|Abstract:||Home automation opened the door for automation of gardens and agricultural land. Which is possible through the use of sensor nodes, that are arranged over the cultivated area. They are very useful because they allow monitoring of environmental parameters that can lead to a worse quality of the yield. By using sensor nodes, the farmer will be given the opportunity to observe the impact of environmental parameters and act on it. In particular, this type of control is useful for larger areas, which are often far from the farmer's residence. With the development of sensor nodes, a new farming concept, called precision agriculture, was introduced. It is based on the observation and measurement of environmental parameters, which makes it is possible to create an automatic supply system for cultivated areas.
Our idea was to use several sensor nodes evenly distributed across the cultivated area to capture environmental parameters. Therefore, the main objective of the project was to make a working prototype circuit of the sensor node for measuring environmental parameters and to test it in a real environment. We have designed a sensor node that can measure air temperature and humidity, soil humidity and presence of the sun or illumination (indirectly via a solar cell). Separate sensors were used to measure the properties of air and soil, and the illumination was measured using the AD converter on the microcontroller. We have designed an autonomous circuit containing a solar cell and a spare battery. In the days of sunshine, the solar cell powers the circuit and charges the battery at the same time. If the weather is cloudy or rainy, the main power source will be a spare battery.
In the framework of the prototype design, we measured the characteristics of the power supply part of the circuit, compared to the characteristics of the solar cells of different manufacturers, programmed the microcontroller and tested the prototype in a real environment. The microcontroller was programmed to perform measurements every ten minutes, and after they are done, the whole circuit goes into sleep mode. The communication between the sensors and the microcontroller is carried out via the communication bus TWI. In our case, the microcontroller has the role of master, and sensors have the role of slaves. After the measurements are made, the microcontroller sends the results to the computer via communication USART.
We measured the change in the charging current of the battery and the efficiency of the power supply circuit based on the voltage on the solar cell. As long as the voltage on the solar cell is higher than or equal to 0.4 V, the circuit uses the solar cell as the main source of power supply and charges the battery. At lower solar cell voltage value, the main power supply source is the battery, resulting in a significant increase in the efficiency of the power supply circuit. We also measured the change of current in the microcontroller according to its mode (sleep or measuring). As expected, the current was much smaller when the microcontroller was in sleep mode. For a better overview of the real functioning of the sensor node, we tested the prototype in a real environment. The results clearly show that the air humidity increases slightly at night, while air temperature decreases. The rapid transitions on soil moisture curve appeared we watered the plant. Since our sensor unit was on the west side, we can see that it was not illuminated during all day even though it was sunny. Short drops of voltage on the solar cell indicate short-term cloudiness. For a more detailed overview of voltage variations on the battery, we performed a three-day measurement, where we logged the voltage values on the battery and solar cell. The results show that the battery voltage does not decrease much (less than 5%). By creating a prototype for the sensor node, we met all the requirements of the project.|