How do photoelectric sensors work? How exactly does
the shadow a person creates "open" a door, for example?
As you might guess, photoelectric sensors use electrical components
(usually diodes) which are sensitive to light. If a big change
in light level occurs, the diodes changes properties (I think they
usually become more conducting when the light level goes up). This
is just like a switch in the wall - when it is up it lets current
through, and when it is down it does not. So whatever the switch
controls, such as a door, is operated when the light level changes...
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Update: June 2012