This work describes a two-year project carried out in an Italian technical high school. Through integrated work, the activity involved the efforts of different teachers and students, particularly those in the Chemical, Electronic, and IT departments. They merged their complementary skills to create an inexpensive device that quickly measures water hardness. The pedagogical aspect of this work aimed to support and reinforce development of the key competences for lifelong learning recommended by the European Commission, especially by strengthening the logical-mathematical, digital, and social skills of the students. The device made is based on an Arduino board connected to a red LED and an optical sensor. The device is able to read the intensity of light transmitted through the sample and to independently convert this output into the water hardness expressed in French degrees. The color change used as information transducer is due to the aggregation of negative gold nanoparticles induced by the bivalent cations in solution. Working in school laboratories under teacher supervision, students connected and troubleshot the hardware system, synthesized and characterized the gold nanoparticles, and developed the software. Finally, the system was assembled inside a box composed of a rigid container, a sample holder, and a touchscreen display. Instructions for replicating this instrument are reported in the paper, while further details and explanations for instructors and students are provided in the Supporting Information.