A wind vane is normally a spinning object with a strong bias to align itself into the wind. An arrow is the classic device with this characteristic: it has a wind catching feature on one end, i.e. the tail feathers, which keeps it moving into an air stream. Using it as a wind vane requires mounting the arrow with an axle through its center of gravity. This allows it to spin smoothly. The feather(s) at the far end places its aerodynamic center-of-pressure well behind the center-of-gravity. The result is it will always spin so it faces directly into the oncoming wind.

It is possible to sense the orientation of a wind vane just like an anemometer, but with many points on a disk attached to its axle. In this case, we would need to keep track of the motion of points as the vane moved. Or each position of the vane could have a printed code we could read optically or magnetically. Another method attaches a potentiometer (a variable resistor) to the axle.

In any case, if energy efficiency is important, we’d like to use a technique that only consumes energy when the wind direction changes.