Residential smoke alarms have four parts:
- A sensor that can respond to smoke
- A horn to alert you when the sensor detects a preset level of smoke
- A battery or connection to your homes electrical wiring
- A circuit board that controls the electrical flow between them. (Some electrically-wired units also contain a backup battery).
There are two kinds of sensors available, an ionization-type and a photoelectric-type.
The ionization sensor gets its name because it measures the electrical balance in the air between positive and negative charges, or ions. Inside the sensor, a tiny piece of radioactive material creates a small electrical current in the air that flows through the sensor chamber. A computer chip on the circuit board monitors the electrical current. When smoke particles enter the sensor chamber they upset the balance between the positive and negative electrical charge. This changes the current flow. As the smoke gets more dense the imbalance increases. When the imbalance reaches a certain threshold, the horn will sound to alert you.
The photoelectric sensor is named so because it uses a light source and a light sensor to measure smoke density. The light source is constantly on but the light beam is angled away from the light sensor. When smoke enters the chamber, the particles scatter some of the light toward the sensor. As the smoke density increases, more light is scattered toward the sensor. When the amount of light scattered into the sensor reaches a certain threshold, the horn will sound to alert you.
Both types of sensors pass the tests required by Underwriters Laboratories for residential smoke alarms, but they do differ in their sensitivity to certain kinds of smoke. For a more detailed explanation of the differences, see Smoke Alarm Facts & Questions.