A: All smoke detectors consist of two basic parts: a sensor to sense the
smoke and a very loud electronic horn to wake people up. Smoke detectors
can run off of a 9-volt battery or 120-volt house current.
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 photo detector that can "see" the light.
When you cross the beam of light, you block it. The photo detector 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,
and 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:
In the normal case, the light from the light source on the left shoots
straight across and misses the sensor. When smoke enters the chamber,
however, the smoke particles scatter the light and some amount of light
hits the sensor. The sensor then sets off the horn in the smoke detector.
Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress.
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