# A Low Power IoT Sensor Node Architecture for Waste Management Within Smart Cities Context
## 4 Node Architecture
### Filling Level Sensor  
Since the main purpose of the IoT node is to measure the filling of trash bins, the most suitable sensor to perform this measurement has to be chosen. In order to define the features of the sensor, the following assumptions can be made:  
1. [leftmargin=*]
2. The filling level of trash bins cannot be calculated measuring the weight of the introduced waste. Indeed, a heavy weight does not always mean a large volume. Metallic materials can be very heavy yet leaving the trash bin almost empty, while a large amount of paper can totally fill the trash bin though being very light;
3. The inner part of a trash bin is subject to a high level of fouling, then preventing the use of optical sensors;
4. Trash bins are used for both very small and large objects. This means that solutions based on the count of thrown items do not provide an accurate filling estimate. In addition to all these requirements, two further features have to be provided by the sensor. The first one concerns the cost. Indeed, the sensor is expected to be cheap and since the node has to be deployed in very large quantities (even some thousands), then its overall price is expected to be in the order of few tens of euros. This means that the cost of the sensor should be at least lower than 5 euros. The second requirement concerns the energy consumption. The current absorption of the sensor is expected to be the lowest possible. As it will be explained later on (see Table 2), all the employed components account for a maximum 28 mA absorption. This should be considered as an upper bound also for the sensor, even if a lower value is expected to improve the overall node efficiency. All these constrains significantly narrow the range of possible solutions: all weight sensors, regardless of the type (piezoelectric, load cells, etc. ), can be excluded due to the first constrain, \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Frequency Band (Europe) & 863–870 MHz \\ Frequency Band (US) & 902–928 MHz \\ Transmission Power & 14 dBm \\ Sensitivity & \(-\)134 dBm \\ Channels (Europe) & 8 \\ Channels (US) & 13 \\ Distance & 22 km (13.4 mi) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: SX1272 module characteristics. while the second one leads to the exclusion of optical sensors like cameras (that may be also excluded due to the higher costs and power consumption) and infrared sensors that could have been used to measure the proximity of the trash level to the top of the bin. The last constrain also leads to the exclusion of magnetic or mechanical sensors that could have been applied to the opening of the bin counting the number of openings. Following all the previous considerations, the choice for the best solution fell on ultrasound proximity sensors. These devices measure the distance from a target through an ultrasound emitter (the trigger) and a receiver (the echo): the trigger emits a discontinuous ultrasound signal that hits the target and then it is reflected backwards. The echo detects the reflected signal: by the knowledge of the time interval between the emitted and the reflected signal, and through the value of the speed of sound in the air, it is possible to calculate the distance from the sensor to the target which can be easily translated into the filling level of the bin. These sensors are compliant with all the previous requirements:  
* They do not measure the weight of the trash but, if placed in the internal upper surface of the bin, they measure the actual level of the garbage layer by calculating its distance from the top of the bin;
* They are immune to fouling since ultrasounds are not perturbed by the presence of dirt or dust on the sensor surface;
* Since they detect the actual volume occupied by trash, they measure the filling level regardless of the number of introduced items;
* They have very limited costs (under 2-3 euros) and a limited current absorption (usually <15 mA; there exist also sensors absorbing 3-4 mA but unfortunately they are significantly more expensive, though).