Can I ground a digital pin on an Arduino to provide input?
A simple diode to the relay should work for you if you use the pull-up on the Arduino input. The diode prevents the fan power from driving into the pull-up. If you are using a 3.3V Arduino you may need to use a germanium diode.







simulate this circuit &ndash; Schematic created using CircuitLab

A small series resistor would not hurt too just in case you happen to drive the pin with the relay closed.