PM2.5 are suspended particles in the air that contain solid and liquid; they are generated by artificial pollution or natural factors. Staying in areas with a high concentration of PM2.5 for extended periods will cause a lot of harm to people. This is why PM2.5 was an essential indicator for monitoring air quality around the world in recent years. As technology develops, products used to test air qualities have evolved from desktop types to portable types and are even further combined into wearable air quality detectors; some are also equipped with networking functions to update the data to the cloud in real-time. Currently, most portable detectors use light scattering principles (Mie scattering) to calculate the concentration of PM2.5; it includes a light source control circuit and fan control interface used to adjust the air circulation that enters the measuring channel to improve the accuracy. Therefore, the PM2.5 sensing device's main microcontroller must be equipped with accurate ADC sampling capability and stable PWM control.

Nuvoton's NuMicro^® M451 series microcontroller is based on the Cortex-M4 core with a high as 72 MHz computing speed. It is equipped with 1 set of 12-bit ADC and supports as many as 16 channels; it also has 16-bit PWM that can provide 12 channels of output and has DSP and FPU floating-point unit built-in, able to increase the calculation speed and accuracy of PM2.5. The M451 series has rich peripheral interfaces; it provides three sets of SPI interfaces that support external memory devices and TFT-LCD (for data display) and offers five sets of low-power UART to connect various wireless modules, combined with real-time transmission and collection of data. The M451 series microcontrollers support 2.5V to 5.5V operating voltages and - 40℃ to 105℃ industrial-grade operating temperatures; besides, the M452 USB series also provides 1 set of USB 2.0 FS OTG/Device, and the M453 CAN series offer 1 set of USB 2.0 FS OTG/Device and 1 set of CAN 2.0, allowing PM2.5 sensors to be connected to more applications!

The following is a schematic diagram of the application reference for the development of PM2.5 sensors using the NuMicro^® M451 platform: