building LED tail lights

[WS6]

I'm looking to do this for my motorcycle but I am sure it will apply to cars as well. The issue that I am running into is wiring. How can I have the LEDs go from dim to bright just like a normal bulb or factory LEDs? The bulb has a second filament. An LED does not have any filaments. The only thing I can think of is to run an relay. The normally open position supplies power that is heavily resisted to the LEDs. This makes them on but dim. When I hit the brakes, it triggers the relay closed. That then sends power to the same LEDs only with less resistance there by making them brighter. I would put diodes on each power line as well to prevent any back feeding into the unused circuit. How does that sound? I think it will work but I wonder if there is another way. I was hoping maybe someone knew how the factory did it so I could look into that way. I'm worried the factory uses solid state electronics though.

Thanks

[shizzy]

I have seen a lot of motorcycle LED taillights run two seperate sets of LED's, one set for the "run" and the second set + the first for the "stop" function

[WS6]

I've seen that too even on cars. To me that's cheap and doesn't look good. My bike is an 00 SV650. In 03 they put LEDs on the back end and they work like I am describing. I'd really like to build mine like that. Obviously it can be done, I just need to find out how. Thanks for the reply.

[HsvToolFool]

Okay, this may be more info than you wanted...

LEDs are either on or off. They stay full-off until enough current is applied then they switch to full-on. They do get a teensy bit brighter if you increase the power and overdrive them, but this drastically reduces their lifespan. And they burn out the instant you put too much power through them.

So LEDs can only be dimmed by turning them on and off very fast. I think 150 to 200 times per second is probably adequate to fool your eye and avoid a strobing effect. As the ratio of on-time approaches 100%, the LED appears brighter. As the ratio of off-time approaches 100%, the LED appears dim. Obviously, you need a digital controller to accomplish the above technique, and that's a pretty ambitious project.

Simulating how an old-fashioned incandescant bulb fades on and off is a pretty subtle effect, and most people don't think it's worth the trouble. It's much cheaper and easier to just have lots of LEDs and only turn on some for a "dim" level and all for a "bright" level. In LED brake light kits, one circuit is driven by the running lights while a second circuit is driven by the brake lights.

If you've never worked with LEDs, I recommend web searching and reading. You can't just connect them up to 12V DC in your car or they'll pop. The power requirements for LED chips are very specific, but 4V to 5V at 350 milliamps is typical for the brighter, high-power LEDs you might use for a taillight. I'd look for pre-made LED "drivers" which output the correct voltage and current for your LEDs.

Oh, one last thing: high-power LEDs get HOT and need a proper heat sink or they'll die.

[WS6]

I've worked with LEDs before but only simple stuff. You can put a larger than needed resistor inline with the LEDs and they will be on but dim. I just finished a circuits class last semester and we had an adjustable voltage source so we got to play. I plan to use this resource for my testing.

These aren't hi powered LEDs, just bright ones. Here's an example
http://cgi.ebay.com/ebaymotors/50p-10mm-SUPER-BRIGHT-RED-LED-LAMP-70K-mcd-50-Resistor_W0QQcmdZViewItemQQcategoryZ33713QQihZ022Q QitemZ350019454238QQrdZ1QQsspagenameZWDVW
I just bought some yellows from another company to use for turn signals in the back. That's easy though since it's only on or off, not in between. I'll be using similar ones for the tail light.

Thanks

[shep]

http://www.dprg.org/tutorials/2005-11a/index.html a simple PWM circuit for running lights. I think this should work. Connect the brake circuit to the low resistance side of the POT when the brakes are applied the PWM will go to 100%. Adjust the POT for desired brightness on the running lights.

[WS6]

looks like a more sophisticated approach than what I was thinking. Is POT potentiometer? Is there a knob to turn to adjust or does it adjust a different way? Thanks for the help everyone. I do appreciate it.

[Paul_J]

Yes POT is short for potentiometer. SOme use a knob like the volume control on a Fender Strat and some use a screw slot. I agree that the simplest solution is to only turn on a few LEDs to achieve the dim running light and turn them all on for stop or turn signal. If the light is properly diffused it should provide the required effect. maybe a mix of low and high powered LEDs. And yes, you need to provide a current limiting resistor to both circuits.

[WS6]

OK, I talked with an LED company and they told me how they do theirs. It's even easier than I thought. Simply have two power wires coming into the LED circuit. The one is for running lights and has a diode and large resistor on it. Right after the resistor have a second power wire coming in through another diode that comes from the brake switch. Make sure it has the proper resistor on it for max power and away you go. The bike wiring already kills the running light wire when you hit the brake lights.

[Paul_J]

Good info, Thanks. If it just a plain old LED isn't the min. voltage just .6V to get it to turn on?

[megaladon6]

.6v is the standard bias voltage. some LED's are properly built and will take quite a bit of power before they break down. some won't. just make sure you don't exceed the rated voltage. and that they don't overheat.

[jwvess00]

Hello,

For LEDs, the forward bias voltage is usually between 2V and 2.5V. The spec. sheet for the parts will say.

A normal silicon diode forward bias is 0.7V, which is what you may be remembering.

The LED spec. sheet will also tell you what your maximum forward bias current is. Most LEDs are 20mA (0.020A).

To calculate the series resistor value:
Resistor in ohms = (Supply voltage - forward bias voltage) / (maximum current in amps)

So, for a battery voltage of 14V (use the charging system voltage here, not battery voltage, since that is what the LED will actually see when the car is running), a forward bias voltage of 2V, and a maximum current of 0.020 A,

Resistance = (14V-2V)/(0.020A) = 600 ohms.