## 2 Materials and Methods
### The Fundamentals
#### 2.1.2 Sensing, Data Acquisition, and Wireless Communication  
The primary objective of this work is to investigate different operation scenarios of a low-power sensor node, capable of monitoring the quality of the surrounding environment, using vibration-based energy harvesters. This includes monitoring ambient temperature, air pressure, environmental humidity, and the levels of Volatile Organic Compounds (VOCs), which serve as indicators of air quality. These environmental parameters play a crucial role in determining the suitability of an environment for various activities and also serve as early indicators of potential hazards. The sensing and data acquisition components of the system necessitate the use of sensors, which can be integrated into a single package or distributed across multiple packages. Additionally, an Analog-to-Digital Converter (ADC) is employed to convert the analog data from these sensors into digital numeric values. To process and represent these data in a meaningful format, a small digital computing processor, commonly referred to as a microcontroller, is utilized. In our pursuit of selecting the most suitable communication protocol for low-power applications, we choose Bluetooth Low Energy (BLE) as the preferred option. This decision is based on several compelling factors, including its widespread availability, ease of implementation, and favorable power-to-performance ratio. A literature survey [39; 40; 41] has reaffirmed that BLE boasts the lowest power requirements and the highest power efficiency compared to alternative communication protocols, such as ZigBee and ANT, as represented in Figure 2.  
Furthermore, BLE has now become an integral component of modern smartphones and laptop computers, which are essential for detecting and processing the broadcast data transmitted by our sensor node. This seamless integration with contemporary consumer \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{2}{c}{**Reference Design**} & \multicolumn{2}{c}{**Design 1 (Optimized)**} \\ \hline  & **Mode 1** & **Mode 2** & **Mode 1** & **Mode 2** \\ \hline Base acceleration [g] & & & 0.5 & \\ Frequency [Hz] & 65.03 & 77.78 & 72.12 & 76.85 \\ Voltage output (2151A) [V] & & 2.5 & & \\ Current output [mA] & 0.638 & 1.074 & 3.398 & 1.537 \\ Power output [mW] & 1.595 & 2.685 & 8.495 & 3.842 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the available generated power from the energy harvesters used in this research, under a 0.5 g base-acceleration according to [38]. Figure 1: Geometry description of the reference design (**a**) and the optimized design labeled as “Design 1” (**b**) of the vibration-based energy harvester used to power the sensor node [38]. devices enhances the practicality and accessibility of our system. On the other hand, Near-Field Communication (NFC) is excluded from our consideration due to its requirement for close proximity, which is not suitable for our application where remote wireless data transmission is a necessity. In this research, we focus on programming and testing three popular BLE-enabled microcontroller platforms within the ultra-low-power range. One of our objectives is to thoroughly investigate the impact of broadcast frequency on the functionality of the sensing system and data transfer when powered by the energy harvesters discussed in this section. In terms of the communication channel, the transmitting end of our setup comprises a sensor node that includes a microcontroller, sensors, an energy harvesting module, a power management circuit, and optionally, a rechargeable battery. This battery serves as a backup for worst-case scenarios in which the harvesting scheme may fail. On the receiving side, we have the option to use either a smartphone equipped with a suitable BLE application or a computer with the necessary BLE device.