You’d Never Have to Plug in This Battery-Free Cell Phone

The no-frills battery-free phone prototype is powered by ambient radio signals or light. (Mark Stone/University of Washington)

The no-frills battery-free phone prototype is powered by ambient radio signals or light. (Mark Stone/University of Washington)

In 1945, a hand-carved wooden copy of the Great Seal of the United States was hung in the residential study of the U.S. ambassador to the Soviet Union. The detailing was remarkable—each feather on the eagle’s wings delicately defined, its beak curved just so. But this wasn’t what made the piece astounding.

It was also a bug.

The seal was presented to the ambassador as a peaceful gesture by a delegation from the Young Pioneer Organization of the Soviet Union. But tucked within the artful work, just under the bird’s beak, was a listening device. It had no battery or active electronics, making it virtually undetectable. Indeed, it wasn’t found for seven years.

Now, a team of researchers at the University of Washington in Seattle are once again harnessing similar technology for an equally astounding, but less covert, operation: battery-free cell phones.

Though it’s unlikely the design will ever replace our power-hungry smart phones, the technology has the promise to help in a pinch. “Let's say you're stranded somewhere and your phone runs out of battery,” says Vamsi Talla, who worked on the project while he was a research associate at the University of Washington. “You still could make a 911 call, which could be a lifesaver.”

With a regular cell phone, there are two things that consume a lot of power, says Talla. First, your phone has to convert your voice (an analog signal) into a digital signal. Second, it transmits that signal in the form of radio frequencies to a tower.

So the team stripped these components from their redesigned phone. Instead, they used the basic principals of the Soviet-era technology to rework how the device functions.

In the case of the Great Seal Bug, voices within the room caused a small diaphragm hidden in the structure to vibrate and change the resonance of the internal cavity. To listen in, eager Soviet agents on the street would just need to focus the right frequency of radio waves in the direction of the seal, which activated an antenna inside the bug. Then they could collect the reflections bounced back—also known as backscatter.

In the case of the battery-free phone, a radio signal is constantly emitted from a remote base station. When a person speaks into the device, these frequencies are reflected back to the base station. It is then the base station’s task to connect to the cellular network and transmit the call.

“You can't break the laws of physics,” says Talla.

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