My first “rack” was made of wood. It monitored, meaning displayed and alerted, on some metrics on my Linux server. And controlled a few lights and the alarm system in my room.

I really enjoyed making the computer control the lights.

Get 5 inputs and 8 outputs from a computer printer port using only a few transistors and resistors.

Simple alarm circuit to monitor up to eight inputs, from e.g. production equipment. If any inputs are triggered an alarm is sounded. Uses the AVR AT90S2313 microcontroller.