## 2 Materials and Methods
### Hardware Design for DYU Air Box
high VOC levels due to cooking, food consumption, exhaled breath, or salty sweat can be measured well. Based on the Mie scattering theory, the PSM7003T particle concentration sensor [23] adopts laser light to radiate the suspended particles in the air and produce laser scattering light to a certain degree depending on the particle size as shown in Figure 3b. Both equivalent particle diameter and the number of particles with different diameters per unit volume can be calculated by a microprocessor based on Mie theory. Knowing the main absorption wavelength of 4.26 \(\upmu\)m for CO\({}_{2}\) gas, the SCD30 CO\({}_{2}\) sensor [24] is an industrial nondispersive infrared (NDIR) CO\({}_{2}\)-sensing device which acquires the CO\({}_{2}\) concentration in the range of 400-10,000 parts per million (ppm). As shown in Figure 3c, the SCD30 CO\({}_{2}\) sensor uses an infrared (IR) lamp to direct IR light waves through a tube filled with a sample of air, the CO\({}_{2}\) gas molecules absorb the specific band of IR light while letting other wavelengths of IR light pass through, the remaining light is absorbed by an optical filter except the wavelength absorbed by CO\({}_{2}\) molecules in the sample tube, and a fingerprint is detected by the following CMOS device to identify the CO\({}_{2}\) molecules. A built-in SHT31 temperature and humidity sensor is mainly used to compensate the CO\({}_{2}\) reading; furthermore, the ambient temperature and humidity readings can be used for environment information. It should be noted that CO\({}_{2}\) concentration is one of the most important indicators for IAQ standards. A BH1750 ambient light sensor was used to detect the brightness of light in the range of 1-65,535 lux with high accuracy of 1 lux in the environment [25]. As shown in Figure 3d, the BH1750 light sensor mainly adopts a photodiode (PD) which is a light-sensitive semiconductor device with a p-type-intrinsic-n-type (P-I-N) junction structure in which an intrinsic semiconductor is sandwiched between p-type and n-type ones. The PD devices work in a reversed mode and absorb visible light to produce a current through the effect of photon impact. Then, the photo-generated current is converted to voltage through a two-stage operation amplifier (OP Amp) by integrating an integral OP Amp and an inverter OP Amp. Finally, both an analog-to-digital converter (ADC) and digital circuit with I\({}^{2}\)C interface convert the light intensity into digital data according to the I\({}^{2}\)C serial communication protocol. The main specifications of the sensors are tabulated in Table 2. I\({}^{2}\)C is synchronous and a multi-master/multi-slave serial communication bus. On the other hand, UART has asynchronous serial communication by sending data frames from the least significant to the most significant with both start and stop bits at precise times. Figure 2a illustrates that only the PMS7003T dust sensor is arranged with the GPIO 26 (Rx) and DGPIO 27 (Tx) pins with the UART interface of the programmed sensor shield for the TIGO ESP32 Wi-Fi module. The others are assigned to GPIO 21 (SCL) and GPIOv22 (SDA) pins in any one of the I\({}^{2}\)C interfaces which can be extended with the same I\({}^{2}\)C-interface devices. One SPI interface was well planned for the built-in 1.14-inch ISP display with a resolution of 135 \(\times\) 240. It is well known that the UART protocol is a master-slave configuration and only one device can be connected to the TIGO ESP32 Wi-Fi module. Furthermore, the I\({}^{2}\)C protocol is a multi-master/multi-slave serial communication protocol extending up to 128 devices with the I\({}^{2}\)C interface. Moreover, the SPI protocol is similar to the I\({}^{2}\)C one with multi-master/multi-slave serial communication but only one device can be enabled to transmit data at a single response time.