Ultrasound could waken a sleeping smart home

The home of the future, we are assured, will be swarming with tiny sensors: security cameras, carbon monoxide detectors, speakers and everything else. Few need to be running all the time — but how do you wake them up when they’re needed if they’re off in the first place? Ultrasound.

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. That way the Internet of Things really only consumes power when they’re actively in use.

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.

Ultrasonic sensors, on the other hand, are incredibly power-efficient and require very little space. 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. It’s a bit like the difference between a sensor that’s sensitive to nearly intangible x-rays versus one that detects ordinary visible light.

Rekhi, a grad student in electrical engineering working under Arbabian, describes their approach in a paper just presented at the International Solid-State Circuits Conference in San Francisco. It’s a simple idea in a way — a small switch that hits a bigger switch — but the results are impressive.

The system’s ultrasound receiver is efficient even for an efficient class of sensors; the tiny, super-sensitive microphone was developed at Stanford, as well, by the Khuri-Yakub Group. The receiver is always on, but draws an amazingly small 4 nanowatts of power, and is sensitive enough to detect a signal with a single nanowatt’s strength. That puts it well ahead of most radio receivers in terms of power consumption and sensitivity.

There’s one from a study last year that has it beat on both… but it’s also more than 50 times bigger. The ultrasonic sensor only takes up 14.5 square millimeters to the radio chip’s 900. That’s valuable real estate on an embedded device.

You wouldn’t be able to activate it from across town, of course — ultrasonic signals don’t travel through walls. But they do bounce around them, and the wake-up system’s sensitivity means even the smallest fragment of an ultrasonic signal will be sufficient to activate it.

It’s just a prototype right now, but don’t be surprised if this sort of mega-efficient tech gets snatched up or duplicated by companies trying to squeeze every ounce of life out of a watt-hour.

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