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This article presents a microcomputer-based ultrasonic temperature sensor system to measure the temperature of air conditioner (AC) in an automobile. It uses the ultrasonic measurement of the changes in speed of sound in the air to determine the temperature of bulk air. The changes in speed of sound are calculated by combining time-of-flight (TOF) and phase shift(PS) techniques. This method can work in a wider range than using phase shift alone and is more accurate than TOF scheme. In the proposed system, we use 40 2 ± kHz ultrasonic transducers and adopt the single-pass operation. A 89c51 single-chip microcomputer-based binary frequency shift-keyed (BFSK) signal generator and phase detector are designed to record and calculate TOF, phase shift of the two frequencies and temperature. These data are then sent to either an LCD display or to a PC for calibration and examination. Experimental results show that the proposed measurement system has