Comparing Ionizing Smoke Detectors with Photoelectric Smoke Detectors

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Updated on Feb 08, 2012

Grade Level: 6th -8th; Type: Chemsitry


During the process of learning how different types of smoke detectors work, students will learn about ions, every day applications of radiations and how particulate matter in the air can disrupt sensitive systems.

Research Questions:

  • How does an ionizing smoke detector work?
  • How does a photoelectric smoke detector work?
  • Which type of smoke detector is more effective?
  • How does a radioactive compound create an ionic field?

Photoelectric smoke detectors optically detect particulate matter in the air. A light sensor receives light from an LED (light emitting diode). If smoke obstructs the path between the LED and the sensor air and scatters the light from the LED, the alarm is trigger. Most of the cheap smoke detectors on the market work by the principal.

Most inexpensive smoke detectors on the market are ionizing smoke detectors. Ionizing detectors have two electrodes coupled to different poles of a battery. Underneath the negative electrode is a small amount of americium-241. A current passes between the electrodes because the americium-241ionizes the oxygen and nitrogen between the two electrodes, causing an electron to be knocked off these elements, creating ions. However, if smoke or particulate matter is in the air, substances in the smoke bind to these ions, disrupting the current between the two electrodes and triggering the alarm.


Most supplies are available in discount stores or are already around the house. Ask your friends if you can check out their smoke detectors.

  • Ionizing smoke detector
  • Photoelectric smoke detector
  • Stick of intense
  • Matches
  • Meter stick
  • Stopwatch

Experimental Procedure

  1. Mark 200mm, 400mm, 800mm, 1000mm, 2 meter, 4 meter distance from an ionizing smoke detector.
  2. Light a stick of incense. Disturbing the surrounding air as little as possible, place the lit incense 200mm from the ionizing smoke detector and start the stop watch. How long does it take to trigger the ionizing smoke detector? At some point during your procedure, take a photo of your setup.
  3. Repeat step #3 at 400mm, 800mm and 1000mm, 2 meter, 4 meter distance, making sure the air clears before every trial.
  4. Repeat step #2 and #3 with a photoelectric smoke detector.
  5. Make two graphs of your data. The x-axis can represent time and the y-axis can represent distance. Which smoke detector was more responsive to the smoke? Which do you think would be more likely to create nuisance alarms?

Terms/Concepts: Ionizing smoke detector, photoelectric smoke detector, LED, Ion, Particulate matter, Radiation



How Stuff Works: How Smoke Detectors Work How Do Smoke DetectorsWork? Photoelectric & Ionization Smoke Detectors Chem4Kids: Looking at Ions

Cy Ashley Webb is a science writer. In addition to having worked as a bench scientist and patent agent, she judges science fairs in the San Francisco bay area. She loves working with kids and inspiring them to explore the world through science.

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