MOTOR ROTATION DETECTING CIRCUIT

PURPOSE:To enhance rotation detecting precision and reliability, and to avoid a dangerous state like runaway, by detecting an abnormal state with the change of the height of rotational pulse. CONSTITUTION:In order to detect the rotational frequency of a motor 1, a rotary shaft 2 is provided with the light reflecting surface 11 and non-reflecting surface 10, and light generated from a light generating element 3 is reflected on the reflecting surface 11 and is permitted to reach a photodetection 4. The rotational pulse is waveform-shaped by a Schmitt trigger IC 5, and is counted by a counter 6, and the rotational frequency is computed by a micro-computer 7. Besides, in order to detect the abnormal state of the rotational output pulse of the motor 1, the normal value threshold voltage value of motor rotational- output pulse height is determined by resistors 14, 15, and the voltage value is set to be of the comparison voltage of a comparator 12 and is compared with the rotational output pulse. As a result, the abnormal state of the rotational output pulse is detected.