Understanding Smoke Detectors

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Are you curious about how smoke detectors work? Slobo / Getty Images

Smoke detectors are a cost-efficient innovation that can save thousands of lives each year. You can easily purchase one for as little as $7, and it is recommended that every floor of your home has one for added safety. They consist of two main parts: a sensor to detect smoke and an electronic horn to alert people.

Smoke detectors can run on either a 9-volt battery or 120-volt house current. This article will explore the two most commonly used types of smoke detectors: photoelectric detectors and ionization detectors. We will also take a closer look inside an ionization detector.

Let’s begin with photoelectric detectors.



Photoelectric Detectors

Have you ever walked into a store and triggered a bell as you crossed the threshold? This is often due to a photo beam detector being used. One side of the store’s entrance has a light and lens or a low-power laser, while the other side has a photodetector that can “see” the light. When you walk through the beam of light, it triggers the photodetector to sound the bell.

Photoelectric smoke detectors work in a similar way. Inside the detector, there is a light and a sensor positioned at 90-degree angles to one another. Typically, the light from the source misses the sensor. However, when smoke enters the chamber, it scatters the light, causing some of it to hit the sensor. This triggers the horn in the smoke detector, alerting people to the presence of smoke.

Photoelectric detectors are better at detecting smoky fires, such as a smoldering mattress.

Did you know? Fire sensors originated from NASA technology used to enhance photos of distant planets. Discover more cool NASA innovations in this interactive animation from Discovery Channel.

Ionization Detectors: Ionizing Radiation

Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is more commonly used as it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires.

Inside an ionization detector is a small amount (perhaps 1/5000th of a gram) of americium-241. This radioactive element has a half-life of 432 years and is a good source of alpha particles.

In a smoke detector, the amount of americium-241 can be described as 0.9 microcurie. A curie is a unit of measurement for nuclear material, where one curie indicates that 37 billion atoms in the sample are decaying and emitting a nuclear radiation particle each second. Radium generates roughly one curie of activity in one gram of the element. An ionization chamber is a simple device consisting of two plates with voltage across them and a radioactive source of ionizing radiation. Alpha particles from the americium ionize oxygen and nitrogen atoms in the air of the chamber. Smoke entering the ionization chamber disrupts the small amount of electrical current that the ions and electrons create, alerting the smoke detector to sound the horn. The radiation emitted by the americium in a smoke detector is predominantly alpha radiation, which cannot penetrate a sheet of paper and is blocked by a few centimeters of air. It is crucial to avoid disturbing the americium in any way, as it could pose a danger if inhaled. The smoke detector comprises a printed circuit board, an ionization chamber, and an electronic horn.

The ionization chamber is a container made of aluminum that holds the ionization source. The container has openings to allow air to flow through it. The container acts as the negative plate of the ionization chamber.

Beneath the container is a ceramic holder that holds the positive plate of the ionization chamber. Below that plate is the ionization source, which needs to be handled with care to avoid disturbance.

If you want to learn more about smoke detectors and related topics, you can find links on the following page.

More Information

Related HowStuffWorks Articles

  • How Fire Works
  • How Fire Extinguishers Work
  • How Nuclear Radiation Works
  • How Atoms Work
  • How to Write the Chemical Reaction that Takes Place in a Smoke Detector
  • Why Does Smoke Come from a Fire?
  • How Do Smoke Detectors Intercommunicate?

More Great Links

  • Charlotte Fire Department: Smoke Detectors
  • USFA: Home Fire Safety
  • Smoke Detectors Save Lives
  • Smoke Detectors and Americium


1. What is a smoke detector and how does it work?

A smoke detector is a device that senses smoke and alerts people in case of fire. Modern smoke detectors use two main types of sensors: ionization and photoelectric. Ionization detectors use a small amount of radioactive material to ionize the air in the sensing chamber. When smoke enters the chamber, it disrupts the ionization process, triggering the alarm. Photoelectric detectors use a beam of light that is scattered by smoke particles. When the light beam is disrupted, the alarm goes off.

2. Where should I install smoke detectors in my home?

Smoke detectors should be installed on every level of your home, including the basement and attic. They should be placed in each bedroom and outside sleeping areas, as well as in common areas such as hallways and living rooms. It’s also important to install smoke detectors near kitchen areas, but make sure they are placed away from cooking appliances to avoid false alarms.

3. How often should I replace the batteries in my smoke detectors?

It’s recommended to replace the batteries in your smoke detectors twice a year, or whenever the low-battery warning chirps. You can also test your smoke detectors monthly by pressing the test button. If the alarm doesn’t sound, replace the batteries immediately.

4. How long do smoke detectors last?

Smoke detectors typically last for about 10 years before they need to be replaced. However, it’s important to regularly test your smoke detectors to ensure they are working properly. If your smoke detector is more than 10 years old, it’s a good idea to replace it with a newer model.

5. Can smoke detectors detect carbon monoxide?

No, smoke detectors cannot detect carbon monoxide. Carbon monoxide detectors are a separate device that should be installed in your home to detect the presence of this deadly gas. It’s recommended to install carbon monoxide detectors on every level of your home, as well as outside sleeping areas.

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