Method of determining detector lifetime using a stepped resistor network

摘要

A method and apparatus for sensing the presence of a document moved to a printing station in a printer and for detecting skewness (if any) of the document at the printing station. A first light-detector pair and a second light-detector pair are used, with the sensitivity of each of the detectors in the light-detector pairs being adjusted each time the printer is turned on. A controller and a stored program are used to adjust the sensitivity of the detector along with a resistor network providing steps of resistance. The particular resistance step to which a detector is adjusted also gives a measure of the useful life left for the detector.