A description of the thought process and decision-making of why certain electronic components were selected for use to build the prototype sensors and central hub.

Microcontroller:

The brain of the central hub and sensors must be able to control all the electronics and implement machine learning models while being small and low-powered and cheap, microcontrollers are the most suitable option to fit this category. The microcontroller needs to have enough pins to support all the electronics and enough memory to be able to hold a sufficiently large program but not too large that it is underutilised.

There are many options for microcontroller boards, the most popular ones being Arduinos. Due to their popularity, these are more expensive and generally have very small memories. The ESP32 line of microcontrollers are fast, powerful, and have WiFi capabilities. ESP32 chips can be bought in bulk for less than £5 each, and have an excellent amount of memory, easily having enough capacity for this project. This microcontroller is an excellent first microcontroller to build the prototype around and it has plenty of information and examples available which will speed up the design process.

Touchscreen Display:

The user needs to be able to interface with the electronics without any prior knowledge. The most simple way to do this is to use a display, which would enable the central hub to show sensor readings to the user but also take in touchscreen inputs. As this project is largely focused on cost, the display has to be very small. It should only be as big as it needs. And also because the system will be powered by a microcontroller, normal display connections like HDMI are not available, it will have to be controlled by SPI. Now it has been defined that it must have a touchscreen, SPI, be small and low-powered there are only a few options available.

The 2.8 Inch ILI9341 240x320 TFT LCD fits all the criteria, and for only £20 it will be satisfactory to enable a GUI-controlled system for the central hub. The libraries available for this device are excellent, giving plenty of examples for implementations of GUI design elements like buttons, fonts, styling, etc. which should enable the GUI to easily have a professional look.

Sensors:

Two of the five aspects farmers should pay attention to (according to the source below) are phosphorous levels and potassium levels.

Misuse of fertilisers are a big contributor to losses in crop yield. The majority of fertilisers contain nitrogen, phosphorus, and potassium. By using an NPK sensor, the soil health can be measured and the farmer can take appropriate action.

In order to use an NPK soil sensor a converter module (MAX485 Modbus Module) will be required.

DHT11 is a low cost temperature and humidity sensor which allows the cost of the overall product to be low. It is simple to use and provides accurate readings so it can be easier integrated in the circuit.

RF: