Honda Civic LED 3rd Brakelight Upgrade

The 3rd brakelight on the Civic, at least on my '99, is not the most impressive piece of equipment. It works, but I felt it needed more power! Home Improvement, anyone?

Anyways, I started the project a couple years ago, making a rather simple LED assembly. I had it in the car for some time and it worked to this day. The problem was that I bought the LEDs at a local store, so they were expensive. I decided to build a bigger and better LED brakelight, this time, with on-line parts. I was able to research some various LEDs to find ones which were not only bright, but also featured a wide viewing angle.

The picture to the right shows the original iteration of my LED brakelight. It featured 15 LEDs, which are clearly visible. While it was bright, it wasn't wide-ange, and it didn't fill the entirety of the red diffuser lens that Honda put in there.

The Original LED Design

As you can see on the right, the closeup of the original reveals a lot of gaps between the LEDs. These were very bright, but very directional LEDs that I bought locally. There were four resistors, as the entire thing was a series/parallel circuit. I had four LED/resistor circuits wired in parallel. Three of them had four LEDs, and the last one only three. The last resistor had to be of a different value, to accomodate the slight change in circuitry. More on the gory details when we come to the new design...

The Original Design - Up Close

In order to fully light up the red diffuser lens on the brakelight assembly, I had to create a bigger board than the original design. In the original, I glued the board in place, inside the factory bulb housing. This allowed me try the LED setup without major modifications or investment in the project.

For this new design, I opted for a bigger board, that I custom cut to fit the factory mounting holes. The picture to the right shows the front and back of the board. The front was painted red, to give a finished look to the assembly.

You'll notice two holes near the edges. These fit into the same holes as the holes on the plastic "ears" in the image above. I'm still using the factory mounts, but will use longer screws and stand-offs to get the board an inch or so away from the factory diffuser lens.

The Board - Front and Back

Here's a picture of the finished brakelight board. The LEDs I decided to use are wide-angle and very bright. They are made by OPTEK, Inc. and have been quite impressive thus far. Here's a link to the spec sheet for these insanely bright LEDs.

The LEDs have an absolute maximum current of 70ma (0.070A or Amps). I decided to run the circuit at 50ma for each LED. I felt this would keep the LEDs from being overly stressed and still provide excellent light output.

Populated with LEDs

The back of the board looks quite busy. The LED array is 19 LEDs wide and 3 LEDs tall. Each vertical group of LEDs is controlled by one resistor. If you're new to wiring LEDs, a resistor is an absolute must. If you just wire them up to your car's electrical system, the LEDs will burn out instantly.

In this design, each vertical column of 3 LEDs had one resitor limiting the current to 50ma. The slanted ends were wired together, sot hat the single LED was wired in series to the adjacent column of 2 LEDs. So, I wound up with 17 resistors and 19 columns of LEDs.

Back of the board with current-limiting resistors installed

A little bit of basic electronics is required to make your own LED setup, for whatever project you choose. Let's look at how to do this...

First, you have to be able to calculate the current in a circuit, based on a particular voltage and what the LED can handle, current wise. The basic formula (Ohm's Law) is E=IR, or (in English) Voltage (E) = Current (I) x Resistance (R). Voltage is measured in Volts. Current is measured in Amps, but for this exercise, we'll also use milli-Amps or ma. A milli-Amp is one thousandth of an amp (not much, i know). 1000ma equals 1A or Amp. Resistance is measured in Ohm's.

Second, you have to know a bit about the LED(s) you'll be using. Two ratings are important. One is known as the forward voltage. This is the voltage (or Vf) that will be dropped across the LED when it's running. For my LED, at the design current, the spec sheet said about 2.35V. The other thing to decide on is how much current to use. For my LED, the spec said the max was 70ma (0.070A). I decided to run 50ma, just to be safe.

All 51 burning brightly

So, armed with some specs and a bit of Ohm's Law, let's calculate the proper resistor to use for this setup...

My Civic , while running, produces about 13.8V in the eletrical system. I'll use this number as a starting point. In order to properly calculate current flow through our current-limiting resistor, we have to take into account the forward voltage of the LEDs. I decided to wire three of them in series, each having a forward voltage drop of about 2.35V. So 2.35 x 3 = 7.05V. So, 7.05 Volts will be dropped over the 3 LEDs, while what's left over will drop on the resistor itself. Taking our 13.8V from the car, substracting 7.05V for the LEDs.... and we get 6.75 Volts.

I've said earlier that I wanted to run 50ma through the LEDs. Since I now know the voltage across the resistor, and I know the current I'd like to have flow, we can calculate the resistance required. The basic formula isn't convenient for this, so let's re-arrange it a bit. If we divide voltage by current, we'll get the resistance required. So, dividing the 6.75 Volts across the resistor by 50ma, gives us a resistance of 135 Ohms. That's 6.75/0.050=135. To get the correct result, you have to convert milli-Amps to Amps. Just move the decimal point three spots to the left and you're all set. 50ma equals 0.050A. A 135 Ohm resistor is tougher to get, but a 130 Ohm is a standard resistor value. This slightly smaller value resistor will push the current up slightly. If we recalculate this, we should now have a circuit that runs at about 52ma. Since I'm not pushing the LED to the max, the extra 2ma won't matter in this case.

The array also functions as a flashlight!

With the old design, heat was not an issue. As I soon found out here, it's to be respected...

I tested this design with three LEDs and one resistor. I basically made a one-column version of this array. It ran all night and only got slightly warm. I felt this warmth was acceptable, so off I went building my array. I went with 1/2W resitors, as the current in the circuit was dissipating about 340mW in the resistor. Everything seemed well in spec...

...Until the whole thing got put into the large array. While insanely bright (too much, really) it also got quite hot. Both the LEDs and the resistors got hot. The resistors got too hot to touch after two minutes. Given I sit in traffic, I can't have a brakelight that has a melt-down from continuous operation. The light can maybe take the heat, but I'm not so sure about the piece of plastic it sits inside of.

Calculating how much power is dissipated in a resistor is quite simple. Using Watt's Law, where Wattage equals to Voltage times Current. So, taking the previous 6.75V across the resistor and multiplying it by 0.050A, we get 0.3375W or Watts. It may not seem like a lot, but a third of a Watt being generated by a small part can get it quite hot! This applies especially when there are a lot of parts dissipating this much heat. If you multiply the 0.3375W across 17 resistors, that's 5.7W! That's half the wattage of a small soldering iron!

Back to the drawing board... I tried limiting the current to about a third of the original design and the light still produced very good brighness, without the crazy heat. I had two choices, replace the original resistors, or add a couple high-wattage ones at the input to the whole thing. I went with the latter, as I came up with another idea that would go well with the additional resistors.

That's very, very red.

My engineering snafu gave me an opportunity to make the whole thing nicer. I wanted to reuse the original bulb base, as it would allow the brakelight to plug into the factory harness without modifications. I used my trusty Dremel tool to cut the necessary shapes.

The board mounts with two screws into a couple of factory mounting holes in the brakelight assembly. I experimented with resistors to find what value would produce the correct brightness, without producing too much heat. I found that about 25 Ohms at the input to the whole light assembly would do the trick. I purchased two 10 Ohm resistors along with two 5 Ohm resistors. Each is rated at 5 Watts, so no worries on them not handling the heat. I played around with different combinations, but settled on all four of them wired in series. This produces plenty of light, without heating any of the resistors much.

Now, what do those capacitors do? The answer is coming...

The current-limiting board

The capacitors are used to make the brakelight flash briefly, when it first turns on. From earlier on in the project, I mentioned that the original configuration produced tons of light, but too much heat. The new resistors (the big white ones) have turned that down, so the light stays cool in continuous operation.

The capacitors are all wired in parallel with each other, producing 4000uF of capacitance. That "big capacitor" is wired across the input and output of the four resistors. The effect is this... for a fraction of a second after the brakes are applied, the light glows at its full brightness. As soon as the capacitors fill up with energy, they quit flowing current and all current flows through the big resistors. So, for a fraction of a second, those big resistors basically don't exist, as far as the LEDs are concerned.

Those LEDs can't handle the full current for long periods of time, but a fraction of a second is fine. The effect is that te brakelight is a bit more eye-catching, as it turns on at maximum brighness and tapers down quickly to its normal brightness level. The effect is subtle but pretty cool.

The four 1000uF capacitors are the biggest I was able to find in my local electronics store, as this addition to the project was an after-thought. For the flash to last longer, the capacitors would have to be much larger, and therefore would take longer to stop flowing current.

It's really tough to get into all this and not be too technical, but it all falls under some fairly basic electronics. Wiring up a simple led arrangement requires one easy formula and a few part specs. In my case, my original design for this was drawing over 10 Watts of power. That's quite a bit for a bunch of LED packed tightly together. When you decide to do your own design, keep in mind that heat can be a factor when making your supercharged brakelight! Luckily, I tested mine on the bench before putting it in the car. Always be careful when doing this type of wiring, as you don't want to have something melt down on you. 3rd brakelights usually sit in plastic housings, so a lot of heat is not a good idea, even if the electronics themselves can handle it.

The LED array is under the board

The image to the right shows the array in all its glory. The diffuser lens on my car is made up of a whole bunch of tiny lenses. The light actually seems to have more LEDs than it really does. The nice thing is that now, the brakelight is full of light, even at steep angles to the side. It's made the car more noticable in traffic, without being overly bright. I wasn't looking to blind people with it. I want to have good brakelights, without annoying people who sit behind my car at night. I think this setup has worked quite well towards that goal.

Installed and ready to go! ... I mean STOP!

The image on the right is a comparison of the original and the new design. While the older seems brighter in the image, the new one is actually just as bright. The added benefit of filling the whole lens with light makes it "bigger" to the viewing eye. So, this will hopefully mean that I'll never be rear-ended by anyone!

To finalize, I wanted to provide some additional resources...

The OPTEK Inc. LEDs were purchased through Mouser Electronics. They are part number OVFSRAC8 and Mouser part number 828-OVFSRAC8. The LED product page for this LED at Mouser is here. You can find the spec sheet for the LED here.

As an additional tool superbrightleds.com has a neat calculator right on their site. It does what was described above, but it does it for you automatically. The calculator is located here. I still like to do the math myself, but it's a nice tool for verifying your calculations.

To learn more about Ohm's Law, visit here or here.

To learn more about Watt's Law, visit here or here.

Last updated December 26, 2006.

New light on top, old one on the bottom