Ultrasonic motion detectors all rely on the speed of sound in air for the measurement of distance. The first step is to activate the ultrasonic transducer, creating a short burst of ultrasound. We also start a timer at this moment. We then wait for a “blanking period” of about 0.9 ms, the time it takes sound to go 30 cm. During this time, the transducer is still vibrating from being driven. After the blanking period, we monitor the transducer for an incoming signal. It is normally in a low state and goes high when an ultrasound echo is detected. We simply time from the moment of initiating a measurement to the time the echo is received and use the speed of sound to determine the distance sound traveled in that time. This is twice the distance to the object. Additional information, such as velocity and acceleration data, are generated by software based on the raw position data.