Theory of OperationPhaser® 3500 Laser Printer Service Manual8-8Sensor FunctionsPaper Empty SensorMedia supply is monitored by the Paper Empty Sensor. When a tray runs out of paper, thesensor will send a signal to the Main Board, the control panel LED will blink orange, and the“No Paper in Tray {2|3}” message will display on the control panel.Tray 1 Paper Empty SensorThe Tray 1 paper empty sensors monitors paper from Tray 1. If Tray 1 is selected and there isnot paper, a message appears on the control panel. If there is no paper in Tray 2 or Tray 3 andthe printer is in auto-select mode, the printer will look for paper in all the trays and report “NoPaper in Tray 1”.Registration SensorThe Registration Sensor has two purposes. First, it is used to monitor paper movement. If thepaper takes too long getting to the sensor, or it stays at the sensor location too long, the MainBoard will shut down the machine, the control panel LED will blink orange, and a “Jam” errormessage will display on the control panel.Second, the signal generated by the sensor tells the Main Board that the paper is almost at thetransfer area and it is time to start the xerographic process. It is important that the leading edgeof the paper enters the transfer area at the same time as the leading edge of the developedimage on the drum.Fuser Exit SensorThe Exit Sensor monitors paper exit and detects the “Jam at Exit” error if the paper takes toolong getting to the sensor, or it stays at the sensor too long. At this point the Main Board willshut down the printer, the Control Panel LED will blink red, and a “Jam at Exit” error messagewill display on the Control Panel. The signal from the Exit Sensor is also used to control paperfeed to the Duplex Unit for duplex print jobs.Cover Open SensorThe cover open sensor detects whether the top cover is open or closed. When the top cover isopen the “Close Top Cover” message is displayed on the Control Panel.Duplex Unit SensorThe Duplex Unit slides into the rear of the printer above Tray 2. As paper leaves the FuserAssembly and is picked up by the Exit Rollers, the drive direction is reversed to send the paperdown to the Duplex Unit. As the paper leaves the Duplex Unit, it activates the Duplex Sensor.This activates the feed roller to position the paper for printing on the front side of the paper.
