The light emitted by the small bulb turns to 90 "after passing through the half mirror, and then gathers on the rotating body through the convex lens. Because the reflective strip is attached to the rotating body, the reflected light is reflected parallel to the incident light, so the reflected light passes through the convex lens again, and part of it reaches the phototransistor. The signal received by the phototransistor becomes a pulse wave after amplification and shaping.

Hope to adopt, thank you.