We revisit the known experiment of using the solar projection along a tube in order to calculate the Sun’s radius. However, we propose several features that greatly improve the precision in the measurements. We discuss how the shape of the performation hole affect the results, and what is the best way to construct it. We also discuss how the examination of the measurements can be performed through photographs taken with smartphones, and their analysis using different free softwares. We also propose how a general theory can be formulated to determine the radius of the Sun with only measurements made with this kind of experiment, without any external parameter to the apparatus. Its usefulness and applicability is also considered. With all these additions to this known experience, this work is focused in taking advantage of the strength of smartphones and software technologies to get powerful and precise astronomical results as an educational tool.