How smoke alarms work

How does the smoke alarm work? A: The home smoke alarm consists of four parts: a sensor, a horn, a battery device (or connected to the wire) and a circuit board. (When the alarm is connected to a household electric wire, it is also necessary to have a backup battery.) There are two types of sensors to choose from: one is an ion sensor and the other is a photoelectric sensor. Ion sensors work by measuring the balance of positive and negative charges in the air. Inside the sensor, there is a small piece of radioactive material that generates a tiny current in the air flowing through the sensing chamber. On the circuit board, there is a computer chip to monitor this current. When the smoke particles enter the sensing chamber, the balance of positive and negative charges will be disturbed, and this current will also change. As smoke gradually increases, the imbalance between positive and negative charges increases. When this balance reaches a certain limit, the horn will sound.

Photoelectric sensors measure the concentration of smoke through a beam of light and a light sensor. When the device is designed, the beam is off the sensor. When smoke enters the sensing chamber, the smoke particles scatter some of the light beam onto the sensor. As the concentration of smoke gradually increases, more light is scattered onto the sensor. When the beam reaching the sensor reaches a certain level, the horn will sound.

Both types of sensors passed the detection of home smoke alarms by underwriters' research labs, but their sensitivity to different smoke levels was different.

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