How do I install LEDs in my car? This is the question we all ask at some point in time. Well here is the answer: First you need to know a couple of things. Your cars voltage: about 12v and the voltage drop across the LED you want to use. It’s safe to assume 1.7 volts for non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and low-current Red, and 2 volts for Orange and Yellow, and 2.1 volts for Green. Assume 3.4 volts for bright White, bright non-Yellowish Green, and most Blue types. Assume 4.6 volts for bright Blue types. Now for cars with a 12v system a standard Red LED will drop 1.7 volts leaving you with 10.3 volts. The current across the LED should be about 20 mA to make it last but still be bright. So divide your remaining voltage, 10.3 by .02 or 20mA and you current limiting resistor should be about 510 ohms. A standard Yellow LED will Drop about 2v leaving you with 10 volts and requiring a resistor of about 500 ohms. A standard Green LED will drop about 2.1v leaving you with 9.9 volts and requiring a 480 ohm resistor. A standard Bright White LED will drop about 3.4v leaving you with 8.6 volts and requiring a 430 ohm resistor. A standard Bright Blue LED will drop about 4.6v leaving you with 7.4 volts and requiring a 370 ohm resistor. You will notice that an LED has one lead that is longer than the other. Usually the long lead is the positive side of the LED. Most digital volt meters have a Diode checking setting that can be used to verify this. If you don’t have one handy, tape two AA batteries together, + - + -, and touch the wire soldered onto the long lead to the button, the + side, and the other to the - side of the batteries. Your LED should light up. Connect the + side of the LED to the hot wire and the other side to the return or ground. These values are approximate. They can be adjusted by a few ohms if you can’t find the exact value or you want the LED’s a little brighter or dimmer. A larger value resistor will make the light less bright, a smaller value resistor will make the light brighter. But don’t make the resistor value too small or the LED will explode! As an example I used a 538 ohm resistor to make Green LED’s very dim, a little too dim so I had to decreased the resistor to 510 ohms to get a little more light. I was looking for a subtle effect in the lighting of the cup holders in my focus. Auto LED's