January 18, 2009

Custom LED Third Brake Light Retrofit

Ever since I installed the JDM style red/smoked LED tail lamps, I have been a bit bothered by the delayed reaction time of the factory incandescent bulb in the third brake light. Because LEDs turn on and off much faster, the third brake light always lagged behind the tail lights by nearly 1/4 second when I hit the brakes or release them.

So since my buddy was ordering some LEDs for his LED running light project (A la Audi), I took the opportunity to share the shipping cost and ordered me some nice high powered 4-chip Super Flux LEDs in red. These LEDs have a stated 100 degree viewing angle, which is important for automotive retrofit applications. And the wide angle also means the brightness specs mean something. A 20 degree 5mm LED might have more mcd on axis, as soon as you move off that 20 degrees, the brightness drops significantly. The 4 leg Super Flux LED is also shorter than standard 5mm LEDs and it helps a lot in this application as there isn't much depth between the housing and the lens on the sides.

Also important to consider is the colour of the LED used. LEDs produce light in very tight wavelength. This means a white/blue LED will have very little output in the red colour spectrum. And since the red lens on the third brake light is essentially a photo filter that only allows the red spectrum to shine through, if you used white LEDs behind it, you'll lose a lot of light. Test this by shining your white LED source (keychain light, flashlight, whatever) through a red lens, then through a white or clean lens, and see how much of a difference it makes.

So, anyways, onto making the LEDs

1. Remove the third brake light - Take the cover off, which will expose two 10mm bolts on either side. remove them, then un-clip the center from the hatch and unplug the harness from the bulb socket.

2. Remove the bulb and red lens - Twist the bulb holder out. For the lens, there are four clips. Remove the wider, flatter ones first; it's easier. Use a flat head screwdriver (carefully) to help if needed.

3. Cut the circuit board to size - Pretty self explanatory. Cut to desired size to fit the housing and number of LEDs you are using. Keep in mind the board has to fit inside the inner lip in order to clip the red lens back in.

4. Design your circuit - There are various LED calculators available online if you know the specs of your LEDs. And you can design your circuit in series, parallel, or a combination of both. Make sure you know what you're doing here, or get help from someone who does.

4a. Power goes out - What? It didn't happen to you? Well, it happened to me right about here, just before I can test my circuit. Can't just skip to the next step even if I wanted to since the soldering iron needs electricity too. I played some DS and ate some food (love gas stoves) until the power came back an hour and a half later.

5. Test/Make your circuit - Use a breadboard to make sure the circuit will work, and layout how you're going to place the components on the circuit board. then, just solder the pieces in place.

6. Put it together - Place the completed and tested circuit in the third brake light housing. If you cut your circuit board properly, it will sit nicely inside with the pressure from the metal heat shield inside the housing. Then clip the red lens back in, and it should hold the circuit firmly in place.

7. Re-attach third brake light to the car - Run the lead wires out through the bulb hole. You don't need to put the bulb socket back in. Just attach the positive lead to the white wire side of the harness, and the ground to the black. Clip the housing back in and secure with the 10mm bolts. Put the cover back on, and you're done.

I'm using 6 LEDs that have a forward voltage drop of 2.0V, and current draw of 80mA. I want to run each LED in parallel so that if one LED shorts out, no other LED is affected. I also found a circuit board that was printed so that each horizontal line was already connected, making my job much easier as I don't have to solder connections between every LED. I went with a single 150ohm resistor before the path splits to each LED. And according the the LED calculators, I needed a 1W resistor to handle the heat based on a 14v input. So that's what I got. It's enough since the brake light is not constantly on like a taillight or license plate light.

I'm happy with the results. The reaction time now matches the LED tail lights. The viewing angle is very good, and people say it's definitely bright enough to notice driving behind my car. Compare the OEM JDM version which cost $170USD, my retrofit for a fraction of that is a pretty good deal. Plus it was fun making it. I hope you have fun making yours as well.