Ever wondered how a smoke detector works? In this article you will learn the inner workings of a common smoke detector to better understand how it can detect smoke and warn us of a potential fire.
There are two different kinds of smoke detectors. Here we will learn about the photoelectric smoke detector.
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.
The Photoelectric Smoke Detector explained
An effective way to understand how a photoelectric smoke detector works is by the following example:
At some point you have probably walked in or out of a store and an alarm went off as you crossed 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 an alarm 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.
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 smoke detectors are better at sensing very smoky fires, such as a smoldering mattress. The other most common smoke detector is called an ionization smoke detector.
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