This project describes the design and implementation of an electronic system for measuring rotational speed using a magnetoresistive sensor. Initially, different types of rotational speed sensors are studied. This study describes the technologies and the physical operating principles of the sensors and their main advantages and disadvantages. After that, a specific type of rotational speed sensor (a differential magnetoresistive sensor) is experimentally characterized. This characterization provides information to do the design of the ensuing analog conditioning circuit, which blocks the DC component, amplifies, filters and converts the signal to square one. This time-modulated signal is then measured by a microcontroller using the constant elapsed time method. Finally the microcontroller sends the information about the speed to a personal computer via RS232. And LabView program process and display the data.