Electronic – How to automatically turn off a Fan after a fixed amount of time

Since this is for a real world application, not a hobby project or a school assignment, the requirement not to use a microcontroller is just silly.

A micro is the obvious and easy way to do this. A very small micro like a PIC 10F200 could do this, but since this is also apparently a one-off there is no need to economize. Something like a PIC 18F2520 would be easy. That's way overkill, but the extra $3 or so will be made up with easier programming. This part has a internal RC oscillator, so you pretty much feed it power with bypass caps and it can run.

I agree with Wouter in that relays are appropriate for this since it's a one-off and you are switching very slowly. One DPDT relay is used to set the polarity, and a second SPST is the on/off switch. Now you only need circuits to drive the relay coils from two digital output lines from the micro.

This module

uses the switch as input and replaces it in the circuit with a relay. After the switch is switched off the relay remains on for a programmable time between 0 and 10 minutes.