Battery

• The battery is enclosed within the casing of a smoke detector. Even when a home loses power, the battery allows the smoke alarm to keep working. The typical battery required to operate a smoke detector is an alkaline or 9-volt battery. Neither of these batteries are rechargeable, so it's important to check the battery once a month to make sure it's working properly. Batteries should be changed at least once a year.
  
  

Circuit Board

• A smoke detector has a circuit board inside the case that handles the way electricity circulates through the smoke detector. When smoke is detected, it passes to the sensor in the circuit board and then to a horn that will sound the alarm.
  
  

Sensor

• There are two types of smoke alarm sensors. One is an ionization sensor, which detects positive and negative ions within the air when smoke is detected. This type of sensor works to detect fast-moving fires. The second sensor type is photoelectric and detects changes in light when smoke is detected. Photoelectric sensors detect smoldering and smoky fires. Dual-alarm smoke detectors sense both types of smoke and fire.
  
  

Alarm

• The most important part of a smoke detector is the alarm. The sensor senses smoke present and sounds the alarm. The alarm makes a loud, piercing sound that does not stop until smoke is cleared from the area. A waning battery will also make the alarm go off in intervals. When this happens, the battery needs changing.
  
  

Test Button

• On the outside of the smoke detector is usually a read button with the word "test" printed by it or on it. The test button should be pressed periodically to test the alarm. Pressing the test button tests to make sure the battery and alarm are both working properly.