# A Low Power IoT Sensor Node Architecture for Waste Management Within Smart Cities Context
## 4 Node Architecture
### Block A  
Power supply is provided by four 1.5 V AA lithium batteries, hence the node is provided with an amount of 6 V, and considering that the microcontroller and the transmission module require lower supply voltages, 5 V and 3.3 V respectively, it is mandatory to include voltage regulators within the node. For this reason, two voltage regulators are employed: a 5 V regulator which powers Block B of Figure 1 and a 3.3 V one which powers the LoRa module. The latter is within the Switching and powering system for LoRa block and therefore it will be introduced in the relative Section 4.5. Block A is characterized by the block diagram presented in Figure 4. The duty cycle of the sensor node is managed by an integrated circuit: the HEF4060B by Nexperia. It is a 14-stage ripple carry binary counter which is directly powered by the four 1.5 V AA lithium batteries. Although it is the only component of the IoT sensor node which constantly remains active, this is not an issue at all since it absorbs only 36 \(\mu\)A according to its datasheet [70]. This counter has been chosen since it is cheap (its price hovers around 0.10 euros) as well as easy to use and because it has a dedicated reset pin ensuring the possibility to restart the counting process via an external signal that is provided by the microcontroller. The counter drives an N-type MOSFET whose task is to switch on a 5 V step-down voltage regulator (i.e., the Pololu D24V22F5 (Pololu Corporation, Las Vegas, NV, USA)). Finally, the step-down voltage regulator powers the other parts of the sensor node: they are encompassed by Block B of Figure 1. Whenever the measurement as well as the transmission phases are completed, the microcontroller resets the counter via its dedicated pin. This entails, in turn, the interdiction of the MOSFET and the power disabling of Block B. At the same time, the counting process restarts once again involving the periodic accomplishment of the system tasks. In this way, the sensor node is provided with an energy-saving policy that will be described in Section 5.1: the counter is the only component that is constantly powered while the other ones are exclusively activated for the necessary time on any occasion they need to operate. Figure 4: Block diagram of Block A part of the IoT sensor node. Concerning the oscillation frequency of the counter, the HEF4060B features two setting modes: its clock frequency could be determined both by a crystal quartz or by an RC oscillator. The latter solution was preferred since it is cheaper to implement, and bearing in mind that it is requested a measurement of the filling level of the bins per hour, the design of the RC circuit goes as follows. Since the HEF4060B is a 14-stage counter, it is capable to count up to \(2^{14}-1\). Due to the fact that the N-MOSFET is controlled by the Most Significant Bit (MSB) of the counter (which corresponds to pin number 3, \(Q_{13}\), of the integrated circuit) and considering that only one measurement per hour is needed, the clock frequency has to be designed in such a way that the MSB is at a high logic level after 3600 s. In other words, the HEF4060B has to count up to \(2^{13}-1\) in 3600 s hence it would finish its available combinations of digits after 7200 s. Therefore, the oscillation frequency \(f_{osc}\) is:  
\[f_{osc}=\frac{2^{14}}{7200}\simeq 2.2756\,\mathrm{Hz}. \tag{3}\]  
According to the HEF4060B datasheet, \(f_{osc}\) has a direct relation with the components of the RC circuit (\(R_{T}\) and \(C_{T}\)) which is:  
\[f_{osc}=\frac{1}{2.3R_{T}C_{T}}. \tag{4}\]  
A 680 nF plastic film capacitor was chosen for \(C_{T}\). Such a value is due to the fact that the datasheet suggests \(C_{T}>100\) pF and to the fact that a pretty low frequency is desired. Therefore, by equaling Equations (3) and (4), \(R_{T}\simeq 280\) k\(\Omega\) is obtained. In order to avoid a series connection of resistors, a commercial value was selected for such a component, hence \(R_{T}=270\) k\(\Omega\).