I wanted to know if my Air Condtitioning was running or not at a quick glance. So I made this mini turbine that uses magnets to stick to a vent. It spins freely when the AC fan is running.

Printed in Cura at .2 layer height and 15% infill. Used .05 inch diameter solder for the axle. When you put the solder (or whatever) thourgh all the pieces, pull the turbine down (away from the base) a bit enough to bend the axle. This will allow a better spin for the blade. As for the magnets, they were 1/4in X 1/4in cyclinder magnets. Simply put them in the holes and added tape over them.