| 摘要 |
PURPOSE:To eliminate the need for readjustment of the sensitivity even with a change in the kind of the sheet by obtaining a bipolar output from a photodetector corresponding to the front/rear edge and outputting respectively the 1st and 2nd detection pulses from a signal detection circuit when the bipolar output exceeds a prescribed value. CONSTITUTION:When the kind of a sheet 4 is changed and the reflectance of the sheet 4 and a mark 5 is changed, the photodetection output of photodetection areas PD-a, PD-b is changed respectively. For example, when the difference in the reflectance between the sheet 4 and the mark 5 is decreased, the waveform of each part is as shown in dotted lines in figure and it is changing, but if the level exceeds a threshold value + or -Vth of waveform shaping circuits 14, 15, the mark is detected surely without sensitivity adjustment, that is, while the threshold value Vth is remained constant. Thus, no sensitivity adjustment is required even when the kind of the sheet is changed.
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