building LED tail lights

[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.