This is a new stepper designed for use with the power functions servo motor. The output will spin 90 degrees every time the servo fully turns to the right or left and returns to center. The basic concept is that when the servo leaves it’s center position one of the ratchets on the differential  housing disengages. As it continues to rotate the other ratchet stops the housing from moving, rotating the output. When it returns to center the engaged ratchet allows the housing to slip, so the output remains stationary. If you plan on building this yourself and it is not turning reliably check that the ratchets can rotate easily and the rubber band has enough tension.

Looks intresting, can you post a short video of the mechanism in action? I would help us to understand how it works 

12 hours ago, Gray Gear said:

Looks intresting, can you post a short video of the mechanism in action? I would help us to understand how it works 

Sure.

https://streamable.com/eeg14z

So if I understand correctly, when the servo rotates, the rotation goes through the fifferential, as this one is locked in place by this part . And when the servo returns to center, the differential housing rotes, as the friction on the 90° Steppr is greater than the one needed to make the differential move.
Anyway it's quite interesting. I might try to build it to understand how it exactly works.

19 hours ago, Touc4nx said:

So if I understand correctly, when the servo rotates, the rotation goes through the fifferential, as this one is locked in place by this part . And when the servo returns to center, the differential housing rotes, as the friction on the 90° Steppr is greater than the one needed to make the differential move.
Anyway it's quite interesting. I might try to build it to understand how it exactly works.

That’s pretty much exactly how it works. If you do end up building it let me know what you think.