## 2 Materials and Methods
### Firmware Design for Wireless Multi-Sensor EMIoT System with DYU Air Box  
The design of the proposed DYU Air Box for monitoring environmental conditions of classrooms on Da-Yeh University campus can be programmable through Arduino platforms under the open-source Arduino integrated development environment (IDE) which supports the code programming languages C and C++ with specified rules of code structure. After the installation of the CP210x USB-to-UART Bridge driver and third-party Wi-Fi platform drive packages in the Arduino IDE environment, the Arduino 1.8.16 software for the proposed wireless DYU Air Box for monitoring the AQI index/level, PM\({}_{1.0}\)/PM\({}_{2.5}\) and CO\({}_{2}\) concentration, brightness, ambient temperature, and humidity in classrooms was developed in the same coding configuration and development environment. The libraries of the BME680, PMS7003T, SCD30, and BH1750 sensors as well as WPA2 Enterprise connection available online were included in the beginning of code programming as shown in the flowchart of Figure 4. The observation data were simultaneously displayed in the built-in ISP display of the TTGO ESP32 module and sent to the open-source Thing5 peak IoT platform through the DYU-802.1X network with the standardized ThingHTTP protocol. It is well known that HTTP was designed to make documents available over a client-server protocol across the internet, which both run over the transmission control protocol (TCP). On the other hand, the other MQTT protocol was initially created for IoT applications. The main differences between HTTP and MQTT protocols are tabulated in Table 4.  
Furthermore, the flowchart of data acquisition, transmission, and demonstration for the proposed wireless DYU Air Box and the DYU EMIoT system on the public Thing5 peak IoT platform for monitoring environmental conditions of classrooms is depicted in Figure 4. It should be noted that the sending of the observation data to the public Thing5 peak IoT platform is directly programmed and they are downloaded into the TTGO ESP32 Wi-Fi module. The DYU EMIoT system in the public Thing5 peak IoT platform for environmental  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**AQI Level** & **AQI Value** & **Air Quality** & **Impact (Long-Term Exposure)** \\ \hline
1 & 0–50 & Excellent & Pure air; best for well-being \\
2 & 51–100 & Good & No irritation or impact on well-being \\
3 & 101–150 & Lightly polluted & Reduction of well-being possible \\
4 & 151–200 & Moderately polluted & More significant irritation possible \\
5 & 201–350 & Severely polluted & More severe health issues possible if harmful VOCs present \\
6 & \(>\)350 & Extremely polluted & Headache, additional neurotoxic effects possible \\ \hline \hline \end{tabular}
\end{table}
Table 3: AQI criteria.  
monitoring of classroom conditions can be configured based on the well-designed format for channel configuration.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Items** & **HTTP** & **MQTT** \\ \hline Architecture & Client–server & Publish/Subscribe \\ Command targets & URIs & Topics \\ Underlying protocol & TCP/IP & TCP/IP \\ Secure connections & TLS + username/password & TLS + username/password \\ Client observability & Unknown connection status & Known connection status \\ Messaging mode & Synchronous & Asynchronous, event-based \\ Message queuing & Application needs to implement & The broker queues messages for \\ Message overhead & 8 bytes minimum. Header data are text-compressed & disconnected subscribers \\ Message size & Over 256 MB is normal & 2 bytes minimum. Header data can be binary \\ Content type & Text (base64 binary encoding) & Any (binary) \\ Message distribution & One to one & One to many \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparisons between MQTT and HTTP technologies.  
Figure 4: The flowchart of the proposed wireless DYU Air Box.