Vamsi Talla and colleagues, University of Washington, Seattle, USA, have developed a mobile phone without a battery. The phone requires only a few micro-watts of power and operates by harvesting energy from surrounding radio frequency (RF) signals or surrounding light by using tiny photodiodes.

The most power-consuming step in telephoning is usually the conversion of analog sound signals into digital data. The new cell phone uses a technology called backscatter to transport digital packages to a base station that is approx. 9 m away. This means, the mobile phone captures the radio signals of a nearby base station or a configured Wi-Fi router, converts them into electricity with which it modifies and reflects the signal with encoded data. From the base station, a custom base station receives the digital packages and connects the digital number using in this case Skype. The battery free phone receives the signal from the other phone using zero power amplitude modulation. The audio is transmitted using zero power analog backscatter, meaning the voice signals from the microphone are sent on existing radio waves to a receiver.

Operating the phone requires only a current consumption of 3.5 micro-watts. Using power harvested from surrounding light with tiny photodiodes, the device can communicate with a base-station that is 15 m away.

First tests with a prototype show how well this works. The prototype consists of simple electronic components, available everywhere, mounted on a printed circuit board. The researchers are working to increase the range of their cell phone and to encrypt the signals. Optical displays based on e-ink technology and video streams will also be possible in the future, they say.

• Battery-Free Cellphone,
  Vamsi Talla, Bryce Kellogg, Shyamnath Gollakota, Joshua R. Smith,
  Proc. ACM Interactive, Mobile, Wearable and Ubiquitous Technologies 2017, 1(2).
  https://doi.org/10.1145/3090090