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How Do Smoke Alarms Work? | Understanding Different Types of Smoke Detector

There are two main types of smoke alarm: ionization detectors and photoelectric detectors. Some are hardwired while others are battery-powered. 

What are the differences, and how do they work? Find out how smoke alarms function, the differences between the two types, and what one is best below.

How Do Smoke Alarms Work?

Smoke alarms work using either ionization detectors or photoelectric detectors, while combination alarms use both for maximum security.

Ionization Detectors

A small piece of radioactive material ionizes (charges) the air between two plates. This creates a current that flows constantly. Particles that enter the device will weaken the current, while smoke will reduce it below a threshold, sounding the alarm.


  • Responsive to flaming fires (black smoke)
  • Cheaper, and most common type in homes


  • More unresponsive to smoldering fires (lighter, larger, reflective particles)
  • Older technology

Photoelectric Detectors

Instead of using radioactive material like an ionization detector, a light beam is shone inside a chamber. It won’t strike the detector if the chamber is free of particles. If smoke is present, the amount of light reflected will pass a threshold, causing the alarm to sound.


  • Responsive to smoldering fires (light smoke)
  • Newer technology


  • More unresponsive to aggressive fires (black smoke)
  • More expensive than ionization detectors.

Which Smoke Alarm Do I Need?

Ionization Detectors are more responsive to flaming fires (i.e. fires with lots of black soot, which will quickly disrupt the current), while Photoelectric Detectors are more responsive to smoldering fires, which produce larger, lighter colored particles than flaming fires. 

Smoldering fires might be more common in modern homes because plastic is common in furnishings, objects, and construction materials..

Both will work just fine in both circumstances, but since there is evidence to suggest they have these different strengths, a combination of alarms would be ideal if possible.

Hence, dual ionization and photoelectric detectors are recommended by the US Fire Administration. 

The Difference Between Smoke Alarms and Fire Alarms

Smoke alarms, smoke detectors, and fire alarms are all terms used interchangeably. What’s usually being referred to is either an ionization or photoelectric smoke detector. The ‘alarm’ part of the device is technically a fire alarm; the smoke detector triggers the fire alarm.

Smoke DetectorAn ionization or photoelectric detector that is able to trigger a fire alarm if smoke enters the chamber.
Fire AlarmA loud ringing sound that can either be triggered by a smoke detector or by being pulled by hand (often seen in large public buildings like schools and shopping malls).
CO DetectorA CO detector is able to detect unsafe carbon monoxide levels in the air. Like a smoke detector, it can then sound an alarm.

Hardwired vs. Battery-Powered Smoke Alarms

Aside from the differences in how smoke detectors do their job, there’s also differences in how they are powered.

Hardwired Smoke Detectors

Wired to the mains electricity. They also are required to have a back-up battery in case the mains electricity goes out.


  • Last upwards of ten years (every alarm should be replaced every ten years)
  • Much more dependable


  • A nuisance to install, may require an electrician
  • Can be awkward to turn off a false alarm

Battery-powered Smoke Detectors

Not connected with a wire, and are instead powered solely by replaceable batteries.


  • Mobile
  • Easy installation


  • A single power source means it’s much less dependable
  • Lifespan is significantly shorter than a hardwired option


Is photoelectric smoke detector better?

Photoelectric smoke detectors are better than ionization smoke detectors at detecting light, smoldering smoke. This might be more useful for modern buildings because so much of what is in newer homes is now made out of some form of plastic, which ‘smolders’. 

But ionization smoke detectors are much better at detecting the thick, black smoke caused by fast burning fires. It is therefore recommended to have both types in the home.

Is it better to hardwire a smoke detector?

It is better to have any kind of smoke detector rather than none at all. Hardwired smoke detectors last longer than battery-powered ones, meaning less annoying chirps and beeps in the long run. They also usually have a backup battery inside them, meaning they are far more reliable than ones powered solely by batteries. However, hardwiring is a lengthy install process, and it can’t be moved as easily as one powered by batteries.


There is still a need for both ionization and photoelectric smoke detectors, so neither can be described as ‘better’ than the other. 

It is advisable to have both types of smoke detector in the home to deal with both smoldering, slow fires, and fast, smokey fires. There’s no telling what kind of fire could break out, so it is better to be prepared.