To identify the impeller types automatically for an overhead stirrer operation, a digitalization of the analog magnetic signal of the permanent magnet arrays was performed. The permanent magnets were installed into the impeller shaft in NS-SN array, and their magnetic fields were measured by a hall sensor while the impeller was mounted on the stirrer. Results of the experiments and finite element analysis showed that the number of peaks of magnetic field signal was observed corresponding to the number of magnets. Considering the consistency of the number and the magnitude of magnetic field peaks, it was found to be more advantageous to measure the magnetic field in the axial than in the radial direction of the impeller. For eliminating the influence of the noise included in the analog signal and connecting it with the micro-controller of the stirrer, the analog magnetic field signal was transformed to the digital-pulse-patterned signal through DC offset, amplification, absolute value, and the comparator circuit. Through these processes, the error in identifying the impeller type could be significantly reduced compared with directly using the magnitude of the analog magnetic signal.