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| How Do Photoelectric Smoke Detectors Work? |
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 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, 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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