioned earlier I'm working on a little extra circuitry to be mounted to the board that will rapidly flash the brake light (or perhaps the whole thing?) a few times when you apply the brakes then remain steadilly on. Another idea I'm cogitating upon is removing the lens entirely, placing a carefully shaped thin, black plastic sheet over the board with holes cut for the LED's, then mounting a thickish (say 10mm) piece of clear perspex as a lens. This would show the LED's as individual dots rather than having the lens diffuse the light, but I'll have to play with it and see how well it works. You'd probably lose most or all side-on visibility, and having the number plate lit then becomes a problem. I'd also need to add a reflector for legality since the standard one's part of the lens. Anyway, I'm rambling... enjoy your light. :)