LIGHT BEAM SCANNER

PURPOSE:To allow the control of the revolving speed of a revolving shaft and to contrive the simplification of constitution and the reduction of a cost by providing the transmission parts for a light beam which are a part of a revolution detecting mechanism on a rotating substrate previously coaxially mounted with good accuracy to the revolving shaft of a driving means. CONSTITUTION:The light beam transmitted through a hologram pattern 2 scans on a projecting surface when the rotating substrate 3 and a hologram disk 1 is rotated by the revolution of the revolving shaft 9 of a motor 8. The other light beam divided by a mirror 13 transmits the transmission parts 7 on the outer periphery of the rotating substrate 3 and is made incident on a photodetector 15. The output of the photodetector 15 is shaped in waveform to the pulse waves of the frequency proportional to the rotating speed of the motor 8 by a waveform shaping circuit 16. This output signal is subjected to phase comparison with the output signal from a reference signal genera tor 17 in a phase comparator 18, by which the output of the voltage proportional to the phase difference is obtd. A motor driving circuit 19 controls the driving of the motor 8 in accordance with this output. The revolving speed of the revolving shaft of the driving means is controlled in this way by the simple constitution at the low cost.