anorton:
vasquo:
Compute your resistor for 10mA forward current and go from there... adjust if it's too bright or too dim.What do I do about forward voltage?
Nothing really, just use a 500 ohm series resistor between the led and wire that network between ground and the 5V pin on your arduino. The led will light up if you have the polarity of the led wired correctly. Once you see it lite up you can measure the voltage drop across the led with your digital multimeter and you will then know what it's forward voltage drop value is. That can then be used to calculate a series resistor value to run the led at whatever current you wish. Most common leds are rated for 20ma maximum continous current, but they are useful indicators lights at any current from 3 ma up to 20ma. There is no reason you have to run an led at a full 20ma, many people spend way to much time worrying and fussing about that, just run them at 10ma and life gets simple. You will find that the color of the led is what has the biggest effect on it's forward voltage drop rating. Red are around 1.5vdc.
Lefty