Brother MFC620CN Service Manual page 128

• There are four ink cartridge sensors, one for each color. The sensor actuator inside the ink
cartridge usually blocks the light path to indicate the presence of ink. When ink runs low (near-
empty), the arm moves out of the beam, activating the sensor. The same signal also indicates
whether there is an ink cartridge present.
• The large-volume ink cartridge sensor detects whether a large-volume ink cartridge is loaded.
• The hook switch sensor detects whether the handset is on the handset mount.
Most sensors are photointerrupters consisting of a light-emitting diode and a light-sensitive
transistor. The only exception is the paper width sensor, which uses reflective tape. The illustration
on the next page gives the sensor and actuator locations.
Actuator's end
Light-emitting Light-sensitive
diode transistor
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