There are countless “guides” online for air purifiers, from purchasing to usage. Most of the content is stereotyped, teaching users how to buy and use them. However, after reading so much, can you really master the air purifier in your home? Today, Luftmy editor will focus on a small component that is standard in most purifiers but ignored by 99% of users——the PM2.5 dust particle sensor.

I. Introduction to Sensor Working Principle

In the current field of air purification, PM2.5 dust particle sensors have almost become a standard accessory for purification equipment. Their role is to monitor the concentration of particles such as PM2.5 in the air. The working principle is as follows: a constant light source (such as an infrared LED) is set inside the sensor. When air passes through the light, the particles in it scatter the light, causing attenuation of light intensity. The relative attenuation rate is proportional to the concentration of the particles.

A light detector (such as a phototransistor) is placed on the other side diagonal to the light source. It can detect the light reflected by the particles and output a PWM signal (Pulse Width Modulation signal) according to the intensity of the reflected light, thereby determining the concentration of particles. For particles of different sizes (such as PM10 and PM2.5), it can output multiple different signals to distinguish them.

This seemingly simple process actually involves light scattering, reflection, light intensity attenuation, and complex algorithms. The reason why we can intuitively see the air quality index on the sensor in different colors or numerical forms is thanks to the sensor.

Currently, the mainstream sensors on the market are divided into two types: infrared particle sensors and laser particle sensors. In terms of working principles, the difference between the two is not too large, but the structures are quite different.

II. Comparison of the Two Mainstream Sensors

Although both are mainstream products, the structural differences between infrared sensors and laser sensors are considerable. The internal structure and circuit design of infrared sensors are relatively simple, while laser sensors are more complex.

Different designs lead to differences in measurement accuracy. Infrared sensors use infrared light-emitting diodes as light sources, while laser PM2.5 sensors use more stable laser diodes.

During the sensor's operation, one of the necessary conditions is that flowing air passes through the intersection area between the light source and the receiver. To drive airflow, infrared sensors use resistive heating to utilize hot air to drive the flow of surrounding gases; laser sensors have a fixed fan inside.

In terms of signal output, the phototransistor inside the infrared sensor can only output a pulse width modulation signal (PWM signal). This signal cannot intuitively display the concentration of particles in the air and requires further calculation to obtain the particle concentration range. The photoelectric effect of the photodetector inside the laser sensor generates a current signal. After being amplified and processed by the circuit, the particle concentration value can be obtained, and the signal is generally output via a serial port.

This explains why some purifiers can only indicate air quality through lights of different colors, while other purification products can display the specific air quality index in digital form.

In addition, infrared sensors use resistive heating to drive airflow, resulting in a small number of particle samples and slightly insufficient test accuracy. Laser PM2.5 sensors use fan drives, providing a large enough data collection volume to ensure data accuracy to a certain extent.

Of course, high precision also has side effects——the lifespan of laser sensors is shorter than that of infrared sensors. However, with continuous technological improvements, most sensors currently show good performance.