The light sensor is a fundamental tool in the cultivation of microalgae, since these microorganisms are photoautotrophic, and require an exact amount of light to carry out photosynthesis and maintain a high growth rate for the species. However most of the renowned sensors in the market are of great financial investment. Therefore, this work developed and built a light sensor using an electronic component, called LDR ( light depent resistor). It was based on Arduino, a platform accessible to all and allows the creation of much more economical sensors. An optical density ratio as a function of luminosity during an acidophilic Chlamydomonas culture was proposed and its data were adjusted linearly to fit the model. Such a tool simplifies the sampling procedure to follow the growth curve of this microorganism and ensures a better cost-benefit.