What does this do?
This is a fan that will automatically come on when i sit at my workbench and off when i leave.
It uses an Ultrasonic Distance Sensor,
or commonly called a "Ping " sensor for detection.
An ATtiny2313 to handle the processing.
A power supply from a router. ( Wall wort )
A relay to switch the AC power.
The ping sensor works by sending out radio waves and times how long it takes for the radio waves to bounce off of an object then return. From this time we can calculate distance. I know it is more complicated than this but this is the simplified version Video Link