## 3 Experimental Studies and Discussions  
### Experimental Setup  
The experimental setup of the proposed multiple-layer PEH is shown in Figure 3. In this research, we mainly focused on illustrating the features of the proposed harvester experimentally. [PERSON] developed a detailed analytical electromechanical coupling model in Ref. [31]. The PEH contained three piezoelectric cantilevers with connections at both the fixed and free ends. The spring steel substrate of the piezoelectric cantilever had dimensions of 75.18 \(\times\) 20 \(\times\) 0.16 mm\({}^{3}\). Six piezoelectric transducers (15 \(\times\) 20 \(\times\) 0.2 mm\({}^{3}\)) were glued to the root and free end of the cantilevers using epoxy resin DP460 (3M, St. Paul, MN, USA). The lead zirconate titanate-5 piezoelectric transducers were purchased from Jiayeda, Co., Ltd., Changde, China, with Young's Modulus of 106 GPa and piezoelectric coefficient d\({}_{31}\) = -171 pC/N. The proof mass was attached at the free end of the PEH. The proof mass was made of permanent magnet blocks. Permanent magnets were glued onto the cantilever as the primary proof mass. Additional permanent magnets were attached to the primary one to tune the natural frequency of the harvester to the operational frequency of 100 Hz. As the permanent magnets would provide attractive force to each other, the additional permanent magnets were added to the fixed location at the free end of the cantilever. The number and sizes of the magnets were carefully selected to adjust the resonant frequency of the model to 100 Hz. A 200 N shaker was adopted to provide the base-movement excitations. The root mean square (RMS) output voltages of the piezoelectric transducers were measured by a NI-DAQ device (PCIe6343) (National Instruments, Austin, TX, USA) with a sampling frequency of 20 kHz. In addition, the amplitude of the base-movement was measured by an accelerometer. In order to validate the performance of the self-powered CO\({}_{2}\) monitoring, a self-powered wireless sensor node was designed consisting of an STM32F103C6 microcontroller (STMicroelectronics, Geneva, Switzerland), an analog CO\({}_{2}\) sensor, a Zigbee wireless module CC2530 (Texas Instruments, Dallas, TX, USA), six LTC-3588 voltage rectifications/regulators (Sparkfun, Boulder, CO, USA), and energy storage capacitors, as shown in Figure 3. The LTC-3588 was proven to have good performance for rectifying and regulating the output voltages from piezoelectric transducers [31]. It outputs 3.3 V to power the microcontroller,  
Figure 3: Experimental setup of the proposed PEH. sensor, and Zigbee wireless modules. In Figure 3, three LTC3588s are drawn in the sketch due to the space limitation of the figure. The other ones are represented by ellipses. To guarantee a sufficient and stable power supply for the wireless sensing node during operation, this study employed 16,000 \(\upmu\)r capacitors for energy storage. A microcontroller was used to manage the sensor and Zigbee module, with its clock frequency set to 16 MHz to minimize the overall power consumption. Opting for a low-power microcontroller, such as the STM32L or Attiny85 (Microchip Technology, Chandler, AZ, USA), could further decrease the power usage of the sensor node. An ADC port was selected to retrieve the analog signal from the CO\({}_{2}\) sensor. Acknowledging that the wireless module and the sensor are the primary consumers of electrical energy in the circuit, we designed the power supply to these modules through multiple GPIO ports, which have the capability to be shut down instantaneously to conserve energy. The receiver, a secondary CC2530 Zigbee module, and a laptop were included in the host computer, as shown in Figure 4b. Typically, the Zigbee wireless data transmission module can have a theoretical communication distance of up to 250 m outdoors. In the experiment, the receiver was placed in the vicinity of the wireless sensor node for the conceptual illustration.