[Al Skierkiewicz]

Robert,
As you can see from the above reference, most LEDs are near 2 volts when turned on. The exceptions are the blue and white and combinations of those. To make the resistance calculation, subtract the LED voltage from the power supply voltage. In our case, 5volts-2volts(LED)=3 volts. This is the voltage the resistor must drop. A good rule of thumb for most general purpose LEDs is 20ma. Using Ohm's Law solving for resistance R=V/I=3/20ma=3/.02A=150 ohms. 150 is a standard value of resistance but if you can't find one, 180 ohms is also a standard value. Using Ohm's law for current I=V/R=3/180=.016=16ma. Although the LED would be not as brite at 16ma, you might never see the difference. As pointed out in the documentation, the output circuitry is limited to a maximum amount of current so using the 180 ohm resistor is better for the circuit but gives a dimmer output for the LED.