CHAPTER 3 EXPOSURE SYSTEMCOPYRIGHT © 1999 CANON INC. CANON NP6320 REV.0 DEC. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON) 3-11F. Document Size Identification1. OutlineThe copier is equipped with auto paper selection and auto reproduction ratio selection mecha-nisms. These mechanisms call for identification of the document size as described below.To enable such identification, three circuits are attached under the copyboard glass (Figure 3-305); when the copyboard cover is lowered, the DC controller checks the output of these circuitstwice, thereby identifying the document as A3, A4, A4R, or A5R.The document identification circuit causes its LED to emit light to illuminate the document;the reflected light is then checked by the phototransistor, whose output is sent to the DC controller.2. MechanismThe DC controller checks the output of the document identification circuit when the copyboardcover is lowered to about 20°. (SW4 ON) and about 1 second after the over has been lowered toabout 5°. (SW5 ON).The DC controller recognized the presence of a document if the two measurements are identi-cal; conversely, it recognizes the absence of a document if the measurements are different.Figure 3-305When a document is present, the light reflected by the document is checked by the documentidentification circuit; therefore, the output of the circuit is constant regardless of the angle of thecopyboard cover.When no document is present, the output of the circuit from the first measurement is low ornone at all; the output of the circuit from the second measurement, on the other hand, is highbecause the light is reflected by the document sheet.If the document is too thick and, therefore, if only SW4 goes ON, the document size is identi-fied based on the current output level.Copyboard coverDocumentSW4SW520°5°ONOFFSW4SW5ONONDocumentidentificationcircuitDC controller PCB DC controller PCBOutputOutput(constant)Output(varying)Output(The thickness of the arrows represents the degree of the level.)
