When it comes to air purifiers, from purchasing to usage, various “guides” online are a dime a dozen. Most of the content is cliché, simply teaching users how to buy and use them. However, after “studying” for so long, can you really master the air purifier in your home? Setting other things aside, today let’s focus on a small component that is standard in most purifiers but ignored by 99% of users——the air quality sensor, represented by the PM2.5 sensor. This article will explain in detail how this seemingly inconspicuous component actually works.

In the current field of air purification, air quality 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 light-emitting diode) is set inside the sensor. When air passes through the light beam, the particles within 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 side diagonal to the light source. It can detect the light reflected by the particles and output a PWM signal (Pulse Width Modulation signal) based on the reflected light intensity, 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 work process actually involves light scattering, reflection, light intensity attenuation, and complex algorithms. The reason we can intuitively see the air quality index on the sensor, either through different colors or in digital form, is due 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 principle, the difference between the two is not too large, but the structures are quite different.

Different designs lead to differences in measurement accuracy. Infrared sensors use infrared LEDs as the light source, while laser sensors use more stable laser diodes.

During the sensor’s operation, a necessary condition is the flow of air through the intersection area between the light source and the receiver. To drive the airflow, infrared sensors use resistance heating to utilize hot air to drive the surrounding gas flow, while laser sensors have a fixed fan inside.

Regarding signal output, the phototransistor inside the infrared sensor can only output a Pulse Width Modulation (PWM) signal. This signal cannot intuitively display the concentration of particles in the air and requires further calculation to derive the concentration range. The photoelectric effect of the photodetector inside the laser sensor generates a current signal. After circuit amplification and processing, the particle concentration value can be obtained, and the signal is generally output via a serial port.

Additionally, because infrared sensors use resistance heating to drive airflow, the number of particle samples is small, leading to slightly insufficient testing accuracy. In contrast, laser sensors use a fan drive, allowing for a large enough volume of data collection, which ensures data accuracy to a certain extent.

Of course, high precision has its trade-offs——the lifespan of laser sensors is shorter than that of infrared sensors. However, with continuous technological improvements, most current sensors demonstrate excellent performance.

Luftmy is a high-tech company focusing on the R&D, production, and sales of PM2.5 sensors. If you are interested in our company’s sensor products, please feel free to contact us, or download the relevant specification sheets on our website. We look forward to cooperating with you.

Application areas for laser PM2.5 dust sensors: air purifiers, air conditioners with purification functions, PM2.5 detectors, range hoods, smoke alarms, fresh air systems, specialized PM2.5 sensors, air detectors, wearable devices, etc.