The University of Washington researchers have developed a new approach called “power over Wi-Fi or PoWi-Fi” for drawing enough power from standard Wi-Fi signals to power a security camera and recharge a fitness tracker. The team behind the project believes its techniques will be useful for powering the many devices expected to form the “internet of things”.
The team’s approach simply connecting an antenna to a temperature sensor, place it close to a Wi-Fi router and measure the resulting voltages in the device. The voltage across the sensor is never high enough to cross the operating threshold of around 300 millivolts.
The problem is that Wi-Fi broadcasts are not continuous. Routers tend to broadcast on a single channel in bursts. This provides enough power for the sensor but as soon as the broadcast stops, the voltages drop. The result is that, on average, the sensor does not have enough juice to work.
To fix this, the research team modified standard wi-fi hotspots and routers to broadcast noise when a channel was not being used to send data. This meant the power of the wi-fi signals stayed constant and, though low, was high enough to power some components.
The security camera connected to the antenna was capable of producing 174 x 144 pixel black and white images at an energy cost of 10.4 milliJoules per picture. An attached low-leakage capacitor activates when charged to 3.1V, cutting out again on a drop to 2.4V, with images stored in 64kb of non-volatile ferroelectric RAM.
The camera succeeded in taking an image every 35 minutes initially, and the addition of a wirelessly-rechargeable battery increased the router distance to seven meters. The router could even power the camera through a brick wall, demonstrating that it would be possible to attach the device outside while keeping the power supply inside.
To demonstrate the potential of PoWiFi as a low-drain charging unit, Valla and his team created a 2dBi Wi-Fi antenna and used it to charge a Jawbone UP24 fitness band, increasing its charge status from zero to 41% in 2.5 hours, from Wi-Fi signals alone.
Later the researchers fitted six homes in a metropolitan area with these devices and then monitored the impact on the users. Four of the users did not perceive any difference in the user experience and one user said their online experience actually improved.
According to MIT Technology Review, “Talla and co say this was because their modified router replaced one of particularly low quality. The final user reported a slight deterioration in her YouTube viewing experience, which Talla and co say was probably due to interference affects with other devices.”
The ability to deliver power wirelessly to a wide range of autonomous devices and sensors is hugely significant. But the real icing on the cake here is the ability to do this with ordinary technology that is commonly available all over the developed world and beyond. As such, PoWi-Fi could be the enabling technology that finally brings the Internet of things to life.
