Photoelectric

• Photoelectric smoke detectors contain a light source that is designed to just miss touching a sensor pad inside the device. Smoke particles in the air cause the beam of the light source to deflect onto the sensor, which sounds the smoke alarm, according to "Electrical Systems."
  
  

Ionization

• If your smoke detector works through ionization, then it contains a small amount of radioactive material. The material releases electrons, which create an electric current inside the detector. Smoke particles interrupt the electrical current when they pass through it, which causes the smoke alarm to go off.
  
  

Considerations

• Ionization smoke detectors are the most common type found in residences. They tend to be better at detecting small fires or those that do not give off much smoke. Photoelectric smoke detectors react faster to very smoky fires. Neither type reacts to heat or flames if there is no smoke.