16vCorey
16vCorey SuperDork
8/20/10 2:15 p.m.

So I'm making my own tail light/brake light for my motorcycle. I got a super freakin' bright 9 led strip from a police car's light bar. It's got a positive and a negative wire, you hook 12v to it and it lights up red, and very, very bright. Pretty simple. Here's my thought process. If I put a resistor on the tail light power wire and run the tail light power wire and brake light power wire to a wiring diode like this one: Then run the wire coming out of the diode to my led panel, I should have a dim tail light and a bright brake light, right? Assuming what I've said so far is true, what kind of resistor should I use? I'd like the tail light to be at most half as bright as the brake light, because at full 12v this sucker is BRIGHT. Is my logic correct? Any ideas on how to size the resistor?

DILYSI Dave
DILYSI Dave SuperDork
8/20/10 2:17 p.m.

Pretty sure you can't use resistors to dim LED's.

16vCorey
16vCorey SuperDork
8/20/10 2:21 p.m.

Thanks a lot dream crusher. Any ideas on what I CAN use to dim an led?

turboswede
turboswede GRM+ Memberand SuperDork
8/20/10 3:01 p.m.

You can't dim LED's The way LED brake lights work is that they have two circuits of lights. One circuit is about half or less of the total number of lights and is used for the marker lights, the rest are used for the brake light circuit.

The tricky bit comes with spreading and mixing the marker LEDs out across the light housing so that you get good wide light dispersal and enough difference in lighting output to know when the brakes are applied.

Here's some interesting LED projects:

http://hackaday.com/category/led-hacks/

http://www.instructables.com/id/Motorcycle-LED-Tail-Light/

http://www.instructables.com/id/DIY-LED-car-headlights!/

http://www.instructables.com/id/LED-CAR-TAIL-LAMPS/

http://www.instructables.com/id/Givi-Motorcycle-Trunk-LED-Mod/

mad_machine
mad_machine GRM+ Memberand SuperDork
8/20/10 3:09 p.m.

there you have it... unfortunately. Got room in the housing to run a standard lamp for the taillight and the LED for the brake?

BoxheadTim
BoxheadTim GRM+ Memberand Dork
8/20/10 3:12 p.m.

The way to 'dim' LEDs is normally by pulsing them and manipulating the pulse width. If you do it quickly enough, the human eye doesn't see them flashing but perceives a 'dimmer' LED.

For brake lights, just give them full power instead of pulsing them.

16vCorey
16vCorey SuperDork
8/20/10 3:44 p.m.

OK, but the part that doesn't make sense to me is (in my experience with using leds on some sculptures) if you hook multiple leds to the same power source, the more you add the less each individual led shines, and they also they get dim as the battery gets low. Why couldn't either scenario be simulated?

WilberM3
WilberM3 Reader
8/20/10 4:00 p.m.

not sure if LEDs would alter their output enough, but what if you fed the running lights with voltage from a 5v regulator triggered by the light switch and 12v by the brake switch?

Grtechguy
Grtechguy SuperDork
8/20/10 4:14 p.m.

I use a 4.5 semi trailer LED light from napa. Prewired to be a brake/running light. but, it will about blind whomever is following at a close range.

very effective against tailgaters

Dr. Hess
Dr. Hess SuperDork
8/20/10 4:40 p.m.

Grtechguy, I bought a pair of those recently for just that purpose from a truck stop. I plan on mounting them inside a lens. One for each bike. Just haven't bought the extra lenses yet.

erohslc
erohslc Reader
8/20/10 6:58 p.m.

You can use a resistor to 'dim' an LED, but the results are non-linear, and vary from unit to unit. As stated, you can pulse an LED, but that's also problematic, because the of way the human eye works. Basically, a bright light pulsing at a high rate 'appears' the nearly the same intensity as the same bright light without pulsing. It's what makes movies, TV, and fluorescent lamps appear to be flicker free. Designers have used this 'trick' for years to reduce costs and more recently to save energy. Wave an LED alarm clock, and notice the flicker. Virtually all the runway marker lights at airports are pulsed LED's nowdays, move your head quickly to see the show.

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