The home use smoke alarm consists of four parts incorporating one sensor, one loudspeaker, one battery device (or being connected to the electric wire), and one circuit board. When the smoke alarm is connected to the household electrical wire, one backup battery is also necessary. The sensor is optional with the ion transducer and photoelectric type.
The ion transducer works by measuring the balance of positive and negative charges in the air. Inside the sensor, there is a small piece of radioactive material which can generate a tiny electrical current in the flowing air in the sensing chamber. On the circuit board, a computer chip is in charge of monitoring the current. When the particles in the smoke enter the sensing chamber, they would disturb the balance of positive and negative charges, and make the current change. When the smoke concentration increases, the imbalance of positive and negative charges would strengthen. When the imbalance reaches a certain level, the loudspeaker would sound.
The photoelectric sensor measures the concentration of smoke via a beam of light and a light sensor. It is designed with the light beam deviation from the sensor. When the smoke enters the sensing chamber, the particles in the smoke would scatter part of the light onto the sensor. With the increase of smoke concentration gradually, more and more beams of light would be scattered onto the sensor. When the light reaching the sensor comes to a certain degree, the loudspeaker would emit the sound.
These two kinds of sensors pass the test of the insurer research laboratory to the household smoke alarm. But they have different sensitivity to different smoke.