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.
When you cross the beam of light, you block it. The photodetector senses
the lack of light and triggers a bell. You can imagine how this same type
of sensor could act as a smoke detector. If it ever got smoky enough in
the store to block the light beam sufficiently, the bell would go off.
But there are two problems here:
1.It's a pretty big smoke detector.
2.It is not very sensitive.
There would have to be a LOT of smoke before the alarm would go off --
the smoke would have to be thick enough to completely block out the light.
It takes quite a bit of smoke to do that.
Photoelectric smoke detectors therefore use light in a different way.
Inside the smoke detector there is a light and a sensor, but they are
positioned at 90-degree angles to one another, like this:
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