## 5 Wireless Sensor Node Design  
The EM converter provides a sustainable power source to satisfy the energy requirements for the wireless sensor node. In practice, the electronic specifications of the wireless module, such as the communication module, the sampling rate, and the resolution of the ADC, influence its overall energy consumption [3]. The choice of the microcontroller (MCU) is crucial in the operation of the wireless sensor node and represents the main energy-consuming component [31; 32]. Therefore, the MCU must enable a high level of performance while maintaining a low level of energy consumption. In particular, the choice of the MCU is based on the applications' requirements, such as the computational capabilities of the node, power consumption, and size. Considering our application's requirements, the system-on-chip microcontroller CC430F5137 [33] integrated with an RF transceiver core CC1001 [34] is selected to collect the vibration information and forward it to the user. The CC430F5137 MCU works at a frequency of 868 MHz and consumes a minimum current of 0.5 uA in the deep sleep mode with a 3 V supply voltage. In particular, CC430F5137 MCU enables three main working modes: active, sleep, and idle. It consumes around 13 mA and 36 mA for the receiving and the sending of a data packet, respectively. During the runtime, the wireless node collects the vibration information from the converter and then transmits it through the RF transceiver to the base station node, which is connected to the computer. The main characteristics of the wireless node are presented in Table 2. Figure 10 presents the node hardware used in this work. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline MCU & CC430F5137 (MSP430 core + CC1101 radio SOC) \\ \hline Operating input voltage & 3–3.3 V \\ \hline Operating frequency & 868 MHz \\ \hline \hline \end{tabular}
\begin{tabular}{c} \hline \hline Sleep: 1–2 uA \\ Tx: 36 mA (max) \\ Rx: 18 mA (max) \\ \hline Sending power & +12 dBm (max) \\ \hline ADC & 12 bits \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Hardware specification of the wireless sensor node. In our application, the wireless sensor node was powered using the harvested energy of the EM converter. For this, given the difference between the EH and communication link budgets, investigating the energy consumption and ideal efficient current for the node's operation was required. To do so, a small wireless network was created using two wireless nodes as a proof of concept. One node is identified as the transmitter node, which is connected to the harvester output, responsible for following the change in the voltage output. The collected voltage output is later sent to the base station node, which is interfaced with the computer. Periodic voltage sensor data are collected from the connected pin and transferred to the base station. The energy consumption of the node during its different working modes was evaluated to extract the necessary tuning parameters for the energy-management circuit. To characterize the needs of the wireless sensor node in terms of power consumption, the wireless module was investigated over its different working activity modes. The energy consumption during the run time of the wireless node, the Agilent Keysight E5270b, was used to characterize the current-voltage consumption [35]. In practice, the four-wire, resistance measurement method was used to quantify the energy consumption of the wireless module during its different working modes (active and sleep). Continuous data measurement and transmission were considered to identify the complete working cycle of the node in a normal scenario. To do so, an accurate energy measurement is necessary to characterize the wireless module, which can be generalized according to the consumed current in each state, which is defined in Equation (6).