The house of the future, we are assured, will be teeming with small sensors: security cameras, carbon monoxide detectors, speakers and all the rest. Few people need to run all the time – but how do you wake them when they are needed if they are off in the first place? Ultrasound.
It is the idea that Angad Rekhi and Amin Arbabian are pursuing at Stanford, anyway. Their approach to the problem of devices that can not stay, but can not be all the way, is to minimize the amount of energy needed to send and receive a "wake up" signal. In this way, the Internet of Things only consumes energy when it is actively used.
The radio, which of course all these small sensors use to transmit and receive information, is actually quite expensive in terms of power and space. Keeping the antenna and signal processor ready and listening uses more energy than these devices need to save if they have to last for years on a charge.
Ultrasonic sensors, on the other hand, are incredibly energy efficient and require very little space. Ultrasound – sound waves over the human range of hearing, 22KHz or more – is a much more physical phenomenon and detection is easier in many ways than the detection of radiofrequency waves. It's a bit like the difference between an almost intangible X-ray sensitive sensor and a sensor that detects ordinary visible light.
Rekhi, a graduate student in Electrical Engineering working under Arbabian, describes their approach in a paper that has just been presented at the International Conference of Solid Circuits in San Francisco. It's a simple idea in a way – a small switch that touches a bigger switch – but the results are impressive.
The ultrasonic receiver of the system is effective even for an efficient class of sensors; the tiny, hyper-sensitive microphone was developed at Stanford by the Khuri-Yakub group. The receiver is still on, but draws an astonishing little 4 nanowatts of power, and is sensitive enough to detect a signal with a single nanowatt force. This puts it well ahead of most radio receivers in terms of power consumption and sensitivity.
There is one from a study last year that beat both … but it is also over 50 times bigger. The ultrasonic sensor takes only 14.5 square millimeters to the 900 of the radio chip. This is a valuable real estate on an embedded device.
Of course, you could not activate it on the other side of the city – the ultrasonic signals do not cross the walls. But they bounce around them, and the sensitivity of the waking system means that even the smallest fragment of an ultrasonic signal will be enough to activate it.
It's just a prototype right now, but do not be surprised if this kind of mega-efficient technology is ripped off or duplicated by companies trying to squeeze every ounce of life on an hour of watt.
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