From AVRFreaks Wiki

[edit] The LED interface

LEDs are controlled by a sysfs interface. It lives in

/sys/class/leds

There is one folder per LED here.

[edit] Attributes

Each LED has a lot of attributes

 brightness  device      power       subsystem   trigger     uevent

The interesting ones are just "brightness" and "trigger". If you simply want to turn the LED on and off, just use the brightness attribute like this (example for LED "a"):

#: echo 1 > /sys/class/led/a/brightness    #turn it on
#: echo 0 > /sys/class/led/b/brightness   #turn it off

[edit] Triggers

An LED may be triggered by a number of events. For example it my come on when there's hard drive or MMC card activity, it may pulse to the systems's "heartbeat" or a number of others. To see the trigger options and the currently selected one, read the trigger attribute of the LED you're interested in.

#: cat /sys/class/led/a/trigger
[none] mmc0 timer heartbeat default-on

This shows that the "none" trigger is selected and there are 5 triggers available. To make the LED flash like a heartbeat

#: echo "heartbeat" > /sys/class/led/a/trigger

To make it flash when the mmc card is being accessed

#: echo "mmc0" > /sys/class/led/a/trigger