Abstract: An optical sensor unit for detecting a predetermined object to be detected comprises an optical element, a connector for connection to a wiring cable of an external equipment, and a printed circuit board having a predetermined printed wiring thereon for connecting the optical element to the connector while the optical element is completely sealed by the printed circuit board at the lower end thereof and by the case at the upper portion and sides thereof whereby reduction of quantity of the light emitted from the optical element due to collection of dust can be prevented and whereby the attachment and detachment operations of the optical sensor unit can be effected with ease.

Abstract: In a medium detecting system having a plurality of optical sensors, each including a light-emitting element and a light-receiving element whose current varies with the amount of light received, and a load resistance unit provided in common for the sensors, the resistance of the load resistance circuit can be varied in accordance with load resistance selecting data supplied thereto. A sensor selector is responsive to sensor selecting data for selecting one of the sensors and connecting the light-receiving element of the selected sensor to the load resistance unit. A rewritable memory stores the sensor selecting data and data used for selecting the load resistance.