A DC motor assembly includes a motorized unit for generating a rotatable power at an output shaft, and a step adjusting control arrangement including an optical grating and a photocoupler. The optical grating, which is operatively coupled at the output shaft, has a plurality of light transmissible portions and a plurality of light blocking portions alternating with the light transmissible portions. The photocoupler is activated to send out an impulse signal in responsive to a phase shift between the light transmissible portion and the light blocking portion of the optical grating, wherein the output shaft is controllably driven to be rotated and stopped in a sequent manner as a stepping movement thereof in responsive to the impulse signal so as to controllably adjust the rotational speed of the output shaft.