FWIW, Atmel AVR (the Arduino microcontroller) I/Os are typically beefy enough to drive the 20 mA that LEDs usually like to get nice and bright. It was one of th

FWIW, Atmel AVR (the Arduino microcontroller) I/Os are typically beefy enough to drive the 20 mA that LEDs usually like to get nice and bright. It was one of their early differentiators. Depending on the LED, I would have no problem driving it directly with just a current limiting resisitor.