# InoT Sensor Network for Wild-Animal Detection near Roads
## 3 Design of DSN Hardware
### Energy Consumption and Harvesting of the DSM Node  
The main source of energy for DSNs is solar panels (photovoltaic cells). During the night, the devices will be powered by the battery. The average current generated by solar panels throughout the day is:  
\[I_{agg}=(T_{d}\cdot I_{sp})/24, \tag{1}\]  
where \(T_{d}\) is the length of daylight (day) in hours and \(I_{sp}\) is the average current generated by solar panels during the day. The energy balance was estimated for the worst-case scenario, during a cloudy day on the shortest day. In December in Lithuania, the length of the day \(T_{d}\) is only a little longer than 7 h, so in the following calculations \(T_{d}\) = 7 h was accepted. The experimental assessment of the expected average solar panel current was performed under various lighting conditions in March-April 2021. The current generated by polycrystalline solar panels with an area of 66 cm\({}^{2}\) (batteries of a similar size are expected to be used in DSN prototypes) was recorded when the voltage generated by it was higher than 3 V. The long-term data-acquisition results are shown in Figure 6.  
The average current \(I_{sp}\) during daylight hours in cloudy and rainy weather is about 6 mA, while on a sunny day, it can reach 60 mA or more (depending on the orientation of the solar panel). The experimental results are shown in Figure 7.  
When calculating the resources of the average generated current in the worst-case scenario, it was assumed that \(I_{\text{rd}}\) = 6 mA. In December, under overcast weather, the average daily generated current will be only:  
\[I_{v}=(7\times 6)/24=1.75\text{ mA} \tag{2}\]  
The charge current can be even lower in the shade of forest trees, or when the surfaces of the solar panels are contaminated and obstructed. So, it was assumed that the current used by the DSN in standby mode, when only the most necessary node (sensors) is active, \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Lens Dimensions**} & **Object 1 Average** & **Object 2 Average** & **Object 3 Average** \\  & **Distance, m** & **Distance, m** & **Distance, m** \\ \hline LENS 1 (d = 25 mm) & 19.5 & 20.5 & 21.1 \\ LENS 2 (d = 16 mm) & 35.6 & 36.2 & 36.8 \\ LENS 3 (d = 29 mm) & 40.2 & 41.3 & 41.1 \\ LENS 4 (d = 27 mm) & 37.8 & 39.1 & 39.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental results of lens-diameter influence on object-detection distance. should not exceed 1 mA. Due to limited energy resources, it was decided not to use sensors based on the light-beam barrier in the prototypes, because their lights require a current of >20 mA. The batteries are designed to power the DSN during the dark hours of the day. The majority of the batteries perform poorly at low temperatures--their capacity usually decreases and internal resistance increases. Since the DSN will have to work in a wide range of temperatures, it is necessary to choose suitable batteries that can be used properly in both high and low temperatures. The capacity of lithium-ion batteries can decrease up to 55% at negative temperatures (\(<\)-20 \({}^{\circ}\)C) [45; 46; 47]. There are rechargeable cells on the market that do not degrade as much in the cold, but their price is high (\(\sim\)525 [48]). Lithium-based batteries require specialized charging and discharging circuits to keep their voltages around 2.6...4.2 V. They can catch fire if overloaded or overcharged, therefore they often need additional protections to shut them down at temperatures higher than the manufacturer recommends. NiMH batteries are safer than lithium (cannot self-ignite) and can be fully discharged and recharged (small current \(<\) 0.1 \({}^{\circ}\)C). The disadvantage of many NiMHs is their fairly significant self-discharge, but there are also NiMH batteries with an extremely low self-discharge current [49].