# Integration of Autonomous Wireless Sensor Networks in Academic School Gardens
## 2 Materials and Methods  
The experiments have been carried out in the orchard of the 'Camino de Santiago' primary school, located near the city of Pamplona. The orchard has an educational role, as the students learn how to grow different kind of vegetables such as cucumbers, pumpkins, onions, garlic, tomatoes, beans, zucchini, corn and so forth. The orchard is 25 m long and 9.5 m width and it is located within the school yard, near the school building, as can be seen in Figure 1, where the orchard is delimited by a red rectangle. The 18 yellow dots within the red rectangle that can be seen in the figure represent the positions where the sensor devices of the proposed WSN have been placed. Figure 1 shows a real picture of the orchard under study. The proposed WSN for monitoring the orchard has been built based on EnOcean's energy harvesting modules. Specifically, the STM 330 modules have been employed, which employ the ISO/IEC 14543-3-1X proprietary standard for wireless communications. The STM 330 module makes use of the European operating frequency (868 MHz), receiver sensitivity value of \(-\)96 dBm@125 kbps and expected current consumption values ranging from 0.6 mA to 130 mA depending on charging and luminosity conditions. These wireless modules have integrated a 16 MHz 8051 CPU with 32 kB FLASH and 2 kB SRAM. They also have incorporated a temperature sensor (range 0 to 40 \({}^{\circ}\)C) and provide the possibility of equipping them with a humidity sensor, which has been used in this study as it provides interesting information for the purpose of the presented application. The humidity sensor is HSM 100 module, which has a measurement range from 0% to 100% with a resolution of 0.4% and a typical accuracy of \(\pm\)5% (for values between 30-70%). In Figure 2 a simplified device block diagram is presented. Figure 2 shows a picture of a STM 330 module alongside the optional humidity sensor. In the same way, Figure 2 shows a picture of the opposite side of the same module, where the solar cell used for the energy harvesting can be seen. The power supplied by the solar cell is managed by an energy management circuit to bridge periods of darkness. Specifically, if the energy storage is fully charged, the operation time in darkness is typically 4 days (which could vary depending on the temperature), when the information transmissions are made every 1000 s. The module also provides  
Figure 1: (**a**) ‘Camino de Santiago’ public school’s upper view; (**b**) A picture of the school’s orchard. user configurable cyclic wake-up functionality. After wake-up, a radio message is transmitted in case of a significant change of measured data is detected. Besides the reduced size of the modules, which provides an easy-to-deploy feature, it is very important to note that the energy harvesting technology of the devices (they are self-powered by the small solar cell) avoids the maintenance task of replacing batteries that common WSNs usually need. This is a very important feature of the EnOcean devices and it is worth noting that from the literature, there are very few solutions where self-powered devices are employed [13; 33], the rest need to be powered externally as the use development boards such as Arduino or Raspberry Pi. Regarding the radio characteristics of the STM 330 modules, they operate at 868.3 MHz, which can provide longer distances than common devices operating at 2.4 GHz (such as ZigBee and Bluetooth) due to lower radio propagation losses. They provide a low data rate of 125 kbps (in comparison, ZigBee at 2.4 GHz transmits 256 kbps), which is enough in order to transmit the required information. The transmitted power level is between 5 and 8 dBm.