This little quirk of physics means that anything with an antenna can convert radio signals into small, but not insignificant, currents. That way, they can activate and run without the use of any batteries or electronics to speak of.
The UW team of engineers employed this in their endeavor, and 3D printed objects that mixed plastic with copper. So that’s the conductive antenna sorted, but how does the device know when something’s changed within it, or in the surrounding environment if it’s not always “on”?
Remarkably, much like those aforementioned watches, the objects included small gears that moved about whenever provoked by a physical motion – say, the laundry soap exiting the container – which causes mechanical teeth to click into place at various rates, and register how much or how little is left.
“As you pour detergent out of a Tide bottle, for instance, the speed at which the gears are turning tells you how much soap is flowing out,” senior author Shyam Gollakota, an associate professor in the Paul G. Allen School of Computer Science & Engineering at UW, said in a statement.
At a certain point, an electrically conductive switch will poke the antenna, forming a larger circuit. This will change how the antenna reflects radio frequency signals, allowing the WiFi router to continuously track the state of whatever the device is monitoring.
The laundry soap device is the team's piece de resistance right now, but we’d expect them to manufacture a few more technological wonders sooner rather than later, given their obvious zeal.