We propose a method to measure the speed of sound in air using a motion detector. The experiment is based on the principle of a motion detector used to measure distances. This device measures the time of flight of alternating ultrasonic waves between the observer and the target. By assuming a fixed speed of sound, the time of flight can be converted into a distance. The medium, including its temperature through which the sound travels, influences the speed of sound. A motion detector has been used to show the effect of CO[subscript 2] concentration on the resulting distance measurement, and to estimate the linear change in speed of sound per degree Celsius. The distance measurement using a motion detector has to be calibrated according to the medium's condition. The Logger Pro 3 features allow adjustment of the temperature setting and addition of an offset value to correct the motion detector reading.