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Battery-Free Cell Phone Demonstrated

Battery-free phone

Battery-free phone prototype (University of Washington)

6 July 2017. A computer science and engineering team demonstrated a prototype cell phone that sends and receives calls, but operates with power from ambient light and radio signals. The phone is described by its inventors from University of Washington in Seattle in the June issue of Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies (paid subscription required).

The researchers led by computer science and engineering professor Joshua Smith are seeking a solution to an issue affecting everyone with a cell phone: keeping the phone powered during the day. Many phone users carry chargers and power cords, and look for spare outlets or USB ports during the day to recharge their devices. Previous attempts at battery-free devices conserve power by intermittent periods of “sleeping” and harvesting power from ambient sources. The Washington device, on the other hand, is designed to operate continuously.

“The cell phone is the device we depend on most today,” says Smith in a university statement. “So if there were one device you’d want to be able to use without batteries, it is the cell phone.”

The prototype phone, designed by doctoral candidate Bryce Kellogg, is configured to operate with a bare minimum of power, about 3.5 microwatts. That energy is captured from ambient radio-frequency signals emitted by a base station, which provides power to the phone within 31 feet. The phone also has a tiny solar cell that offers enough power for the device to communicate with the base station within a 50-foot range.

The Washington team achieved its battery-free phone by devising an alternative method of converting sound waves in the human voice to digital signals, and back again to audible sound. Today’s phones convert the captured analog audio into digital signals, then back to analog sound, which requires a good deal of power. The prototype phone, on the other hand, senses minute vibrations in the microphone or speaker and encodes or decodes those vibration patterns into or from the radio signals emitted by the base station.

The device senses human speech and actuates attached earphones for listening. When talking, vibrations from the microphone are encoded into the radio signals sent to the base station, and when listening, signals from the base station are decoded into audible vibrations on the earphones. The prototype device requires the user to toggle between speaking and listening modes, much like a walkie-talkie. This process is called ambient backscatter, which Science & Enterprise described in an earlier version of the technology developed by Smith and co-author Shyam Gollakota in 2013.

The prototype phone is built on a single circuit board with off-the-shelf components, including a numeric keypad. In a demonstration, the researchers used Skype to transmit speech and data using basic phone functions, such as dialing a number, receiving incoming calls, and placing a caller on hold.

The team is working on extending the range of the device from a base station, as well as encrypting voice calls and adding video transmissions. Developing a commercial version of the device will likely require Wi-Fi routers and cell phone towers to add the base station technology to send and receive signals with battery-free phones.

Gollakota is co-founder of the company Jeeva Wireless, which in June of 2016 received a $225,000 Small Business Innovation Research grant from National Science Foundation to design a prototype version of its low-power technology to operate in an Internet of Things network. Kellogg is leading that project for the company.

The following brief video demonstrates the battery-free phone.

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