MG Sports Cars

Jim, Could take a picture of the LED light behind the lens, It would be interesting to see what a solid bulb looks like compared to multiple. Just to give people an idea between the two.

One thing that is going to effect the LED light is the lens. With the stock tail lights, they use regular bulbs and the brightness of the bulbs is directly affected by how shiny the tailight case they are in. By bouncing the light around in the housing, it gives the appearance of being pretty bright! However, LED’s do not use this concept. LED’s have a set degree (angle) of light output which is determined by the size and type of LED bulb, and can vary anywhere from 20*-360*. For this reason, they generally do not light up surrounding areas inside the tail light housing like the standard bulbs do, and it can make them seem less bright, However with thanks to miniaturization that is where the 1157 bulb using SMD LED's is becoming more effective replacement than a board type application, just plug in ready

When considering LED's, while they are relativly the same they to do have major differences. One of the main differences with A SMD(surface mount device LED like in a 1157 bulb, the flat looking bulb) and the type I used in my application a (5mm LED) is the beam pattern of the LED. Most of the size of a 5mm or 3mm indicator LED is the epoxy package–the actual LED junction is quite small. With SMD LEDs, you don’t get much extra packaging, just the glowing bits. Without the epoxy package focusing the beam, they tend to have wide viewing angles. The bulb I used, has a viewing anlge of 110-120 Degree's, and most SMD have a much wider pattern of 140 or greater. So when picking LED's you have the look at the LED Characteriscs and how they are formed.

Another difference is handling, handling SMD led is difficult. They come in a tape reel packaging designed for automated assembly. Solder reflow is the usual way to connect these to your circuit board, but for home hobbyists with steady hands, a fine-tipped soldering iron may get the job done for you. Be quick–five seconds at 260°C can cook them. And I must say very time consuming with the something that small, the 5mm were time consuming enough.

It really comes down to preference and what you want to do (design, function, safety, and it is really your preference). You have more creativity with a LED Bulb and how it is used than a smd led and I think the circuit design would be simplier for a hobbyist. There are multiple bulbs out there you just have to do some research and how you are wanting to apply it to your application. I am working on a HOW TO and will post it soon! This application has multiple uses and it is always fun to try and learn something new.

The LED I've got is a flat matrix of perhaps 20 small square grids, each of which probably have hundreds of light sources, if not more. The whole pattern is about 1/2" square and sits on a small plate with connector tabs for power.
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These new lighting elements have a lighting angle specification of 140 degrees but I can tell you that if you can see the element at all it will put spots in front of your eyes so 180 is probably closer. Clearly you don't need more than that for a tail light. It brightly illuminates every edge of the late MGB amber tail light lens when the lens is held in front of the LED with no reflector structure or anything else. I'm telling you, it's a LOT of light.

The stock lenses are designed for a point source of illumination. They look a little odd with multiple discrete LEDs but work ok. This is a point source. It will work but will require the original socket (and reflector maybe) to be replaced with a heat sink.

Jim

Nice Job.

I know plenty of people who swear they can build them for $10 for a set but think you have a more realistic idea of $30 per unit.

Did you add any spike protection or else? Are both boards used for brake or only one for brake and one for turn?

-BMC.

Brian, I know you've been the point man on LED tail lights and have done a great job with the 1st generation units. Really great looking tail lights btw.

My second 25w LED came in, I think I'll use these for the backup lights. Can't be done for $10, no how, no way. Each LED array cost ten bucks by the time you pay shipping from China. But there's no way you can buy a brighter backup light bulb that's even remotely feasible. The next step is to fashion an acceptable heat sink that can take the place of the stock bulb socket. Commercial heat sinks are available, but they are HUGE. I think probably just a thick aluminum plate, maybe 5/16 or 3/8" thick would do the trick, no longer than they will be on, and may dissipate enough for continuous use. These are the right voltage for automotive use so no power supply is required, though with a higher voltage alternator they might not last. I think anything up to about 15 volts they'll be happy.

As for other uses, this array can be oriented horizontally and should be good for fog or supplemental lighting. A heat sink in the form of a piece of aluminum strap would work fine and a number of them could be lined up above or below the bumper. The issue would be to come up with a lens or clear weather cover, plus a mask if they are intended for use in traffic. The bumper (CB) could serve the function of a mask. (They are definitely bright enough to puts spots in your vision. Big ones.) Bear in mind, these are 25 watt bulbs and more efficient than incandescent. The low beam of some headlight bulbs is only 35 watts, so it is a serious light source, far too bright for tail lights, and marginal for back-up lights.

However... lesser wattage single LED elements and arrays are available. The trick is to find something in a lower wattage that runs on automotive voltages. I would guess that a 5 watt for the brake lights and a 1 watt for the running lights would be in the ball park, and maybe a 3 watt for the turn signals (especially if it uses an amber lens) but the only bulb in that range that I have so far is a black light in the far ultraviolet wavelength so it isn't a good indicator of light output.

I suspect that you could use a flat plate inside the bodywork for the bulb holder and heat sink with threaded holes for the lens screws. The trim I'm not so sure about, as I don't have an early set of lenses to play with. The electrical connection could be via pigtail with a sealed connector which would insure continued retention of system smoke for long term reliability, and soldered connections to the LED elements since they have a near infinite life, provided system voltage remains at nominal levels.

That leaves voltage. Most LEDs can tolerate a variation in operating voltage of 25% without difficulty, and will function with variations of 50%, though perhaps with some output reduction or reduced life. That means that a simple dropping resistor of suitable wattage and value can make them perfectly usable in an automotive environment. This would be fine for a back-up light. But a voltage regulator is clearly much better. Where the trouble comes in with a resistor, especially with arrays, is when an element burns out. If it is a series parallel array, one series will go dark and the current draw will drop, which raises the voltage, cascading the failure. Same mechanism with discrete bulbs. This was the problem with the original 1157 LED bulbs and led to a short circuit which could then burn out the vehicle wiring. Inexpensive tab mounted voltage regulators can easily handle 5 watt loads at a cost of around a buck or so and would be suitable for use in a tail light assembly where reliability is a major concern. The regulator will hold the correct voltage regardless of how many element may fail, allowing the remaining ones to continue to operate correctly. Overall light output will drop off, but the bulb itself will continue to work.

For the 5, 3 and 1 watt bulbs there are two options. The best is to find arrays that operate at automotive 12v levels so that no dropping resistor is needed. Then if one series of the array fails it is not a problem because each series needs the nominal 12v supply, which does not change. The next best option is the regulator chip which requires additional soldered connections, mounting to the heat sink, and protection from the elements. The dropping resistor is an option but not a good one unless a separate resistor is used with each individual LED element, all of which are in parallel with the supply voltage. Spike protection typically consists of a voltage limiter such as a zener diode which is cheap and rather easy to add to the input wires, one for each circuit. The Zener has a breakdown voltage, say 16v for instance, and conducts anything above that to ground. These were common in the charging systems of British motorcycles a few decades back and vastly simplified the circuitry, essentially replacing the entire voltage regulator with one simple solid state component. The problem was that it had to handle a lot of current on a continual basis. We wouldn't have that problem here.

I will send an inquiry to the LED sellers I've bought from, maybe they have something suitable. I think a single 5, 3, and 1 watt warm white LED with diffuse integral lenses mounted on a single backplane centered in the original bulb centerlines should make an excellent taillight replacement for the MGB. I'm not especially interested in competing with Brian for the limited market on this, but I am willing to help in the development.

Jim



Edited 3 time(s). Last edit at 03/25/2012 11:21AM by BlownMGB-V8.

Brian,

Thanks for the compliment. Currently I have them set up on a parallel circuit with just resistors, I did look at putting in a voltage regulator in for voltage stabilization, decided against it and wanted to see how they would work before I put anything else on them. I plan on redoing them after awhile and maybe make some changes. Currently boards are set up one for brake and one for turn. But If I decide to convert them for both that should be pretty simple. Thanks for replying