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World's Smallest Solar Powered Sensor Invented by University of Michigan Researchers

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University of Michigan engineers developed the smallest solar powered sensor system, having only 9 cubic millimeters, that can harvest energy from ambient light to operate.

The sensor system has a microprocessor powered by solar cells, and it also has batteries, all contained in the tiny frame measuring 2.5 x 3.5 x 1 mm. Its uses range from biomedical implants to home, building and bridge monitoring devices, improving the efficiency and cost of current environmental sensor networks that detect movement or track air or water quality.

An ARM Cortex-M3 processor stands behind the calculations, but the processor isn’t always on, but rather standing by most of the time. Only during a few moments it wakes up every few minutes to take measurements, having an average power consumption of less than 1 nW (one-billionth of a watt).

“Our system can run nearly perpetually if periodically exposed to reasonable lighting conditions, even indoors,” said David Blaauw, an electrical and computer engineering professor. “Its only limiting factor is battery wear-out, but the battery would last many years.”

“The ARM Cortex-M3 processor has been widely adopted throughout the microcontroller industry for its low-power, energy efficient features such as deep sleep mode and Wake-Up Interrupt Controller, which enables the core to be placed in ultra-low leakage mode, returning to fully active mode almost instantaneously,” said Eric Schorn, vice president, marketing, processor division, ARM. “This implementation of the processor exploits all of those features to the maximum to achieve an ultra-low-power operation.”

Having ultra-low power devices running on solar power is not so important regarding the energetic part, but it is rather important due to the material economy made – wires, chemical batteries that would have to be recharged or replaced all the time. That’s why even they are small, they are still green gadgets.

The developers say the key innovation is their method for managing power. The processor only needs about half of a volt to operate, but its low-voltage, thin-film Cymbet battery puts out close to 4 volts. The voltage, which is essentially the pressure of the electric current, must be reduced for the system to function most efficiently.

“If we used traditional methods, the voltage conversion process would have consumed many times more power than the processor itself uses,” said Dennis Sylvester, an associate professor in electrical and computer engineering.

Inside the human body, the sensor could find its application in harvesting energy from movement or heat, rather than light, the engineers say.

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