Tiny battery-free ultrasonic tags for smart homes ^{CEFR B2}

Engineers at Georgia Tech have created tiny, battery-free metal tags that produce a short ultrasonic pulse when struck. The tags are metal disks with a central hole and cutouts around the edge; mounted on a small 3D-printed base, they attach to cabinet doors, doorframes, faucets or other surfaces. When a tab on a moving part hits a disk, the impact generates an ultrasonic pulse that humans cannot hear but that a nearby wearable microphone can detect and log.

The exact shape of each disk determines its resonant frequency, so each tag produces a distinct acoustic "fingerprint" that can be used for activity recognition, as Yibo Fu explains. Possible applications include counting repetitions on gym equipment, monitoring faucets or toilet lids to track water use or signal that an elderly person may need help, and button-style tags to start timers or log actions. A video Fu posted on Instagram went viral, drawing about 1.6 million views, 150,000 likes and hundreds of comments, with viewers suggesting other uses.

Bolei Deng led the vibration modeling and simulation work. The team’s simulations produced nearly 1,300 initial designs that each generate a unique ultrasonic frequency above 20 kilohertz, and the researchers tested 15 designs. With careful design the set could expand to dozens, hundreds or even thousands of distinct tags. Because ultrasound is easy to detect in noisy environments and does not travel far, only nearby microphones hear a tag. The team avoided complex machine learning and instead used a simple hardcoded algorithm, which reduces computational and electrical demands. The project was a collaboration among engineering and computing researchers and appeared in the Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies.