Matt Reynolds, an assistant professor in the electrical and computer engineering department at Duke University, wears other hats, too - including that of co-founder of two companies. These days, his interest is in a real hat now in prototype: a hard hat with a tiny microprocessor and beeper that sound 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, installed to keep track of their locations. The microprocessor monitors the strength and direction of the radio signal from the construction equipment to determine if the hat's wearer is too close. Dr. Reynolds designed this low-power hat, called the SmartHat, with Jochen Teizer, an assistant professor in the school of civil and environmental engineering at Georgia Tech. They are among several people devising devices and systems that consume so little power that it can be drawn from ambient radio waves, reducing or even eliminating the need for batteries. Their work has been funded in part by the National Science Foundation.