Custom LED Brake Lights for 1991 BMW
I'm building custom LED brake lights for my car (1991 bmw 318is) thats essentially what i would like it to look like. the 'bars' you can see (where the lights are separated) are about 4&3/4 inches long by 1 & 3/4 inches tall. so I would need roughly 11 of these per side. I need them to be wide angle led's, and as bright as possible but also as cheap as possible. The would need to light up as all the yellows together, the middle reds together, and the outside reds together, and the white by itself. its going to be automotive so a 12v system will be what everything is running on. thats essentially all the information i have so far, i don't know what kind, size of leds to buy, i don't know what kind of board to buy, or resistors.
Red LEDs for Tail Lights
It sounds like your main objective is going to be finding a fairly bright yet inexpensive LED for the tail lights. Osram Opto Semiconductors Inc offers a nice surface mount LED in a 2-PLCC package. This LED has a wide beam rated at 120 degrees, low cost under $0.10 each, but with an intensity rating of 140 mcd. When used in clusters, it can be sufficient for many LED tail light applications. This LED is designed for and tested at 20mA, so it would be possible to drive it using a simple current limiting resistor. The LED also comes in orange and yellow, and Digi-Key offers cut tape if you do not require a full reel of 2000 LEDs.
If you find that this particular LED is not bright enough, then Osram offers a step up, the LS E67B-T2V1-1-1. It comes in a 4-PLCC package and offers 630mcd at 50mA drive current. Either surface mount LED will require precise assembly techniques onto the circuit board. Although normally done by machine in volume, it would not be difficult to complete the surface mount assembly using a soldering iron, for low volumes.
White LEDs for Back-up Lights
White surface mount LEDs tend to be much more expensive than red, by a factor of ten or greater. Since cost is a primary concern, you may want to consider reverting back to a through-hole LED package. However, this will restrict you to narrow beam angles generally between 10 to 70 degrees. For reverse lights, this may not be an issue.
Resistors in LED Taillights
If you plan on using current limiting resistors, two white LEDs per resistors is always a good combination in automotive circuits. Expect your white LEDs to each drop around 3.2 volts and 7.4 across the resistor. For 15mA current, a 490 to 500 ohm resistor, rated at 1/4 watt, should work fine. For your red and yellow, plan on four LEDs per resistor. A 290 to 300 ohm resistor should limit current around 20mA, and will drop 5.8 volts. This would also be a 1/4 watt component. The LED datasheets show the recommended solder pad layout, so you can use these drawings while searching for a compatible circuit board, or for a new board layout.
Taillight LED Spacing
You may want to experiment with various LED spacings. One possibility would be to start with a one inch spacing between LEDs. If this does not yield the consistency or spread you are looking for, you may need to decrease your spacing by as much as 50%. You may also require derating with a decreased spacing, to maintain a junction temperature within normal limits. Too many LEDs too close together can result in the generation of too much heat.