How Smoke Detectors Function

Key Points to Remember

Smoke detectors are one of those incredible inventions that, due to mass production, are incredibly affordable. You can find a smoke detector for as low as $7. Despite their low cost, they save thousands of lives annually. In fact, it's advised that every home have at least one smoke detector per floor.

Every smoke detector is made up of two key components: a sensor that detects smoke and a very loud electronic horn designed to alert people. Smoke detectors are powered either by a 9-volt battery or 120-volt household current.

In this article, we will explore the two most widely used types of smoke detectors today: photoelectric detectors and ionization detectors. We will also take a closer look inside an ionization detector.

Let's begin by discussing photoelectric detectors.

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Photoelectric Detectors

Sometimes, when you enter a store, a bell rings as you cross the threshold. If you observe closely, you'll often find a photo beam detector in use. There's typically a light (either a white light with a lens or a low-power laser) on one side of the door, and a photodetector on the other side that can 'see' the light.

As you pass through the light beam, you block it. The photodetector detects the absence of light and activates the bell. You can imagine how a similar type of sensor could be used in smoke detectors. If enough smoke entered the store to block the light beam significantly, the bell would sound. However, there are two issues with this system:

A large amount of smoke would be needed before the alarm activates—the smoke would need to be dense enough to completely block the light. It requires quite a bit of smoke to achieve this.

Photoelectric smoke detectors, on the other hand, utilize light in a different manner. Inside the detector, there is both a light source and a sensor, but they are positioned at right angles to each other, like this:

Normally, the light from the left-side light source travels straight across and misses the sensor. However, when smoke enters the chamber, the smoke particles scatter the light, and some of the light reaches the sensor:

The sensor then triggers the alarm in the smoke detector.

Photoelectric detectors are more effective at detecting smoky fires, like a mattress that's smoldering.

Ionization Detectors: Ionizing Radiation

Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. These detectors are more widely used because they are affordable and more effective at detecting the smaller amounts of smoke produced by flaming fires.

Inside an ionization detector is a tiny amount (about 1/5000th of a gram) of americium-241. Americium, a radioactive element, has a half-life of 432 years and is an excellent source of alpha particles.

Another way to express the amount of americium in the detector is to say that it contains 0.9 microcurie of americium-241. A curie is a unit used to measure nuclear material. Holding a curie of material means that 37 billion atoms are undergoing nuclear transformations every second, emitting particles of nuclear radiation (like alpha particles). One gram of radium generates about 1 curie of activity (Marie Curie, after whom the curie is named, conducted much of her research with radium).

(For a detailed explanation of nuclear materials and radiation, refer to How Nuclear Radiation Works.)

Now, let's examine the ionization chamber.

Ionization Detectors: Ionization Chamber

An ionization chamber is quite basic. It consists of two plates with a voltage across them, along with a radioactive source of ionizing radiation, as shown here:

The alpha particles produced by americium have this characteristic: They ionize the oxygen and nitrogen atoms in the air inside the chamber. To 'ionize' means to 'remove an electron.' When an electron is removed from an atom, you get a free electron (with a negative charge) and an atom that is now missing one electron (with a positive charge). The free electron is drawn to the plate with a positive voltage, and the positively charged atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector detect the small amount of electrical current generated by the movement of these electrons and ions toward the plates.

When smoke enters the ionization chamber, it disrupts the electrical current -- the smoke particles bind to the ions, neutralizing them. The smoke detector detects the reduction in current between the plates and triggers the alarm.

Speaking of alarms, the mention of "nuclear radiation" often causes many people to panic. However, the amount of radiation in a smoke detector is extremely minimal. It is primarily alpha radiation, which cannot pass through a sheet of paper and is blocked by a few centimeters of air. The americium in the detector could only pose a risk if inhaled. Therefore, you should avoid tampering with the americium in a smoke detector, as disturbing it could make it airborne. For more information, see How Nuclear Radiation Works.

Next, let’s take a closer look inside an ionization smoke detector.

Inside an Ionization Smoke Detector

This is the appearance of a typical smoke detector from the outside.

Once you remove the cover, you’ll see that a smoke detector is quite simple. This model includes a printed circuit board (for a detailed explanation of printed circuit boards, see the article Inside a Remote Control), an ionization chamber (the silver cylinder located near the top right in the picture below), and an electronic horn (the brass cylinder at the bottom right in the image):

Here’s a closer view of the circuit board:

And here is the underside of the board:

The ionization chamber is an aluminum can that houses the ionization source. The can has vents to allow air to flow through it, and it functions as the negative plate of the ionization chamber.

Beneath the can is a ceramic holder containing the positive plate of the ionization chamber. Below this plate sits the ionization source, WHICH YOU WANT TO BE SURE NOT TO DISTURB.