US researchers invent first battery-free cellphone

Researchers at the University of Washington (UW) have invented a phone that harvests the few microwatts of power it requires from either ambient radio signals or light and requires no batteries.

Detailed in a paper published in the Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies, the battery-free cellphone marks a major leap forward in moving beyond chargers, cords and dying phones, Xinhua news agency reported.

The team of computer scientists and electrical engineers eliminated a power-hungry step in cellular transmissions, namely converting analog signals that convey sound into digital data that a phone can understand, which consumes so much energy that it's been impossible to design a phone that can rely on ambient power sources. Instead, the new technology takes advantage of tiny vibrations in a phone's microphone or speaker that occur when a person is talking into a phone or listening to a call.

An antenna connected to those components converts that motion into changes in standard analog radio signal emitted by a cellular base station. The process essentially encodes speech patterns in reflected radio signals in a way that uses almost no power.

To transmit speech, the phone uses vibrations from the device's microphone to encode speech patterns in the reflected signals.

To receive speech, it converts encoded radio signals into sound vibrations that that are picked up by the phone's speaker.

In the prototype device, the user presses a button to switch between the "transmitting" and "listening" modes.

Using off-the-shelf components on a printed circuit board, the UW team demonstrated that the prototype can perform basic phone functions, transmitting speech and data and receiving user input via buttons.