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Glossary PI

Definition of PI

1.  An acronym for photo interrupter. In most scenarios, a black plastic lever, the LED, and the photo-diode are all one unit. When a piece of paper passes through the fax machine, it pushes down the plastic lever. This lever interrupts the beam of light (from an LED or light emitting diode), or allows it to strike the photo-diode. In either case, this change of state signals the control board as to the present location of the piece of paper. When the paper passes by, a counterweight or spring returns the lever to its original position.