Brother HL-5440D Service Manual page 43

Part name
Toner sensor
New toner sensor
Detects the toner remaining in the cartridge. When the toner is
below the specified level, the user is notified that the toner
cartridge needs replacing soon.
Detects the toner volume when a new toner cartridge is set.
(Does not react to the starter toner cartridge even if it is new.)
When a new toner cartridge is detected, the developer roller
counter and the developing bias voltage value are reset, and
the toner cartridge replacement counter increases by one.
2-7
Operation
Confidential