Just like RFID tags, the battery-free sensors turn on only when they encounter a reader. As long as the RFID reader is within range of the device, Smith says, it can collect data and send it to the reader.

Battery-free sensors could be useful in many areas, including medicine, says Zeke Mejia, chief technology officer of St. Paul-based Digital Angel, an RFID tag maker. They could "check the status and certain conditions in the body" at any moment, Mejia says, from glucose levels in people with diabetes to the pH of blood and other body fluids.

In their current form, Intel's sensors need to be within about a meter of a reader to be activated. That's closer than would be ideal for some applications, such as measuring the temperature of foods packed in large crates or vibrations in thick walls. The problem is that while the microcontroller needs only a milliwatt of power to run, it needs three volts of electricity to turn on, and the sensor has to be within a meter of an industry-standard RFID reader to generate that much energy. But with minor changes to the way the microcontroller processes data, Smith says, the group could reduce the voltage requirement to 1.8 volts, thus extending the range to about five meters.

The team's latest prototype incorporates a light sensor, temperature sensor, and even a tilt sensor into one battery-free device. The researchers are working on ways to integrate the microcontroller and antenna into a single chip that would be easier to install in the field. In the meantime, they have developed a visual demonstration of just how much energy an RFID antenna can garner from a reader: they've used it to power the second hand on a wristwatch.

"It's surprising to people that this invisible form of energy –- radio waves -– can actually make a watch hand move," Smith says. And a single tick of a second hand, Smith says, takes about as much energy as sending one bit of data from his sensor.