How do smoke detectors work?

As the saying goes, “Where there’s smoke, there’s fire.” Smoke detectors are amazing: They’re pretty inexpensive, but they save thousands of lives each year. 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. There are two most common types of smoke detectors used today: photoelectric detectors and ionization detectors.

Photoelectric Detectors
You may have noticed sometimes when you walk into a mall or store , a bell goes 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.

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.

Ionization Detectors
Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. Inside the ionization detector is a small amount (perhaps 1/5000th of a gram) of americium-241(CAS number is 7440-35-9), a radio active material.

This type of smoke detector is more common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires.