## 4 Results and Discussions
### Application in Temperature Monitoring System  
In the Simulink/MATLAB environment, the data from the Attiny85 microcontroller was monitored to evaluate its operation in different states. Figure 12 shows the system assembled on a protoboard, where a battery powers both the microcontroller and the LM35 temperature sensor, as well as the indicator LED, allowing the storage of the generated energy.  
To enhance the understanding of the energy harvesting process, Figure 12 illustrates the complete system where the energy generated by the piezoelectric transducers is converted and stored using the LTC3588-1 circuit. This stored energy is then utilized to power the temperature monitoring system. The microcontroller (ATtiny85) and temperature sensor (LM35) benefit from the harvested energy, ensuring efficient power usage and longer operational autonomy.  
For data acquisition, the microcontroller sends temperature information, using the PWM pin, and the microcontroller's state (on or in sleep mode) to the acquisition system. The information is sent via serial communication with a baud rate of 9600 bps (bits per second) and a 4-bit struct data type, with an \"R\" identifier at the beginning for Simulink system identification. The temperature reading occurs at each complete watchdog cycle.  
In the application, the microcontroller monitored the ambient temperature, generating a response signal, as shown in Figure 13.  
Analyzing Figure 13, it can be observed that the microcontroller enters active mode when the temperature exceeds 40 \({}^{\circ}\)C and returns to sleep mode after 3 s if the temperature falls below the setpoint. The microcontroller will only exit sleep mode again if there is a new exceeding of the setpoint value.  
The piezoelectric element, in conjunction with the LTC-3588-1, can generate up to 4.53 mW at its maximum potential, thus maintaining the electrical potential of the system. However, it is necessary to evaluate the autonomy of the monitoring system with the use of the battery.  
Figure 12: Assembly of the monitoring system.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Power**} & \multicolumn{2}{c}{**Power Density**} \\ \cline{2-3}  & **PZT-5H** & **P5-13B** \\ \hline Resistor & 6.49 mW/cm\({}^{3}\) & 4.02 mW/cm\({}^{3}\) \\ Diode Bridge & 1.47 mW/cm\({}^{3}\) & 0.94 mW/cm\({}^{3}\) \\ Buck Boost (LTC-3588) & 3.31 mW/cm\({}^{3}\) & 1.68 mW/cm\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 9: Overall power results for 3 studied cases.  
In this configuration, a fully charged battery has 3.6 V and a cut-off voltage of 2.7 V during use, which is the operating voltage limit of the microcontroller. The system can function until the battery is fully discharged, and, considering that the temperature stays above the setpoint for 50% of the time, i.e., a consumption of 3.5 mA/h (measured), the system would have an autonomy of 21.34 h without charging. With the system charging, consuming 2.3 mA/h (assuming the piezoelectric generator is in ideal condition), the circuit would operate for 35.06 h, representing an increase of 64.3% in the system's autonomy.  
Figure 14 shows the Simulink-implemented simulation of the autonomy of the monitoring system based on this data.  
Figure 14: Autonomy of the monitoring system.  
Figure 13: Operation of the temperature monitoring system. (**a**) Temperature of LM35 and (**b**) state of ATiny85.