HP Color LaserJet 8500 Service Manual - Page 159

around the detection window. The light receiver of PS621 detects the toner. After PS621 detects light, the controller board notes the time until light can no longer pass through the detection window. The controller board signals the formatter that the color toner cartridge has no toner if the time the light passes through the detection window exceeds the fixed time. The printer detects the toner level when the printer is turned on, or while the stirring plate is rotating. Light receiver Light emitter Figure 38. Color toner level detection Color cartridge sensor The color cartridge sensor (PS1901C) detects the color toner cartridge. PS1901C consists of a light emitter and a light receiver. The light from the light emitter is reflected by the cartridge and reaches to the light receiver only when the cartridge is installed. If the light does not reach the light receiver, the controller board signals color toner cartridge failure to the formatter and stops the printer. Also, the printer checks for the cartridge pressure when the power switch is turned on, or right after any cover is opened or closed. EN Consumable detection mechanisms 157