LinuxFocus.org: A serial line computer shutdown button and LEDFeb 11, 2001, 19:00 (0 Talkback[s])
(Other stories by Guido Socher)
"We will build a small device with 2 LEDs and one pushbutton. A green LED will indicate when the pushbutton is active and a slowly blinking yellow LED indicates that the server is working or still in the process of shutting down. To shut down the server you press the pushbutton for 3 seconds."
"When you press the pushbutton then the green LED will go off indicating the shutdown is in progress. The command "/sbin/shutdown -t2 -h now" is now executing. The yellow LED continues blinking while the server is still shutting down. When the yellow LED stops blinking then you should still wait 10 seconds before you power off the computer. The yellow LED stops when all processes are killed and the disks are being unmounted. If your computer supports automatic power off then you do not have to worry about the 10 seconds. Just wait until it has switched off it self."