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Date: 1991 

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...

Arthur Smith

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