# A Low Power IoT Sensor Node Architecture for Waste Management Within Smart Cities Context
## 6 Operation Tests
### Tests on the HC-SR04 Ultrasound Sensor  
The tests on the effectiveness of the sensor were performed by using a sub-set of the node prototype made up only by the Arduino UNO board and the HC-SR04 sensor since the presence of other components would have had no influence on the accuracy of the measurement. In addition, since an Arduino UNO board is equipped with the same microcontroller employed in the node, such a board was chosen for these tests due to its simple usage whenever rapid prototyping is needed and because the same firmware of the IoT node could be exploited. The tests were performed using a real trash bin equipped with a cardboard cover ad-hoc built with the sensor embedded in it (Figure 7). In order to ease data reading, an LCD screen was added on the Arduino UNO board displaying the distance in cm measured by the sensor (Figure 8a). The transmitter and the receiver of the sensor were directed toward the bottom of the bin, measuring then the distance of the upper surface of the garbage layer from the lower surface of the cardboard cover (Figure 8b): that distance is \(\sim\)100 cm if the bin is empty, with an approximation range of \(\pm\)2 cm due to the presence of the trash bag (as in a real scenario) that could be crumpled and not perfectly adherent to the bottom of the bin.  
The accuracy of the measurement with the empty bin was in the order of \(\sim\)1 cm. The trash bin was then progressively filled up with rubbish (mainly paper), and the value acquired by the sensor then compared with the distance from the sensor to the trash level measured with a measuring tape. The filling level was obtained by performing three consecutive measurements, then a mean of these data was computed by the microcontroller and finally displayed on the value obtained through the sensor fell within a 1 cm error range by measuring the distance with the measuring tape, it did not take into account possible rubbish accumulations on the sides of the bin since the level value was measured in its central point. Anyway, due to the reduced internal surface of a trash bin compared to a dumpster, the level of the rubbish can be considered almost constant with variations lower than 5 cm. Moreover, it has to be underlined that, to asses the filling of the bin, 5 cm variations can be considered negligible.  
The accuracy of the sensor was confirmed for almost the whole height of the bin. Inaccurate readings occurred only in the last 5 cm by the proximity of the upper inner surface of the bin: this value was higher than the one expected according to the sensor specifications (2 cm to 400 cm measurement range). Anyway, this result does not affect the effectiveness of the IoT node because when a 5 cm distance is measured, the trash bin can be considered full.  
Figure 8: Experimental setup: (**a**) the LCD screen showing a measured distance value; (**b**) the ultrasound sensor placed in the cardboard cover.  
Figure 7: Experimental setup for the validation of the sensor. 3D model of the sensing structure: (**a**) axonometric view from above; (**b**) axonometric view from below; (**c**) final realization.