A linear or annular optical grating cooperates with a detector unit having two detectors on the front face of a carrier member of rotational symmetry and being either directly rotatable through a rear-face screwdriver slot, or via a cam-operated lever, the cam being rotatable by screwdriver action. The two detectors are aligned along a diameter line of the carrier member and will produce two phase-shifted pulse trains upon relative movement between the detection unit and the grating. The preferred field of application is matrix printers, the grating defining print position.