Adequate power quality monitoring in all voltage levels of the power system has important advantages as the reduction of the losses caused by the different electromagnetic disturbances. However, the high costs of the power quality monitors restrict their use. This paper presents the methodology developed for the design of a module for the recording of voltage sags in low voltage systems as an alternative to expensive equipment. The module includes a signal conditioning, acquisition and data processing, storage, real time clock and communications, it is also able to register voltage sags in single-phase and three-phase systems.