how to find the right resistors for led's First, you need to know the resistance needed. For this, you need Ohm's Law. (Vs-Vf)/If=R where: Vs = Source Voltage (either 3.3v, 5v, or 12v) Vf = Forward Voltage (almost always stated on the package) If = Forward Current (usually 20-30mA, stated as 0.02 to 0.03 in the formula) R = Resistance in Ohms For example, a 2.1v, 20mA LED using a 5v power source would figure like this: (5-2.1)/0.02=R 2.9/0.02=R R=145 Ohms Use this number as a rough guide and purchase the resistor(s) that is the next highest value of R (in this case, I believe it'd be 170 ohms). You also need to know what Watt rated resistor you need. P=If*(Vs-Vf) <--- Power Law P = Power in Watts If = Forward Current from the previous formula Vs-Vf = what you used in the previous formula also Continuing with the example: P=0.02*2.9 P=0.058 watts If P <= 0.25, then use 1/4 Watt rated If P > 0.25 but <= 0.5 use 1/2 Watt rated If P > 0.5 then use 1 Watt rated Using these numbers will help you purchase the resistor(s) you will need for a project working with LEDs. Submitted by Snick