How Do Smoke Detectors Work?

Smoke detectors are pretty amazing. They are small, mass produced products, relatively cheap, and yet they save thousands of lives every year. If you are wondering how these little gadgets do it, read on to find out all about different kinds of smoke detectors and how they work.

Parts of the Smoke Detectors

Smoke detectors are made up of two basic parts: a sensor which detects smoke and an electric horn which alerts people in the case of fire. These can run off a 9 volt battery or a 120-volt house current.

Photoelectric Detectors

kidde smoke detector
Kidde Smoke Detector available at USESI

One type of smoke detector is the photoelectric detector. These are often used in stores and operate with the technology of a photo beam detector. The light (which is either a white light and lens or low power laser) might be located on one side of a door in a building. On the other side of the door is a photodetector that can “see” the light. If smoke starts becoming an issue, to the point where the detector can no longer see the light, a bell is triggered.

Because these detectors tend to be big and not very sensitive, there has to be a lot of smoke present to block off the light and set off the alarm.

Photoelectric smoke detectors use light in a different way, with a smoke detector and light sensor positioned at a 90 degree angle to each other. When there is no fire, the light source on the left shoots straight across and misses the sensor. In the case where smoke is present, it enters the chamber and the smoke particles scatter the light causing some amount of light to hit the sensor which then sets off the horn. Photoelectric detectors tend to be best at sensing smoky fires.

Ionization Detectors: These detectors use an ionization chamber and ionizing radiation to detect smoke. They are more popular types of detectors than the photoelectric type due to their cheaper price tag and increased ability to detect smaller amounts of smoke that a flaming fire might produce.

Inside the ionization detector is a small amount of the radioactive element americium-241. This element has a half life of 432 years and is a good source of alpha particle.

The layout of an ionization chamber is pretty simple. There are two plates with a voltage across them, along with a radioactive source of ionizing radiation.

The alpha particles generated by the americium have the ability to ionize the oxygen and nitrogen atoms of the air in the chamber. These means they ‘knock of an electron’. This results in a free electron with a negative charge and an atom missing one electron with a positive charge. The negative electron is attracted to the plate with the positive voltage while the positive electron is attracted to the plate with the negative charge. The electronics in the smoke detector sense the small amount of electrical current these electrons and ions represent as they more towards the plate.

When smoke enters the ionization chamber, the current is disrupted. This is due to the smoke particles attaching to the ions and neutralizing them. When the smoke detectors sense the drop in current, the horn becomes activated.

It should be noted that the small amount of radiation present in the americium in the ionization smoke detector is not enough to do any harm unless it is inhaled. While it might not be a good idea to poke at the americium, or play with it in any way, it can not do harm when functioning as it should be.

Inside an Ionization Smoke Detector

When you take off the cover of an ionization smoke detector, you will find the layout is pretty simple. It is common to find a circuit board, and ionization chamber and an electronic horn. The ionization chamber is essentially an aluminum can that contains the ionization source. There are slots in it that allow air flow. The can acts as an ionization chamber. Under the plate is the ionization source which should not be touched.

It’s a bit mind blowing, how much technology can go into the workings of a smoke detector, especially when you consider that they are such small and relatively inexpensive gadgets. In any case, we should be very thankful that this technology is available in modern homes. Make sure your smoke detector is functioning properly for the ultimate in protection for you and your family.

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