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Fri 2019.06.21
Hello Jenny,
These tutorials might provide one choice using ultrasonic:
http://www.espruino.com/HC-SR04
http://www.espruino.com/distance_sensing_robotOtherwise I.R. beam (optical) break, such as a TV remote signal, or the radar RCWL-0516 Microwave (radio) Radar Sensor come to mind.
Robin
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