We can then design a photodetector to sense changes in the beam size - and thus figure out when the lens should be moved.
The detector consists of a center circle and a surrounding ring. These are sized so the edges of a parallel beam just barely spill over onto the ring.
If the disk starts to move downward, the beam focuses inward depriving the ring of any light. This change is then programmed to reduce the current to the electromagnet, dropping the lens.
Or, if the disk tries to move upward, the portion of the light striking the photodetector’s ring increases, programming more current to the electromagnet current, raising the lens.
