The use of radioactive isotopes in radioactive decay studies can be a problem due to the manipulation of radioisotopes, expensive equipment, and the difficulty of finding samples with a reasonable half-life (between 1 and 10 min). In this paper, we present a mock-up of a Geiger-Mu¨ller counter plus three fictitious radioisotopes. They form an experimental kit that teachers and professors can use in laboratories of schools and universities. Inside the Geiger electronic box, an Arduino board can recognize the isotope specimen sample the student inserts in the Geiger-Mu¨ller tube support. Once the measuring process starts, the Arduino simulates the particle counting as a function of time for that chosen isotope. The complete kit presented here costs approximately US$ 20.00, and it can be an alternative to other simulations for radioactive decay lessons.