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A basic cell phone communicates with a cell tower, senses, transmits, and receives speech, and works the speakers. University of Washington scientists were able to create a phone, as seen in [Fig. 1], which performs those functions while only consuming a few micro-watts of power (3.5 microwatts of power). As mentioned previously,, scientists created a battery-free cell phone using power harvesting.
The battery-free cell phone functions by using their dedicated ‘base station’ or what we call a cellular tower.
Using radio frequency signals (RF), the battery-free phone uses a rectifier that converts the radio frequency signal into low voltage DC power . A rectifier is an electrical device composed of diodes that converts alternating current (AC) to direct current (DC) . The converting analog signals carry the sound into digital data. Analog signals increase and decrease in waves based on what is being transmitted. A call can be received and transmitted from 31 feet away using RF.
To harvest from ambient light, they used a tiny solar cell about the size of a rice grain. A photodiode harvesting phone can function at very high duty cycles (expanded upon further below) and continues as long as there is plenty of ambient light. A call was received and transmitted from 50 feet away using ambient light.
Since the harvesting system of the battery-free phone has lower power available using the RF or ambient light, it utilizes duty cycling. Duty cycling is the switch between sleep mode and the active mode of the phone. In order to conserve power, the phone enters lower power in sleep mode and efficiently and exclusively harvests energy .
In the end, the battery-free phone uses analog backscatter, which transmits the signal to a nearby cell tower, to directly send the speech from the microphone and also receives speech back from the cellular base station. The scientists put together an analog backscatter microphone and an AM receiver to receive and transmit speech on the battery-free system . Instead of transmitting its own radio signal, it is piggybacking on the back of a radio signal emitted from the cellular base station. The phone encodes the user’s speech in a radio signal using sound vibration.
The battery-free cell phone is able to complete the basic functions of a cell phone using little power. This can save the need for batteries and dead phones. Others might think to solely rely on solar power to recharge phone batteries, but many do not realize that solar-based systems are often expensive and heavy. In order for this phone to be implemented into society, we would need to use the cellular base station created by the UW scientists. The technology is able to be implemented into our cellular towers and even wifi routers today.
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