RF signals, such as TV broadcasts that used to be considered information-only, can now be treated as a power source, thanks to the continual improvements in the energy efficiency of microelectronics, a consequence of Moore's law. The image on the left shows a kitchen thermometer, with LCD display, being powered by RF signals from a TV tower 4km away. We have also developed a newer WARP sensor node that measures light level and received power and transmits the data on a short range (Zigbee-like) radio.