| 摘要 |
PURPOSE:To facilitate the checking of a photo sensor, by reducing current flowing through a light emitting element below that in the normal operation to detect whether the drop in the level of the light receiving element is lower than a fixed level or not. CONSTITUTION:When a sensor checking signal S1 is H, normal light emitting operation is performed in such a manner that current flowing through light emitting elements of sensors 33-37 is only limited with respective resistances R1, R3 and R5. For checking, when the sensor checking signal S1 is turned to an L level, current flowing through the light emitting elements is limited with respective combined resistances R1+R7, R3+R8 and R5+R9. Under such a condition, the output level of output signals S2-S6 of light receiving elements is checked whether to exceeds a fixed value or not. When it is below the fixed level, a photo sensor is determined to be polluted to drive an alarm circuit. |
