Here's an old school rotary dimmer with a drag rotate. It could be automated by placing a transparent object over the right and left sides, assigning actions to them to continue/reverse and continue. For the last keyframe you can set a keyframe one frame after to serve as a backstop by forcing it to jump back to the last keyframe. You can also set a pause at the mid point.
Note that the dimmer elements and the drop shadow are set to shift brightness and contrast
Dimmer.zip (281.9 KB)