Design and construction of an electrically automated chemical sprayer for a vegetable garden.

Abstract

Agriculture in developing countries, such as Uganda, relies heavily on crop production by smallholder farmers. However, conventional spraying methods—like manually operated knapsack sprayers—are widely used and present significant challenges. These traditional methods are labor-intensive, cause muscle strain due to tiresome repeated rhythms, lead to inconsistent spray patterns and chemical wastage due to a lack of pressure control, and continually expose the applicator to harmful chemicals. Alternatively, gasoline-powered sprayers are expensive, heavy, noisy, and require continuous expenditure on fuel. This project, submitted in partial fulfillment of the requirements for the Degree of Bachelor of Science in Mechanical Engineering at Makerere University, aimed to design and develop a functional prototype of a solar-powered automated chemical sprayer for vegetable gardens. The primary goal was to efficiently automate the pressure-building process to enhance the ease and effectiveness of spraying operations. The design process involved determining specific user requirements, such as reduced labor, consistent spray rates (targeting low, medium, and high flow rates up to 2 L/min), affordability, and solar power viability. The final design, Concept A, adopted a knapsack-mounted system with a detached solar photovoltaic (PV) module, optimizing for lower weight and greater mobility. The detailed design integrated a 16-liter chemical storage tank and a 12V DC diaphragm pump (rated at 72W and 6 L/min maximum flow rate). Automation was achieved using an Arduino NANO microcontroller and a BTS7960 motor driver, implementing Pulse Width Modulation (PWM) to systematically vary the pump speed and control the flow rate at three predefined levels. The system was powered by a 5.2 Ah lithium-ion battery pack. A functional prototype was successfully constructed and evaluated. Performance evaluation demonstrated the prototype's capability to deliver variable flow rates. Field trials showed an Effective Field Capacity (EFC) of approximately 200 m²/h and a Field Efficiency of 83.33%. Furthermore, the automated system significantly improved user comfort compared to manual sprayers by eliminating manual pumping strain, achieving an average discomfort score of 4.89 (on a scale where 7 is extreme discomfort). The solar-powered automated chemical sprayer offers smallholder vegetable farmers a sustainable, cost-effective solution that is less labor intensive, provides consistent chemical application, and minimizes chemical wastage compared to conventional methods.