## 4 Conclusions and Outlooks  
In this work, we programmed three microcontroller platforms and successfully developed Bluetooth Low Energy (BLE)-based wireless sensor nodes capable of sensing and transmitting environmental data such as temperature, air pressure, humidity, and air quality over a standard protocol distance of 30 m. The overarching goal of achieving autonomous operation in an ultra-low-power mode through the implementation of a vibration-based energy harvesting scheme has been realized. Various hardware and software power-saving techniques have been implemented and thoroughly studied to minimize the power consumption of our system. This was achieved through the optimization of timing protocols, implementation of multiple power-saving schemes, utilization of different sleep modes, tweaking operating voltage levels, and integrating ultra-low-power external timing devices where necessary. Furthermore, we conducted a comprehensive investigation of worst-case system operation scenarios, where a battery serves as a backup power supply. Our results demonstrated that by employing our energy harvesting source, the battery lifespan can be extended by a factor of 9 times. This feature holds significant potential for reducing maintenance costs and minimizing battery waste. To enhance the system's reliability, we implemented various measures aimed at improving robustness and noise tolerance within the feedback system. As part of future improvements, we propose upgrading the system by substituting the existing external timer with a more dependable timer module. Finally, throughout this work, a 2.45 MHz MCU speed clock was used for all platforms. Exploring the possibility of lowering this value could be another avenue for optimizing the ultra-low-power operation of the sensor node. While potential reliability issues in operation might arise, it is worth studying the tradeoff between a reliable operation, a slower MCU \begin{table}
\begin{tabular}{l l c c c c} \hline \hline
**Configuration** & **E.H.** & \(V_{output}\) [V] & **Br. Time Interval [s]** & \(I_{capacitor}\) [mA] & \(V_{capacitor}\) [V]** \\ \hline ATMEGA328P + TPL5110 TIMER & & & 5 & n/a & n/a \\ TI-CC2650 + INTERNAL TIMER & & & 5 & 3.372 & 2.477 \\ TI-CC2650 + TPL5110 TIMER & & & 6 & 0.877 & 2.476 \\ ADAFRUIT + TPL5110 TIMER & & & 15 & 1.467 & 2.475 \\ ATMEGA328P + TPL5110 TIMER & & & 5 & 18.801 & 2.972 \\ TI-CC2650 + INTERNAL TIMER & & & 5 & 11.598 & 2.972 \\ TI-CC2650 + TPL5110 TIMER & & & 6 & 10.717 & 2.971 \\ ADAFRUIT + TPL5110 TIMER & & & 15 & 3.091 & 2.968 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Overview showing the autonomous operation of the sensor node, which uses different MCU platforms at different broadcast frequencies using the reference energy harvester’s power generated at each mode stored in a supercapacitor. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Configuration** & **E.H.** & **Mode** & \(V_{output}\) [V] & **Br. Time Interval [s]** & \(I_{capacitor}\) [mA] & \(V_{capacitor}\) [V]** \\ \hline TI-CC2650 + INTERNAL TIMER & & I & & 1 & 3.62 & 2.49 \\  & & II & & 3 & 1.12 & 2.48 \\ TI-CC2650 + TPL5110 TIMER & & I & & 1 & 3.44 & 2.48 \\  & & II & & 3 & 2.08 & 2.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Performances of the autonomous sensor node operating using the optimized energy harvester (labeled as “Design 1”), providing higher power levels compared to the reference design. speed clock, and power consumption. Furthermore, the system integration of the overall sensing unit can be enhanced, especially in scenarios with space restrictions. This can be achieved by the development of a dedicated circuit board, which would consolidate the microcontroller, the power management board, and the sensing unit's Integrated Circuits onto a single platform. This approach would result in optimized space utilization by omitting all non-used units, contributing to a more streamlined and efficient design.