Precision farming reduces chemical use and improves profitability. The autosteering and automated section controls help the driver focus on the driving task, such as observing obstacles. The open-source system AgOpenGPS offers a cost-effective and customizable alternative to commercial section control solutions and autosteering. The aim of this study was to develop a control box for a Hardi sprayer, operated via the AgOpenGPS software. The goal was to automate section control, prevent overlapping spraying, and assist in route mapping. The first prototype was a controller box placed on top of Hardi’s display. The next prototype was based on relay boards and controlled with Arduino Nano microcontroller. The system was connected to AgOpenGPS through a USB interface. Testing was carried out both in simulation and in practical field trials, focusing on reliability. The first prototype of relay box proved unreliable due to wiring issues. The second version was simplified, resulting in improved wiring reliability. Field tests showed that the control box responded quickly to section boundaries and significantly reduced overlapping spraying. An open-source AgOpenGPS-based control box offers a flexible and affordable option for sprayer automation. However, it requires technical skills and personal time investment. Building such a system also benefits from having a network of people to consult for advice.