The home of the future, we are assured, will be swarming with tiny sensors: security cameras, carbon monoxide detectors, speakers and everything else.
That’s the idea being pursued by Angad Rekhi and Amin Arbabian at Stanford, anyway.
Their approach to the problem of devices that can’t stay on, yet can’t be all the way off, is to minimize the amount of energy necessary to send and receive a “wake” signal.
Radio, which of course all these tiny sensors use to transmit and receive information, is actually pretty expensive in terms of power and space.
Keeping the antenna and signal processor ready and listening uses more energy than these devices have to spare if they’re to last for years on a charge.
Ultrasound — soundwaves above the human range of hearing, 22KHz or so — is a much more physical phenomenon, and detecting it is easier in many ways than detecting radio frequency waves.