Using radio wavesas a power source

Matt Reynolds, an assistant professor in the electrical and computer engineering department at Duke University in North Carolina, is already the co-founder of two companies. His current interest is in a hard hat with a tiny microprocessor and a beeper that sounds a warning when dangerous equipment is nearby on a construction site.

What's unusual, however, is that the hat's beeper and microprocessor work without batteries. They use so little power that they can harvest all they need from radio waves in the air.

The waves come from wireless network transmitters on backhoes and bulldozers, which had been installed to keep track of their locations. The microprocessor monitors the strength and direction of the radio signal from the equipment to determine if the hat's wearer is too close.