SD2

The SD2 electronic balance-wheel movement is a Chinese movement from the 70s.

It uses a transistorized circuit to energize a coil, which keeps the balance wheel oscillating. The coil is composed of two windings, one is used as feedback and it triggers the next pulse.

This movement runs at 36000 bph, more than most watches of this kind. The citizen cosmotron might be the only other transistorized movement with the same rate.