Some info for the person making LED tail lights... Options

[Hi_Fi_Guy]

The DucatiMonster board I belong to has an enterprising person who is also working on custon LED tail lights. He is using resistors to provide different intensity for running light and brake light... looks likethere is some helpful info posted on his project page.

http://www.sca40.com/led/

[smrz914]

Thanks for the link but that is not the way you do it. On mine I will have some saftey features and will dim them the proper way. I'm not gonna let out my secrets but should have something to show for my work in the next couple weeks and will post accordingly.

I will tell you this. LED's are current devices not resistance devices therefore you shouldn't use resistors to change their intensity. It doesn't work well because LEDs have a turn-on voltage and they don't dim in a linear manner like a bulb does. I have done my homework on this and used LEDs that were made to use in taillights not the stuff you get in radioshack.

I'm just gonna add that I was behind some cars that had incadescent bulbs and an LED third brake light and you could see the time difference in them turning on. I was supprised that you could actually see the difference!

[Hi_Fi_Guy]

Cool, I do not know the tech behind the LED tail lights. Sorry if the link was not useful.

Like you I have noticed the difference in how quickly LEDs light up compared to traditional lights.

Best of luck with your efforts... I know we are all waiting to see your final results.

[smrz914]

It's been a huge learning experience for me and that is the biggest benefit.

[Malmz]

Have you done any calculations regarding resistence and making the flasher relay still functional? It seems like this would be one of the hurdles with LEDs. I know even a cruddy socket or bad license plate light can make some pretty strange things happen in a 914.

Just random thoughts...

sm

[smrz914]

The blinker uses the amount of resistance in the circuit to blink. The less resistance the faster it blinks. I'll work on this when I get there but I have seen a blinker relay thingy online that should take care of this but i don't think it will be a big deal.

[Eddie Williams]

So you could use a pot to vary the blink rate...

[Sparky]

Hey Paul any update on your progress? I'm looking for an LEd solution...


Thanks!
Mike D.

[TonyAKAVW]

To vary the intensity, its three letters....

P W M (pulse width modulation) You can actually run an LED at a higher current than their continuous rated current by pulsing them at a lower duty cycle. Virtually all the professional applications of LEDs use PWM to vary the intensity of the light output. If you take a look at some of the LED taillights on BMWs they purposfully make the PWM slow enough that your eye notices (just barely) the flickering. You can really notice it if you scan your eyes across.

There are a bunch of places selling PWM drivers for LEDs now. They aren't that hard to make, but you can buy the modules if you aren't handy with a soldering iron. If yu are handy, then you can go to places like Linear Technologies and do a search for LED drivers like this:

http://www.linear.com/pub/q_srch.html?targ...duct_family=All

and there are a bunch of LED driver chips with schematics and the whole works. You may have to use external switching transistors to handle the high current levels for some of the larger LEDs (or if you want to run a bunch in parallel). You can control the PWM intensity with analog controls, or digitally through an I2C bus. Lots of options here...

So once you have the intensity control taken care of, you still have to blink the LEDs. In a 914, the wiring harness is set up so that when you engage the blink it puts the blinker module in series with the resistance of the bulbs. If you match the DC impedance of your new LED turn signal to that of the bulb, you should expect no problems. However it may be tricky to do. The better thing to do is build a simple oscillator driving a P-channel FET or other similar high-side switch device (expensive and hard to find sadly). This is all due to the fact that in a 914, you are switching +12, not ground. Alternatively, you could build a small blinker circuit into each bulb assembly since you've already got a bunch of electronics there anyway, and just have a preset flash rate. I can tell you from personal experience that one of the cool things about LED turn signals is the ability to flash them at insanely high speeds. Most LEDs can be turned on and off at very high rates (think TV remote controls, etc.)

Lastly, you need a source of high power LEDs. As I posted in another thread, most LEDs are not bright enough. You need to get the high end LEDs made by Lumileds. Either pirhanas or luxeons. They are costly, but Wierdstuff in Santa Clara (I believe) had a deal (maybe they still do) where you could get a couple hundred red Pirhana LEDs for like $30. Thats enough to do a bunch of turn signals and brakes, at a dramatic savings (would be a few bucks a piece otherwise). If you go with the higher power Luxeon devices, you may need only 3 or 4 to do a brake light and turn signal. They are more expensive, but you need fewer of them..

You could also buy white Luxeons and just use the filters of the turn signals.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewI...3831282699&rd=1

There are also some stores online that sell those as well.

oops, you also need circuit boards with appropriate heat sinking. The luxeon LEDs dissipate more than half their power as heat, so you need to dissipate half a watt from a <1cm square
When making (reliable) large arrays of high power LEDs this becomes non-trivial....

-Tony

[mightyohm]

I think Weird Stuff is out of the Lumileds. I bought a few trays and then I went back and they were gone.

[SpecialK]

Excellent Info TonyAKAVW (IMG:style_emoticons/default/smilie_pokal.gif)

Bookmarked that site.