The units contain an L.E.D. light source and a photo sensor that scans a shaft‑mounted, slotted disc. An internal pulse‑shaping amplifier circuit delivers a rectangular pulse signal from the current sinking output in response to the passing slots as it rotates. They can be direct‑coupled to a machine shaft by means of a flexible‑bellows, spring, or rubber sleeve type coupling that allows for axial and radial misalignment. They can also be coupled with light instrument timing‑belts. Timing‑belt drives also allow convenient gear‑up or gear‑down speed ratio changes that can be useful for obtaining non‑standard PPR rates.