# Development of Multi-Item Air Quality Monitoring System Based on Real-Time Data
## 2 Materials and Methods
### Hardware Process of Multi-Item Air Quality Monitoring System  
The hardware part of the multi-item air quality monitoring system is divided into a sensor part and a communication part. The sensor unit includes nine sensors including a PM sensor, a temperature and humidity sensor, a VOC sensor, an oxygen sensor, a hydrogen sulfide sensor, an ammonia sensor, a sulfur dioxide sensor, a nitrogen dioxide sensor, an ozone sensor, and a carbon monoxide sensor. The carbon dioxide and methane sensors are attached to the separated probe rod and used in connection with the main device. Detailed sensor specifications can be found in Table 1. The PM sensor can measure the PM\({}_{1.0}\), PM\({}_{2.5}\), PM\({}_{4.0}\), PM\({}_{10}\) from 0 to 1000 \(\upmu\)g/m\({}^{3}\) in real time by a light scattering method. The carbon dioxide and methane sensor use the non-dispersive infrared absorption (NDIR) method, and the measurement range is 400 ppm to 10,000 ppm for carbon dioxide and 0 ppm to 50,000 ppm for methane. The oxygen sensor measures 0 to 25% oxygen electrochemically. The hydrogen sulfide sensor uses an electrochemical method, and the measurement range is 0 to 100 ppm. The ammonia sensor uses an electrochemical method, and the measurement range is 0 to 100 ppm. The sulfur dioxide sensor based on the electrochemical method can measure 0 to 20 ppm. Ozone can be measured from 0 to 20 ppm electrochemically. CO based on the electrochemical method can measure from 0 to 100%. The volatile organic compound sensor can measure 0 to 60,000 ppb in the electrochemical method. The temperature and humidity sensor measures the temperature from \(-40\)\({}^{\circ}\)C to \(125\)\({}^{\circ}\)C and the humidity from 0% to 100%. The communication control unit is equipped with Bluetooth and Wi-Fi modules for short- and medium-range communication and a USB memory storage module for data storage. The Wi-Fi module has a communication distance of about 500 m or more in low power. For the Bluetooth module, Bluetooth 4.2 NRF52832 Module, which has been verified for commercialization, is used. Figure 1: Schematic diagram of multi-item air quality monitoring system. Figure 2 shows the external and internal design of the developed monitoring system. The exterior design is shown in Figure 2a. The material of the front case is made of hard polycarbonate. The display shows the measurement value, measurement item, time, operation status, and logo. The ventilation hole was installed to inhale PM and not interfere with other sensors. A capacitive touch button allows the measurement items displayed on the LED display to be changed by touching the button. The interior design is shown in Figure 2b. The main board is designed to make it easy to attach and replace sensors, to attach many sensors to a minimum area, and to avoid interference. The button part was installed by dividing it into a power button and an operation button. It is designed with an interval of 20 mm so that there is no interference between the display board and the main board. For adequate measurement, a fan was introduced to control the air flow. For the power supply, C-type was introduced to make it compatible with existing portable electronic products. The SD card slot was designed to store data on the SD card.