The led option controls the keyboard LEDs. This controls the turning on or off of one or all of the LEDs. It accepts an optional integer, a preceding dash(-) or an 'on/off' flag. If no parameter or the 'on' flag is given, all LEDs are turned on. If a preceding dash or the flag 'off' is given, all LEDs are turned off. If a value between 1 and 32 is given, that LED will be turned on or off depending on the existence of a preceding dash. “xset led 3” would turn led #3 on. “xset -led 3” would turn it off. The particular LED values may refer to different LEDs on different hardware. If the X server supports the XKEYBOARD (XKB) extension, leds may be referenced by the XKB indicator name by specifying the `named' keyword and the indicator name. For example, to turn on the Scroll Lock LED:
xset led named "Scroll Lock"
http://www.x.org/archive/X11R7.5/doc/ma ... set.1.html