# A Low Power IoT Sensor Node Architecture for Waste Management Within Smart Cities Context
## 4 Node Architecture  
This paper is focused on a solution for the real-time monitoring of the filling level of trash bins, placing the optimization of power consumption at the bulk of all the design activity. Beside this primary target, cost containment has been considered as the secondary one. The requirement of low power consumption implicitly implies that no connection to the electrical grid will be available at the deployment site of the node. Therefore, the node is expected to be powered through either energy harvesting solutions (e.g., solar cells) or batteries. The use of renewable energy sources is expected to double the overall cost of a basic IoT node like the one proposed in this paper, and the deployment of these solutions may not always be possible in highly urbanized areas (e.g., trash bins are often employed in historical centers where in narrow alleys direct sunlight is available only few hours per day or even never): for this reason, the architecture proposed in this paper has been customized on battery-powered IoT nodes. In particular, power is provided by four 1.5 V AA lithium batteries whose capacity is 3500 mAh. Since the crucial component affecting power dissipation in sensor nodes is the transmission module, such a design activity will be focused not only on the structure of the sensor node but also on the definition of the network architecture. To this end, the identification of an efficient data transmission technology is the square one towards the realization of an energy efficient sensor node. Beyond this, an investigation in order to determine which is the necessary amount of computational power should be carried out. For instance, if data processing is required on board of the sensor node, then a powerful computing unit is needed; this entails bigger consumption though. On the other hand, if data acquisition is the only operation that the node has to perform, then such a constraint could be relaxed by employing simpler devices. Hence, the choice of the microcontroller/microprocessor ensuring the best trade-off between energy efficiency and complexity has to be selected. Eventually, sensors have to be chosen looking for the best compromise between the accuracy of the measured data and the required energy to perform such task, thus privileging the use of passive sensors since power hungry sensors may become the crucial factor for energy consumption. Once identified the main components of the node, the design has to be focus on their integration, choosing the best electronic components for their interconnection and implementing energy saving policies through the use of procedures and technical solutions that will allow to turn off the node, or parts of it, when they are not used. Such energy-saving policy will be introduced later on in Section 5.1. The proposed IoT node was designed following this kind of approach, starting from the choice of the basic components: a single-chip microcontroller (ATmega328/P by Atmel (San Jose, CA, USA)) rather than a general purpose single-board microcontroller was employed, while for data transmission the choice fell on the LoRa technology, also taking into account an overall network architecture shaped on a Smart City scenario. Finally, a low cost and low power sensor was chosen, providing the best ratio between measurement accuracy and energy efficiency. Once the single components were selected, then the design was focused on their integration into a single board, embedding additional components required for energy management. The final node architecture is described by the block diagram shown in Figure 1: in addition to the main components, two switching circuits turning off respectively the LoRa radio module and the node in its entirety were added in order to implement duty-cycling policies. In particular, a timer controls the latter switching block. The diagram is divided into two main blocks: Block A is in charge of managing the duty cycle of the IoT node, and Block B measures the filling level of the bin and then transmits such a data.