| 摘要 |
PURPOSE:To attain precise mark detection independently of peripheral light intensity by forming a comparator for deciding the level of an output voltage of a reference side sensor by a threshold and switching an offset voltage by the output power voltage. CONSTITUTION:The comparator 6 decides the level of an output power voltage VSR of the reference side sensor 4 by the threshold VT and switches the offset voltage VOF by the output voltage. Namely, the output voltage VSR of the sensor 4 is compared with the threshold VT, and when the former value exceeds the latter value, the offset voltage VOF is increased by DELTAVOF to increase the input voltage V+ of the comparator 6. When the peripheral illuminance is extremely high and the sensor output voltage is included in the level of VS5, a mark side sensor 3 detects the peripheral illuminance, and even if the mark side input voltage V- is not reduced down to a prescribed value, the out put of a comparator 5 is correctly inverted. Namely, always precise mark detection can be attained independently of the peripheral illuminance.
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