2-6 Phaser 6121MFP Service ManualTheory of OperationAutomatic Document Feeder (ADF) Print PathSensorsThe printer contains sensors of various types that perform a variety of functions.One group of sensors track the progress of the paper along the paper path, anddetects if a jam occurs. Other sensors detect the presence of the Toner Cartridges,stop printer activity if a door is open (interlock), detect the presence of media inthe trays, and monitor the fusing temperature.For an overview of sensors in the printer, see “Sensors and Switches” on page 2-11.Sensor TypesThe types of sensors used vary with function. In general, there are three types inuse: photo sensors, microswitches, and thermistors.Photo SensorsTwo types of photo sensors are used, photo-reflective and photo-receptive. Photo-reflective sensors use light reflected back from an object to detect its presence.Photo-receptive sensors use an actuator or the object itself to block the light pathto detect an object or condition.Photo-reflective sensors have the light emitter and light receiver aligned on asingle surface. Output of the photo-receptor is High (> +4.5 V) when light is beingreflected back and Low (< +.3 V) when it isn’t. Photo-receptive sensors consist of aLED in one arm of a U-shaped holder, and a photo-transistor in the other arm.When the sensing area is vacant, nothing is between the arms of the sensor, lightfalls on the photo-receptor sending the signal High. If the light is interrupted, thephoto-transistor goes Low.MicroswitchesMicroswitches are used primarily as cover interlocks. They are in a normally openstate, and close when actuated. Microswitches employ hooks or catches forretention in the bracket or frame.s6121mfp-378
