I had a couple Narco VOR instruments, and spent some time figuring out how they work, then some more time to figure out how to control them from Arduino, and ended up with quite a nice result that I wanted to share:
https://www.youtube.com/watch?v=_UpgLZphPrI
Both the CDI needle and to-from indicators work the same way, and its like bi-directional voltmeter indicator, where if the two reference pins have a certain differential the indicator goes on one side, and if they have the opposite differential they go the other side.
The OBS ring was quite harder to understand, and then even harder to figure out what to do with them in order to get the OBS bearing value. Literally, its like there are four potentiometer, where there is a common reference, and a reference at 0, 90, 180 and 270 degrees.
Using that I could read three points on the scale, that each gives good reading on +- 90 degrees. Using them combined gives the whole 360 degrees with a certain precision; and then it is used with the bearing received (via flight sim, trough link2fs, to the arduino board) from the simulated VOR station, and reacts accordingly, interacting with the user's OBS knob value.
Its actually quite fun to play with. :)