The Slotted Photointerrupter is the black
object through which the brass blade is passing in the right-hand
photograph above. Photointerrupters have an invisible infrared light
source and a detector built into one assembly. When an object passes
between the light source and the detector, the output changes. These
devices are commonly used to home motion systems.
All mechanical devices connected to computers, such as disk drives and
printers, use photointerrupters to tell the controller when the moving
parts have achieved a known location. Photointerrutpers also come in a
variety called "retroreflective", meaning the light source and detector
face the same direction instead of each other. A polished surface on
the moving parts completes the light path.
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