| 摘要 |
PROBLEM TO BE SOLVED: To prolong the service life of a medium detecting circuit and to enable the circuit to detect a shielding medium with a margin by receiving light from a light emitting element and setting the driving current value at the changing point of the presence/absence signal of a shielding medium at a value lower than the maximum value of the pulse-like current of a light emitting element by adding a margin to the driving current value. SOLUTION: A driving current setting circuit 32 sets a timer 36 to a prescribed duty ratio and a driving circuit 33 which drives a light emitting element 34 with a pulse to a driving current value. A light receiving section 35 receives the light emitted from the element 34 by means of a light receiving element 9 and monitors the presence/absence signal of a medium 30 which shields the light. The section 35 also detects the driving current value at the changing point of the presence/absence signal and inputs a set value to a pulse driving current setting section 31 by adding a prescribed margin to the detected current value. The section 31 also sets the driving current of the circuit 32 to a value smaller than the maximum pulse current value. Therefore, the service life of a medium detecting circuit is prolonged by suppressing heat generation and the circuit can perform detecting operations with a margin even when the performance of the circuit deteriorates. In addition, the maintenance and inspection of the circuit become unnecessary. |
