

Wi-Fi Devices That Use Stray Radio Waves as Their Power Source - finisterre
http://www.technologyreview.com/news/529576/mobile-gadgets-that-connect-to-wi-fi-without-a-battery/

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Johnythree
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.

~~~
FatalLogic
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.

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sxp
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](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](https://developers.google.com/gdata/articles/radish?csw=1)

~~~
dctoedt
[http://en.wikipedia.org/wiki/Crystal_radio](http://en.wikipedia.org/wiki/Crystal_radio)

~~~
marcosscriven
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.

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dctoedt
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](http://en.wikipedia.org/wiki/Exoplanet#Indirect_methods)

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gliese1337
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?

~~~
diziet
I wonder if we shall sooner or later have locators~

~~~
nitrogen
We do, kind of: [https://en.wikipedia.org/wiki/Radio-
frequency_identification](https://en.wikipedia.org/wiki/Radio-
frequency_identification)

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lnanek2
Kind of sad the kick starter for some similar technology was closed recently.
Would have been exciting to see if it worked out.

~~~
cjbprime
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 [..]"

~~~
ctdonath
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.

~~~
IgorPartola
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.

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

