2-6 Phaser 6121MFP Service Manual
Theory of Operation
Automatic Document Feeder (ADF) Print Path
Sensors
The 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, and
detects 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 in
the trays, and monitor the fusing temperature.
For an overview of sensors in the printer, see “Sensors and Switches” on page 2-11.
Sensor Types
The types of sensors used vary with function. In general, there are three types in
use: photo sensors, microswitches, and thermistors.
Photo Sensors
Two 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 path
to detect an object or condition.
Photo-reflective sensors have the light emitter and light receiver aligned on a
single surface. Output of the photo-receptor is High (> +4.5 V) when light is being
reflected back and Low (< +.3 V) when it isn’t. Photo-receptive sensors consist of a
LED 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, light
falls on the photo-receptor sending the signal High. If the light is interrupted, the
photo-transistor goes Low.
Microswitches
Microswitches are used primarily as cover interlocks. They are in a normally open
state, and close when actuated. Microswitches employ hooks or catches for
retention in the bracket or frame.
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