# A Low Power IoT Sensor Node Architecture for Waste Management Within Smart Cities Context
## 4 Node Architecture
### Block A
At this stage, by employing Equation (4), the actual clock frequency \(f_{ck}\) can be evaluated: \(f_{ck}\simeq 2.3681\) Hz. As a direct consequence, the HEF4060B counts up to \(2^{13}-1\) in approximately 3460 s which roughly correspond to 57 min. Such a sampling period for measurement and transmission is acceptable though a finer precision could be achieved by employing, for instance, a trimmer in place of \(R_{T}\). The only two components left to design are \(R_{2}\) and \(C_{2}\). As stated in the HEF4060B datasheet, both this relations have to hold:  
\[ \begin{cases}R_{2}C_{2}\ll R_{T}C_{T}\\ R_{T}\ll R_{2}\end{cases} . \tag{5}\]  
Therefore, in order to satisfy Equation (5), \(R_{2}\) and \(C_{2}\) were selected as follows: \(R_{2}=820\) k\(\Omega\) and \(C_{2}=100\) pF as a ceramic capacitor. The described system clearly has a fixed oscillation frequency which leads to a fixed sampling rate of the filling level of the trash bin. As it has been just mentioned, the usage of a trimmer in place of \(R_{T}\) allows the tuning of the oscillation frequency. In so doing, the sampling rate of the filling level would be also made customizable by the users. Since the conditions expressed in Equation (5) must be always satisfied, the resistance value of the trimmer should remain within a 10 k\(\Omega\div 300\) k\(\Omega\) range. For this purpose, a series between a 10 k\(\Omega\) resistor and a suitable trimmer should be set up. It can be easily checked that the extreme oscillation frequencies are, in turn, 63.9386 Hz and 2.1313 Hz respectively implying \(\sim\)2 min and \(\sim\)64 min sampling periods. As it was already mentioned, when the MSB raises its logic level, the N-MOSFET behaves as a short circuit. Indeed, in this application it is exploited as a mere switch controlled by the counter. As a result, the Pololu D24V22F5 supplies the other parts of the sensor node (i.e., Block B). In spite of its expensiveness (its price hovers around 8 euros), the Pololu D24V22F5 was chosen because it is reliable and efficient. Moreover, it can accept a wide range of input voltages (up to 36 V) [71]. There are also some additional features of this regulator that justify its choice though, especially in comparison with a standard linear one (i.e., a 78 xx-type). Indeed, the main difference between these two categories of devices resides in the way they accomplish regulation. First of all, let us remind that power is a conservative quantity hence it has to be accounted in all its contributes throughout the regulation process. Since the input voltage is usually greater than the output one, then an energy conversion process (like the one performed by regulators) is mainly characterized by three quantities: an input power (\(P_{i}\)), an output power (\(P_{o}\)) and an excess power (\(P_{e}\)). It could be summarized in:  
\[P_{i}=P_{o}+P_{e}. \tag{6}\]  
A 78 xx-type linear regulator converts the input voltage into the output voltage by maintaining constant the current. Due to the fact that these regulators are not featured with any storage component, then \(P_{e}\) turns out to be converted into heat and therefore wasted. On the other hand, the Pol'ou regulators are switching regulators, hence they have a control circuitry and other components among which there are also the storage ones like inductors and capacitors. The control circuitry is in charge to turn the device on and off storing energy in inductors and capacitors so to ensure a steady output voltage. Practically speaking, the regulator is switched on long enough to charge the storage components up and to provide the desired output voltage, then it is shut down letting capacitors and inductors discharge feeding their stored energy to the output. As soon as the output voltage drops too low, then the regulator is switched on once again. As a direct consequence, \(P_{e}\) is stored rather than converted into heat, which implies a greater efficiency. Therefore, due to these features, a switching regulator is anyhow worthy in spite of its pretty high price.