| 摘要 |
<p>PROBLEM TO BE SOLVED: To enable exact measurement while eliminating the influence of a sensitivity change caused by temperature changes of a light receiving element or the like by setting a value, for which a prescribed value is added to the bottom level of a photodetecting signal detected by a photodetector, as the threshold value of a comparator. SOLUTION: The quantity of received light made incident to light receiving elements 13a and 13b is obtained as photodetecting signals outputted from photodetecting signal detecting circuits 3a and 3b. An MPU 5 fetches the quantity of light incident to the light receiving elements 13a and 13b for one cycle from an A/D converter as a digital signal and obtains a minimum value (bottom voltage). Next, the MPU 5 adds a prescribed value (margin voltage) to a minimum value thus obtained and outputs a resultant digital value to a D/A converter, which converts the digital signal outputted from the MPU 5 to an analog signal and sets it as the threshold value (reference voltage) for a comparative output of a comparator. Thus, exact detection is enabled while eliminating the influence of a sensitivity change caused by temperature changes of the light receiving elements 13a and 13b or the like.</p> |
