Wi-Fi Devices That Use Stray Radio Waves as Their Power Source | Hacker News Reader

Wi-Fi Devices That Use Stray Radio Waves as Their Power Source

technologyreview.com

48 pointsfinisterre18 comments

Any device which modulates the surrounding radio waves has the possibility of causing radio interference, so it will need licensing and type approval. And something which generates wideband crud will never be approved.

This thing is not passive. It is receiving, modulating, and re-transmitting broadband signals.

The "rusty bolt" form of radio interference is one of the most common that interference investigators have to deal with.

No doubt true. Doesn't make it any less interesting as a cool hack though. If it's specifically trying to pick up "wifi" power in the 2.4/5GHz spectra that's obviously unregulated and approval is mostly a rubber stamp AIUI.

Even a device which merely absorbs radio waves passively could be seen as causing a kind of radio interference, because wifi (and similar short range systems) make use of reflected waves to go around obstructions.

This is an irrelevant loophole in the current regulations, but if you imagine a scenario where everyone is trying to power their smart wallpaper, smart windows and smart clothes by absorbing radio waves, perhaps it would be enough to reduce the reflected signal strength significantly.

A surprising amount of power can be collected from stray radio waves. You wouldn't be able to power a full computer or even a phone, but you can do cool things like power a radio: http://en.wikipedia.org/wiki/Foxhole_radio

However, indoor "solar" power is a more practical idea: https://developers.google.com/gdata/articles/radish?csw=1

http://en.wikipedia.org/wiki/Crystal_radio

I recall as a kid my mind being blown when I realised you could get enough energy out of "thin air" to drive a little earphone, with nothing more than a length of wire, a finished toilet paper tube, and a crystal/diode.

FTA: "To send data to a smartphone, for example, one of the new prototypes switches its antenna back and forth between modes that absorb and reflect the signal from a nearby Wi-Fi router. Software installed on the phone allows it to read that signal by observing the changing strength of the signal it detects from that same router as the battery-free device soaks some of it up."

Sounds just a bit like one indirect way astronomers search for exoplanets: By looking for fluctuations in a star's light caused by a planet's transit 'across' the star. [1]

[1] http://en.wikipedia.org/wiki/Exoplanet#Indirect_methods

Very _Deepness In The Sky_. Even if the battery-less devices don't have that much computing power yet, this still feels like sci-fi technology- the kind of stuff you always thought ought to possible, but just wasn't yet. Pondering: could they come up with less power-hungry communication protocols that could let battery-less devices communicate with each other?

I wonder if we shall sooner or later have locators~

We do, kind of: https://en.wikipedia.org/wiki/Radio-frequency_identification

Kind of sad the kick starter for some similar technology was closed recently. Would have been exciting to see if it worked out.

you remember the name of that Project in kickstarter?

Thanks.

I think the project lnanek2 is referring to is iFind:

https://www.kickstarter.com/projects/yuansong84/ifind-the-wo...

And the takedown: http://drop-kicker.com/2014/06/ifind-rf-energy-harvesting-bl...

It was clearly fradulent. From the TR article:

"Harvesting ambient radio waves can collect on the order of tens of microwatts of power. But sending data over Wi-Fi requires at least tens of thousands of times more power [..]"

So store it. Charge a capacitor for an hour, transmit for 0.1 second (adjust scales accordingly). If you're talking things like thermostats that are monitoring data that really doesn't change much over an hour or so, not a problem.

I've been waiting to see ambient-power products like this since I built a crystal radio when I was 8 or so. Powered entirely by AM radio waves, no problem with listening volume.

So if for every second you transmit you need 10^4 seconds of gathering energy, that means you can transmit just over 3 times an hour for 0.1 seconds. That is assuming perfect storage efficiency. But remember that to transmit over Wi-Fi you need to associate with the AP, obtain an IP address, discover where you are sending stuff (DNS resolution, routing, etc.), receive an ACK, close a connection, disassociate from the AP. Most of the timing of this is going to be dependent not on you but on the other end of the transmission. And if you are transmitting this infrequently, what percentage of time can you afford to have your data lost? Sure, you could optimize the heck out of this by using pre-defined routes, fixing the IP addresses of everything, using UDP to not have to wait for an ACK. I have a feeling you'd still need to actively run the radio for at least a second or two, which now brings you into the territory of transmitting only about 5 times a day.

But all that aside, the biggest reason I think running Wi-Fi off harvested energy is BS is because we are skipping an important step here: nobody is running Wi-Fi off watch batteries yet, and those are much more energy-dense than the typical radio spectrum. That's like seeing a Kickstarter campaign that claims to be able to run a mid-sized sedan off a rooftop solar panel: it is impractical bordering on impossible.

As an alternative, there are lots of protocols other than 802.11 that can transmit at lower power and with lower time frames. With these you can gather all the data onto a bridge which will then talk 802.11 on the other side. Or, you know, just put in a AA battery.

On the other hand, Bluetooth Low Energy is running off small batteries (for a long time), I wonder what the numbers look like there.

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