An interesting experiment was conducted in a laboratory at the University of Washington not long ago. As part of this experiment, a voice message was transmitted to an ordinary Android smartphone. But the fact that someone managed to hear this message at all amidst the airwaves’ noise and interference is remarkable, because the prototype phone used to transmit the message has no battery or other external power source. This device gets all the energy it needs to operate from the environment, literally “out of thin air.
Scattered light indoors can be converted into electricity using photovoltaic cells. Electricity can also be obtained by harvesting the energy of radio waves from television broadcasts, cellular communications and Wi-Fi signals. A hybrid system using the aforementioned components can provide a few tens of microwatts of power, while a cell phone needs about 800 milliwatts during a call.
The prototype phone has been under development for several years by the Vamsi Talla group. The group had already developed a technology called “passive Wi-Fi”, which works by reflecting back the signal emitted by a Wi-Fi router or access point. But unfortunately, this technology also can’t provide the energy in the amount needed to run a mobile device.
“Converting audio to digital signals requires a lot of energy. That’s why we had to develop completely analog technology that uses the reflected signal back,” says Vamsy Tull, “Although to be fair, our system still uses digital signals to dial the number.”
The communication system created by the researchers uses a kind of base station, which emits a signal on a certain frequency in a range that does not require licensing. The prototype “battery-less” cell phone is configured to collect the energy of radio waves of this frequency, so while the maximum range of communication does not exceed 15 meters. The phone itself is a printed circuit board with touch buttons, and the only indicator element on this phone is a tiny red LED, which flashes when you touch one of the buttons.
The quality of communication provided by passive technology also leaves much to be desired. Even leaving aside the high level of interference and the low quality of the transmitted sound, the communication is done in half-duplex mode, by pressing the transmit button, just like in all simple miniature radios.
Nevertheless, Vamsy Tull promises that in the next version of the “battery-free” cell phone, the quality of communication will be improved several times over. In addition, it will already use an electronic ink-based display, which requires very little energy to operate.
“Something similar could be built into home Wi-Fi routers or cell tower equipment in the future. In the latter case, the power of the emitted signals is hundreds of times higher than our system and the passive communication range could be at least one kilometer,” says Vamsy Tull.