# A Self-Powered Basketball Training Sensor Based on Triboelectric Nanogenerator
## 2 Composition and Working Principle  
As shown in Figure 1b, the sensor is mainly composed of a floor, springs, floating plates and LED lights, etc. The floating plate with a size of 200 mm \(\times\) 200 mm is installed in the floor with a size of 400 mm \(\times\) 400 mm \(\times\) 80 mm, and any two adjacent floating plates are supported by springs to form a vertical contact model triboelectric nanogenerator. The upper surface of the floating plate are successively pasted with 0.05 mm thick copper (Cu) and 0.03 mm thick polytetrafluoroethylene (PTFE) as the electrode layer and friction layer, respectively, while the lower surface is pasted with 0.05 mm thick Cu both as the friction layer and the electrode layer.  
The fabrication of the sensor are as follows. The floor is an ordinary wooden floor purchased. According to the design requirements, the installation space of the floating plate is cut with a saw blade, and the cut part is the floating plate. Further, through holes are drilled around the floor to install LED lights, and the wires of LED lights under the floor. Then, blind holes with a size of 60 mm are drilled on the floor and the floating plate to install the springs. Finally, a floor-shaped striped paper is glued on the surface for beautification after the entire sensor is assembled.  
The sensor is installed on the floor or wall when used for the basketball training of \"bounce the ball in situ\" and \"two-hand chest pass the ball\". If the training posture of the beginner children is correct, the sensor can be impacted by a basketball, which will cause the friction layers between different floating plates to rub against each other to generate corresponding triboelectric signals, to realize the function of measurement. Meanwhile, the LED lights at the four corners of the sensor will be lit once in real-time when the sensor was impacted because the working process of the sensor is also the process of generating electricity, so the beginner children can also observe whether the LED lights are lit to judge whether their training activities are standard.  
The working principle of the sensor is further explained in conjunction with Figure 1c. Assuming that the simplified initial state of the two floating plates, as shown in step S0, and there is no charge at this stage because the two floating plates are not in frictional contact. When the floating plate is impacted by a basketball, the two floating plates will contact each other and reach the stage of S1. At this stage, positive and negative charges are generated in the two friction plates because of the triboelectricity, and the Cu friction layer is positively charged while the PTFE is negatively charged because Cu is easier to lose electrons than PTFE [36]. When the basketball impact force disappears, the two floating plates will gradually separate, due to the spring force, that is, the S2 stage. During this process, the change of the induced electromotive force will cause the electrons in the upper floating plate to transfer to the lower, thereby generating a short-circuit current. Then the S3 stage is reached when the separation distance between the two floating plates reaches the maximum, and the charge transfer is completed at this state. Finally, the floating plate will be returned to the new initial stage S4 by the spring restoring force and gravity. At this stage, the reverse movement of the two plates causes the reverse transfer of charges, thereby generating a reverse short-circuit current. In summary, it can be seen that the sensor will output a voltage pulse signal or an alternating current pulse signal during a working cycle, and the signal frequency is proportional to the frequency of the basketball impacting the sensor, so the effect of beginner children basketball training can be measured and evaluated by counting the number of output pulse signals.