Types of LED Taillight Circuits
I am looking to create some tail lights for my truck. Would like to have three arays. One for Tail/Stop, one for Tail/Turn, and one for Reverse. The struggle that I am having is trying to get the same aray to functon as the Tail/Stop and the Tail/Turn. I am looking at uing 5mm leds with 43 lights in the Tail/Stop, 18 lights in the Tail/Turn, and 14 lights in the Reverse aray. I currently have a three buld set up in the truck. One bulb for tail/stop, one bulb for turn, and one for reverse. Would like to plug and play with the new arays to the existing wiring. The wiring and resistor set up is where I struggle. The fabrication of the lenses and any housings is not an issue. ANy help you can give me would be great.
There are two methods for integrating multiple functions into a single LED tail light or LED brake light. The first we will refer to as the independent method. This means that you have separate circuits that are independent from each other. For example, the LED cluster might have twenty total LED bulbs. Ten are specifically for your tail light function (provides constant illumination while the headlights are on). The other ten are for your brake lights or turn signal. This method designates the first ten LEDs for one function, and the remaining ten LEDs for a completely different function. You can still share a common ground for both circuits, without taking any special precautions. Below is a description of an example circuit that utilizes our independent design method.
Example Circuit for Independent Circuit Design
Consider a taillight that contains 12 total LED lights. The LED taillight has three functions including taillights, brake lights, and turn signal. LEDs 1 - 4 provide the taillight function, 5-8 provide the brake light function, and 9-12 provide the left or right turn signal. The LEDs can be interlaced if you would like. In other words, you will have a taillight LED next to a brake light LED, next to a turn signal LED. Then the pattern repeats. There are four total wires leading into the unit. One black wire for common ground, two red wires for brake and taillight functions, and one orange wire for the turn signal function. When the vehicle headlights are turned on, power is normally fed to the taillight circuits. Current passes through your first red wire and into your 1000 ohm (1K ohm) current limiting resistor located on your board. This will limit the current in the circuit near 6mA. This resistor should be connected in series with all four taillight LEDs (1-4). The brake light LEDs (5-8) work with a closely related circuit. The only difference is the resistor value can be lower if desired, to result in a more intense light when the brakes are applied. A 300 ohm resistor in series with the four brake light LEDs should limit current close to 20mA. The last circuit, with four orange LEDs, provides turn signal functionally. The resistor value can be the same as that in the brake light circuit. When the turn signal is activated, current passes through the orange wire and finally through the turn signal circuit. Since our board example only contains one turn signal circuit, you will need two total boards, one for left and one for right. Connect the brake light power wire from the left board to the brake light power wire on the right board. The same goes for the taillight power wires, and ground wires. However, the turn signal power wires must remain separate and would connect to the left and right turn signal power wires on the vehicle. The only disadvantage with this type of design is that you do not utilize all 12 LEDs at once. Four LEDs light up for the brake lights, and another four for the turn signal, etc.
LED Taillight Combination Circuits
The previous example provides a very simple circuit that allows you to include as many functions as necessary onto a single LED board, but it requires that you use independent LED groups for each function. To utilize the board space and LEDs more efficiently, it is also possible to combine all three functions into a single LED group. Thus, All twelve LEDs on each board will function together at all times, but change their behavior based on the input combination. One way to achieve this advanced functionally would be to incorporate a microcontroller. However, this is not required for a basic LED taillight unless you wanted to include some additional functionally such as timers or fancy flash patterns. An simpler combination circuit can utilize a basic diode that will prevent current grounding back through the automotive circuits. Placing a diode in series with each current limiting resistor will offer this function. It is especially useful in this part of the circuit if each LED ground contains more than four LEDs. So suppose the turn signal LEDs are composed of 40 total LEDs (or 10 groups of 4 in parallel). It would not make much sense to include ten diodes, so a single diode can be included at the power input that feeds off to each of the ten LED groups. It is very important to include this diode, otherwise the current will ground backwards through the car's circuits.
Taillight with Brake Lights
The combination circuit example above does not explain how we produce two separate light intensities to correspond with the taillight (dim mode) and brake light (bright mode). The theory behind this relates to the formula used to calculate the total resistance of parallel resistors. To calculate the total resistance of two or more resistors in series, simply add the values together. However, to calculate the total effective value of two or more resistors placed in parallel, you need to use the formula below. If you run as few example calculations, you will see that two or more resistors in parallel will actually create a total effective resistance lower than any single resistor value contained in the circuit. For example, two 100 ohm and one 50 ohm resistor result in an effective resistance of only 25 ohms. This is the secret to creating a taillight circuit that will increase in light intensity when the brakes are applied. The current passing through the brake light power wires passes through a set of parallel resistors. The result of these resistors in parallel with the taillight resistors will lower the total resistance, therefore increasing total current through the LEDs.
Parallel Resistor Formula
RT = 1/((1/R1)+(1/R2)+(1/R3))