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Comparing the effectiveness of ionizing and photoelectric smoke detectors Featured science projectScience project video


This science fair project was conducted to compare the performance of an ionizing smoke detector with a photoelectric smoke detector. The tests were done by placing a lighted cigarette at 200mm, 400mm, 600mm, 800mm and 1000mm from the smoke detectors, and comparing the speed of each detector in detecting the smoke emitted.


The ionizing smoke detector will be able to detect smoke more quickly.

Scientific Terms

Smoke detectors, photoelectric, ionizing, optical detection, LED, light sensors, americium-241 radioactive isotopes


Smoke detectors

Smoke detectors are devices that are used to help in detecting smoke and preventing fires. They are installed in commercial buildings, factories, hotels and homes. These devices will normally be connected to an alarm system which will be triggered when smoke or fire is detected. Most smoke detectors will operate using photoelectric sensing or by ionizing detection. Some smoke detectors will use both for increased sensitivity.

A photoelectric smoke detector uses optical detection. It has an LED light source and a light sensor. Under normal conditions, the light from the LED will travel in front of the light sensor in a straight line. However, when smoke is present, the light from the LED will be scattered, and this will be detected by the light sensor, triggering the alarm.

Ionizing smoke detectors are able to detect smoke particles that are very small and cannot be seen by the naked eye. It has Americium-241 radioactive isotopes and 2 electrodes. Under normal conditions, the radiation will permit a small current to pass in between the electrodes. However, when smoke is present between the electrodes, the ionization is reduced and the fire alarm triggered. Although ionizing smoke detectors are cheaper than photoelectric smoke detectors, they are more prone to nuisance triggers.

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Complexity level:
Project cost ($):
Time required:
1 hour for preparation, 2 hours for observation
Material availability:
Easily found at a hobby store
Safety concerns:

Adult supervision required. Take all necessary precautions to ensure fire safety. Be extremely careful when handling fire.