| 摘要 |
PROBLEM TO BE SOLVED: To enable maintaining the constant value of a current which is made to flow in a light emitting element, by installing an up and down counter circuit which performs count operation corresponding to detected light quantity level, and a current value adjusting circuit which resets a current value supplied to the light emitting element in accordance with the output result of the counter circuit. SOLUTION: A current which is generated in a light receiving element 2 in accordance with light quantity emitted from a light emitting element 1 is compared with a comparison voltage of a voltage source 6b of a comparator circuit 6 via a current voltage conversion circuit 5. A counter circuit 7 up-counts or down-counts a clock signal CK, in accordance with an U/D signal and a control signal EN1 from the comparator circuit 6. Digital output composed of (n) number ((n) is an arbitrary integer) of outputs of the counter circuit 7 which are obtained from result of count is converted to an analog voltage in a D/A circuit 8, and resets a current value of a constant current source 3a. By the operation, the value of a current which is made to flow in the light emitting element 1 can be maintained to be constant. |
