Using LEDs as Tailights and Brakelights

Light emitting diodes are interesting devices. They were not invented intentionally. Some regular glass bodied diodes were being used in a circuit that was operating near some photographic film. The film kept showing some fuzzy bands of exposure that had nothing to do with what it was intended to record. The problem was finally traced to the diodes. No one knew at that time, that solid state diodes emitted light. In this case the light was invisible to the human eye, but the film caught it. Now, of course, light emitting diodes are common place, used in digital displays, flashlights, traffic signals etc.

The ones used in traffic signals are particularly interesting to us, because they are very bright,, come in colors suitable for turn signals, running lights, brake lights, etc., and are UV resistant. moisture resistant, vibration proof. Traffic signal LEDs are 10mm in diameter, that is about .394 of an inch (a little larger than 3/8). A standard intensity red LED is 5 to 15 MCD in brightness, but come in various levels of brightness. I just bought a 10mm Red LED from Radio Shack (part no. 276-214) which is 300 mcd for use in this article, it cost me about $3. The red ones shone below are well over 18000 MCD while the yellow are about 17000 MCD.

These LEDs are colorless when unlit, but light up in their respective colors. Many people think LEDs are efficient sources of light, but that not technically true, though it is true for practical purposes. The actually give of a similar amount of light as an incandescent bulb on a watt for watt basis. LEDs are typically low power devices, so they will run a long lime on little electricity. They do not produce a lot of light, but they appear to produce a lot of light, because the light is focused and emits from only one end of the 'bulb', whereas an incandescent light emits in all directions. However, since turn signal and brake lights are viewed from from the rear of the car, LEDs work out just fine.

LEDs are very long lived, if not subjected to too much heat. As long as the LED is kept within it's designed current parameters, it will probably never need replacing during the lifetime of your car. What are the parameters and how does one create of maintain them? LED's are current driven devices. That is to say that they are not designed for a particular voltage, but are designed to operate dissipating a certain amount of currant. In the case of the Super Luminosity LEDs we are describing here, that current is 50ma. An automobile's electrical system operates over a range of voltages. We use 12 Volt batteries in our cars, but the car's electrical system runs at a higher voltage than that, because a 12 Volt battery will not charge at only 12 volts. The higher the voltage, the faster the battery charges, but to high a voltage will damage the battery and other electrical components of our cars. So, the charging system is usually set up to operate at 13.8 volts, more or less. This is why you notice that you lights are brighter when the engine is running, and maybe brighter still above a fast idle. We need to provide circuitry that provide the correct current to the LED at 13.8 volts DC. The simplest way to do this, is to wire a resistor in series with the LED.

There are other ways to wire LEDs, for example, using a solid state voltage regulator, this might be more cost effective for using a large number of LEDs, instead of adding a large number of resistors. We'll look at that later.

Now that we have a way to properly light up the LEDs, let create a project to utilize them. Let's say we wanted to add a third brake light (a good idea on our low cars). We could put a large array of LED's in a grill on the back panel, simulating the look of an original Ford GT40, except that our grill would light up when we stepped on the brake. If we wanted, we could even spell out the letters "GT 12" if we chose. Perhaps, we might like a wide narrow bar of LEDs across the top of our Mustang back glass, or maybe we are going to install a couple of "roof bumps" for head clearance, and we could mount a third brake light in the back of each one. I am going to show you how to do this, using your imagination, you will come up with other methods or designs.

By the way, this is not only a fantastically written technical article, it is a clever ruse to sell you LEDs. These LEDs are usually $3.00 to $7.00 each (if you can even find them) depending on the quantity purchased. I have even seen these for almost $10 a pop. I am able to offer them for $1.75 each or $1.50 each in quantities of ten or above. $1.25 each to members in good standing of the BBS regardless of quantity, just because I like you guys and want to offer you another perk as a reward for you participation. I do not have unlimited quantities of these, and do not know if I will be able to repeat the offer once I run out. Profits, if any, help to underwrite the cost of providing this site, and of course encourage further improvements. End of commercial.

Occasionally I get new batches, so pricing may vary from above, could be more or less expensive. For most recent pricing information click here.

I am creating an imaginary project. Let's say we wanted to convert an existing third brake light, to led use. We have a housing and a lens, and inside of course is a bulb and a socket. We would probably open the unit up an remove the bulb and socket, then apply our fabrication skills.

Here I have fabricated a piece of Plexiglas, using a saw, a sander, and a drill. I have shaped the Plexiglas to fit the insides of our imaginary third brake light. This took me 35 minutes. It could take more or less depending on your skills and equipment. It could be made from other materials, in other ways. I used a size "X' drill here.

The X size drill worked out just right. The LEDs were inserted into the holes and had to be pushed slightly to get them to go in all the way. If you have to force them in, it is too tight, if they fall out, it is too loose. If it is loose, you can glue them in with a dab of silicone caulking. Avoid using real glue, you want to be able to replace them in case one should ever fail.

The Plexiglas I used is 1/4 inch thick. There is nothing magic about the thickness, but in my experience, 1/4 is easier to work with than 1/8 inch. Many other plastics are easier to work with, you could use wood or metal too, nothing is critical here.

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As you can see, one leg (wire lead) is shorter than the other. that attaches to the negative battery polarity. Even if the to legs were even, you can tell which is negative. If you look closely where the arrow is pointing, you can see a small, flat section. This marks the negative side.

After solving the problem of mechanically mounting the LEDs, we can proceed to the electrical aspects. As we already noted, we can use a resistor with each LED to limit it's current. The current in this kind of circuit, varies with the supply voltage. This means that the LEDs will run brighter with the engine running than with the engine off, and possibly brighter driving that while idling the motor. We would need to choose our resistors to limit the voltage to an acceptable value at the highest voltage we would expect. Now, the voltage needed to provide the proper current to an LED is considerably less than 12 volts. So, we have the option of reducing the number of resistors we use by wiring some LEDs in series. That might look like this:

In the circuit diagram above, we would have to chose a new resistor value. It's wattage rating must be higher, because the power required for 3 LEDs is running through it, whereas only one LED was powered by each resistor before. The resistance value would change also. The resistor value (measured in ohms), must be calculated for each kind of circuit. Each different type of LED has a different required value depending on the total power in the circuit (volts X amps = power in watts), the number of LEDs in the circuit, and the voltage drop and current rating of each LED. In the case of the 10mm Super Luminosity LEDs being uses here, we need a 220 ohm resistor for each LED in the first circuit, with a power rating of at least 1 watt. In the circuit above where we are running 3 LEDs per resistor, we need a value of 100 ohms and 3 watts or better. You might be wondering if we can all 9 LEDs in series from one resistor like this:

The answer is no, not with our 12 volt supply anyway. When objects are wired in series, the voltage divides across the objects. Taking our 12 volts system (really 13.8 or so) we would divide the 13.8 volts by the number of LEDs (9 in this case), and arrive at 1.53 volts across each LED, we need about 2 for them to light. So, we could as many as 6 in series from one resistor at 13.8 volts, but what if our voltage fell to 12 volts? Dividing the 12 volts by six LEDs still leaves us with 2 volts each, right? Wrong, there are not 6 object in series in such a circuit there are seven, counting the resistor, and it takes a share of the voltage as well, so the practical limit would be 5. This would seem like an easy solution. Less soldering, only one resistor to buy, so, there must be a catch. There are a couple of catches. The first one is that as we add more and more LEDs to the resistor, the higher the power rating on the resistor must be. This means the resistor will be less common and more expensive. However, that is not the biggest drawback of this circuit. In this circuit, every LED is depending on the LEDs next to it for it's voltage. So, if one LED should fail, none of them will light, just like a string of cheap Christmas lights! This may not be so critical on a third brake light, but would not be a good idea for your primary brake lights. After all, you will not be able to pull into your local auto parts store for a replacement. In reality, we could not make the above circuit work anyway. The more LEDs we have in series, the higher the resistence of the circuit goes. The higher the resistance, the lower the current draw, and the lower the current draw, the dimmer the light output. In the case above, with 9 LED's, we would need a supply voltage of at least 25 volts. The practical limit for the number of LEDs of this type (wired in series) in a 13.8 volt system is 4 for maximum brightness, and 5 would probably work ok, wired in series with no resistor.

But, there is still another way to wire them. We can wire a number of LEDs in parallel with a single resistor in series. Like this:

Now, this looks pretty easy, doesn't it? In this circuit, the LEDs do not depend on each other, so if one does go out, the others will continue to light. There are catches here too. The first one is that as we add more and more LEDs to the resistor, the higher the power rating on the resistor must be. This means the resistor will be less common and more expensive. The other liability is that all the LEDs are depending on one resistor. If this resistor fails, none of the LEDs will light until you can replace it. This is why I advocate a several resistor approach, that way you have redundant circuits. LEDs are very reliable devices, and like I said, will probably outlive your car, but were safety is a concern, there is no reason to take unnecessary chances to save a dollar or two and an extra 20 minutes work.

I decided to insert a couple of pictures of one of these LEDs lit up, so you can get an idea of their brightness.


Here is a messy computer & workbench. As you can see, I have one of the LEDs lit up. This is in normal room light, picture taken from 10 ft.	Same thing, only this time I turned on the camera flash. As you can see, the LED is still quite visible in bright light.	In this shot, I have moved the LED to a position 4 feet from the wall in the background. The LED is next to the arrow, there is a red spot projected onto a white box at the wall	Same shot, except I switched the lights out. Remember, this is just a single LED. The high intensity part of the spot is about 4 inches in diameter.

Next installment (Part 2 in April) will cover using solid state voltage regulators to limit voltage or current for greater circuit protection and consistent brightness whether voltage is high or low. It will also cover techniques for variable brightness, such as tail lamp and brake lamp usage. Also solutions for difficulties using LEDs with turn signal flashers.

Part 3 (In Mayl) will cover using LEDs for instrument lights, dome lights, engine trouble lights etc.

Don't get ahead of me on this, unless you already know what you are doing. LEDs can be damaged by soldering temperatures or static electricity from your body, even by bending the leads, If you are not aware of how to prevent these problems, be patient and wait for the rest of the article.