## 3 Low-Cost Multi-Sensor
### Hardware Description  
The main blocks each multi-sensor is composed of are shown in Figure 3. From such an architecture, three main blocks can be identified, which are the set of sensors (on the left), the power and connectivity board (in the center), and the microcontroller (on the right). Regarding the sensors, they were selected to monitor all four IEQ domains, i.e., thermal, visual, acoustic, and indoor air quality. Such devices were selected among those commercially available to optimize cost, measurement range, accuracy, dimensions, response time, and power consumption. The main parameters related to the selected  
Figure 2: Process of data acquisition employed in the PROMET&O system. Each multi-sensor acquires several physical quantities, which are denoted as \(x_{1},\ldots,x_{\text{Msens}}\), each one with a given sampling time (\(t_{\text{sampl},1},\ldots,t_{\text{samplMsens}}\)). Acquired data are then aggregated into \(K\) statistical parameters and sent to the server over time intervals of length \(t_{\text{report},1},\ldots,t_{\text{report},\text{Msens}}\). sensors are listed in Table 1, including the default sampling time and report time for each monitored quantity, as discussed in Section 2.2. The only exception is the microphone, whose output is sampled at 22 kHz, but the sound pressure level is evaluated over a 500 ms time window, as reported in Table 1. Most of the selected sensors can be directly connected to the microcontroller through standard digital serial interfaces. Conversely, the CO and NO\({}_{2}\) sensors, which are based on electro-chemical cells, require dedicated circuits to be interfaced with the microcontroller. Such extra circuitry was placed close to the sensing cells in ad hoc designed boards. The second main block is the power and connectivity board, which is responsible to connect all blocks shown in Figure 3, including the sensor array, to the microcontroller, while power supplying all of them. To this purpose, two DC-DC converters were included, to step down the 12 V external power supply to 5 V and 3.3 V required by the subcircuits the multi-sensor is comprised of. Moreover, the WiFi module was included to allow for the wireless connectivity of the multi-sensor. As far as the micronroller is concerned, it acquires data from the sensors, performs preliminary computations and sent data to the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Physical Quantity** & **Ref.** & **Sensor Range** & **Dimensions** & **Cost** & **Sampling Time** & **Report Time** \\ \hline Temperature & [38] & \(-\)40–125 \({}^{\circ}\)C & 1.5 \(\times\) 1.5 \(\times\) 0.54 mm\({}^{3}\) & 4.04 USD & 0.1 s & 30 s \\ \hline Relative humidity & [38] & 0–100\% & 1.5 \(\times\) 1.5 \(\times\) 0.54 mm\({}^{3}\) & 4.04 USD & 0.1 s & 30 s \\ \hline Illuminance & [39] & 0–120 klx & 6.5 \(\times\) 2.35 \(\times\) 3 mm\({}^{3}\) & 4.89 USD & 0.1 s & 30 s \\ \hline Sound Pressure Level & [40] & 122.5 dB (SPL) & 3 \(\times\) 4 \(\times\) 1 mm\({}^{3}\) & 2.13 USD & 0.5 s & 5 s \\ \hline Formaldehyde & [41] & 0–1 ppm & 42 \(\times\) 24 \(\times\) 5.5 mm\({}^{3}\) & 50.23 USD & 3 s & 1 min \\ \hline TVOC & [42] & 0–10,000 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\) & 22.9 \(\times\) 14 \(\times\) 3.15 mm\({}^{3}\) & 21.92 USD & 3 s & 1 min \\ \hline PM 2.5 & [43] & 0–1000 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\) & 52.3 \(\times\) 43.3 \(\times\) 22.3 mm\({}^{3}\) & 27.91 USD & 3 s & 1 min \\ \hline PM 10 & [43] & 0–10,000 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\) & 52.3 \(\times\) 43.3 \(\times\) 22.3 mm\({}^{3}\) & 27.91 USD & 3 s & 1 min \\ \hline Nitrogen dioxide & [44] & 0–5 ppm & 20 \(\times\) 20 \(\times\) 3.8 mm\({}^{3}\) & 19.65 USD & 3 s & 1 min \\ \hline Carbon dioxide & [45] & 0–40,000 ppm & 35 \(\times\) 23 \(\times\) 7 mm\({}^{3}\) & 51.37 USD & 3 s & 1 min \\ \hline Carbon monoxide & [46] & 0–1000 ppm & 20\(\times\) 20 \(\times\) 3.8 mm\({}^{3}\) & 19.65 USD & 3 s & 1 min \\ \hline \hline \end{tabular}
\end{table}