## 3 Design and Implementation Details  
### Derived System Formation  
The exact way that the participating modules are interconnected is shown in Figure 2a. Figure 2b depicts an experimental implementation of the basic component synergy via a breadboard for fast development, while Figure 2c illustrates details of a mature node implementation. A cost-effective, widely used microcontroller, the Atmel 32u4, running at 8 MHz and equipped with a LoRa transceiver, was tuned to orchestrate the data collection and delivery process from the diverse set of soil monitoring sensors. Initially, an Adafruit Feather 32u4 RFM95 LoRa Radio board was utilized, while the pilot deployment also benefited from the LoRa32u4 II development board, which had an attractive price and the additional feature of easily connecting an inexpensive LoRa antenna through a U.FL coaxial connector cable. The aforementioned microcontroller unit had to periodically power on and off the sensors of interest and acquire the data from them using either the simple serial DDI communication protocol or analog-to-digital conversion (ADC) methods, according to the output type (i.e., digital or analog) of the soil instruments (D.S.I. or A.S.I., respectively) connected to it. The dispatching of the power supply in one direction and of the data flow in the other between the soil sensors and the microcontroller is facilitated by the presence of a dual four-channel analog multiplexer/demultiplexer (i.e., a 74 HC4052 chip) and a set of controllable DC-DC conversion units providing a stable 5 V and (up to) 100 mA output by a lower voltage source. A welcome feature of these powering units, also known as DC voltage converters or charge pumps, is that they can shut down their output by zeroing the voltage at a dedicated control pin called En at negligible quiescent current. The specific unit selected was an Adafruit MiniBoost 5 V @ 100 mA Charge Pump. The board hosting the 32u4 microcontroller, despite its low cost, implements a very satisfactory sleep mode  
Figure 1: The proposed soil instrument monitoring platform architecture and data flow. schema, reaching a sleep consumption close to 0.25 mA, approximately. The whole system drains power from a Li-Ion battery unit of the 18,650 type, which can be connected with a small solar panel for periodic energy replenishment. Figure 3a depicts the outdoor environmental condition-resistant box enclosure arrangements of a finished typical sensor node, which is also accompanied by a single Li-Ion battery of the 18,650 type; a small omnidirectional 2 dBi antenna of 50 \(\Omega\) output resistance, vertically polarized and suitable for communication via LoRa in the 868/915 MHz bands; and (optionally) a small solar panel and its miniature charging circuit. The detailed action sequence followed by the logic of the designed sensor node is depicted in Figure 3b and as follows:  
* Initialize the modules of the microcontroller that are necessary for communication with the instruments connected to the inputs of the node. Figure 2: Sensor node details: (**a**) interconnection among the different components; (**b**) early experimental node implementation via a breadboard; (**c**) late “mature” node implementation. * For each participating instrument, power on the instrument, read the measured quantities, process them if necessary, append accordingly to the packet payload for delivery, power off the instrument. * Activate the LoRa radio and transmit the packet being prepared toward the gateway node. * Deactivate the participating modules (and the LoRa radio) to minimize power consumption during sleep. * Execute the sleep action of the system to sleep for a specific period of time. It should be noted that, in case of analog output soil instruments, oversampling was applied (i.e., of 16 times) in order to improve the native 10-bit resolution of the ADC module of the 32u4 microcontroller. Two sensor nodes were implemented, namely, #A1 and #A2.