Occasionally, you will walk into a

Occasionally, you will walk into a store and a bell will go off as you cross the threshold. If you look, you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens or a low-power laser), and on the other side is a photodetector that can "see" the light.

Occasionally, you will walk into a store and a bell will go off as you cross the threshold. If you look, you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens or a low-power laser), and on the other side is a photodetector that can "see" the light.

Occasionally you will, walk into a store and a bell will go off as you cross the threshold. If, you look you will often. Notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light. And a lens or a low-power laser), and on the other side is a photodetector that can "see" the light.
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