Researchers at the Georgia Institute of Technology have created a passive motion/contact sensor that detects motion using only sound. Without electricity, electronics, or batteries. The sensors are small, uniquely shaped metal tags that each emit very specific ultrasonic sounds upon contact. They are smaller than a penny and cost only a few cents each.
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So, it’s a wireless microswitch, but probably not as useful. They’ll no doubt license it to the government for something a wired microswitch would work just as well.
