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Battery-Free Mobile Devices Communicate Pulling Signals From Air

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William K.
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Platinum
How much energy is in the air?
William K.   9/4/2013 2:29:51 PM
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As an experiment I extended the leads of my Fluke meter in opposuite directions with it set to AC volts. I see abot 5 volts developed across about 20Megohms. That is plenty enough field to make a bug hum in a microphone circuit, but there is not a lot of current available. The same experiment with my 50,000 ohms per volt multimeter shows no meter deflection. So while the potential is there the source impedance is in the tens of megohms. Since the avarage battery has much less than an ohm for internal resistance, it becomes clear that harvesting electrostatic fields will provide micro watts of power, not watts. 

78RPM
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Gold
Re: Self Powered Device
78RPM   9/4/2013 3:30:29 PM
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Elizabeth, I recall reading the article you wrote. I say we are not stealing if we just take a small amount of radio signal to power a small device. It would probably have gotten reflected and wasted anyway.  Now if we were to put up a big antenna array or web to get power, that might rob users in line of sight from getting the signal from the tower.

Regarding electric power lines; if someone just charges a cell phone by capturing the magnetic field from a utility line, I doubt that anybody would notice or care.  If 10,000 people did this near the same line, someone would care. I have heard of farmers who laid a half-mile long wire parallel to a power line crossing their property with the purpose of capturing the magnetic flux as AC current. That qualifies as stealing for sure because the magnetic field cannot return to the utility line.

I vaguely recall that a traffic signal in Israel is powered by road vibrations as cars pass over piezo devices.  That is not stealing anything as nobody has a vested interest in road vibration.

Elizabeth M
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Re: Self Powered Device
Elizabeth M   9/5/2013 4:50:14 AM
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Fair point, jhankwitz, I suppose the potential to use this energy has been around for a long time and, as you point out, people have already been doing it. But I think now it will be utilized more on a larger scale.

Elizabeth M
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Re: Self Powered Device
Elizabeth M   9/5/2013 5:03:20 AM
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Thanks for clarifying that and providing that perspective, 78RPM. Yes, it probably would be a big problem if regular electricity service or RF signals were interrupted, but I agree with your points and think these new energy harvesters are quite ingenius and a good way to leverage existing energy that otherwise would be wasted.

JimT@Future-Product-Innovations
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Re: Self Powered Device
JimT@Future-Product-Innovations   9/5/2013 11:09:27 AM
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Jhankwitz-  My hat's OFF to you, and with great admiration, recognize that you had the foresight to understand and prototype this RF phenomenon over 60 years ago.  Kudos --

I say futuristic because this idea, if it were broadly developed and commercialized, would rock the paradigm of powering wireless communication devices. I realize the losses are a big issue, but the concept of wireless charging was just a dream only a few years ago and now is commercialized.  With this, we skip the charging, and go straight to powering.  Seems futuristic to me.

 

JimT@Future-Product-Innovations
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Re: Self Powered Device
JimT@Future-Product-Innovations   9/5/2013 11:38:10 AM
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78RPM- You have lots of good points, spread over several posts:

1.     I remember the story of the Dairy Farmer stealing from the high voltage lines; an EE professor told that story in ~1980.  Suggested he probably would still be doing it today, had he kept his harvest to a smaller trickle.

2.     Different topic of power from road vibrations; An article was right here in DesignNews, a few weeks back.  See http://www.pavegen.com/ .  Great idea - Waiting for their IPO, on the London exchange!

3.     Last, on using Zigbee and sleep mode for extended battery life on wildlife tracking: I was on a team that developed such a device, but funding slumped and the product was never realized.  Such is the success of engineering.

Thanks for several very good discussion points. – JimT.

jhankwitz
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Platinum
Re: Self Powered Device
jhankwitz   9/5/2013 11:55:19 AM
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No foresight on my part.  Every 7th grader learned how to use hand and power tools and built a table and chair, and every 8th grader fabricated an electric motor out of a coffee can in addition to a crystal radio.  Everyone also had to learn how to play a musical instrument as well as dance.  Back then, they prepped kids with a broad spectrum of activities to help them determine their interests and choose their futures.  

Ann R. Thryft
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Re: Self Powered Device
Ann R. Thryft   9/5/2013 12:26:17 PM
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Well, not "every" student. Those first two were what the boys got to do in wood and metal shop, at least in my school. Girls learned cooking and sewing. I'm glad I learned both, but I also wanted to learn all that stuff in shop, too. Not only did this help us--along with music and art for everyone--figure out what we wanted to do, it also gave us some excellent skills. And there was also PE, which helped keep us all healthy.

jhankwitz
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Re: Self Powered Device
jhankwitz   9/5/2013 1:19:51 PM
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You're so right Ann.  Back in the early '50s, just about the only acceptable jobs for women were teacher, secretary, or housewife/homemaker.  It was a whole different culture. The temporary "Rosie the Riveter" was no longer needed and had returned to homemaker status.  

JimT@Future-Product-Innovations
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Re: Self Powered Device
JimT@Future-Product-Innovations   9/5/2013 1:29:32 PM
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Yeah, I was a 'shop' student in the 70's, too. Wood Shop, metal shop, auto-shop, drafting, both machine and architectural. 'Shop-Class' directed me to a career in Engineering. (Acknowledgement to Ann, also, that girls took Home-Economics. Quite sexist, in retrospect, wasn't it?) But the realization that stray RF was a potential power source was not part of the curriculum, and I think it was (and is) an overlooked and misunderstood phenomenon. I think you minimize your early realization of it. You were a smart kid, jhankwitz.

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