[EMS Discuss] radio wave power question from Joshua

Craig Godwin craig at godwinlabs.com
Fri Jan 24 17:07:33 PST 2014

A crystal radio is powered entirely from radio waves.  The power output 
is very low, but it will drive an extremely high sensitivity ceramic 
earphone enough to hear it.   But when I made one as a kid, I had at 
least 100 feet of copper wire strung around the roof of my house to make 
it work.

It seems like it would take a huge antenna to get an LED to light up 
constantly, but flashing an LED would be practical.

Modern watches take a tiny amount of power - a lot less than an LED.  
Some power themselves just from the motion of the person wearing it.  
Absolutely it would be possible to power one from RF, but the question 
is how big of an antenna would be required?  I don't have a good answer 
for that, but my guess is it could be made small enough to wear.


On 1/24/2014 4:22 PM, Rick Osgood wrote:
> Do you mean you can power an LED from radio waves like from a radio 
> station?  I wouldn't think that would even be enough to power an LED 
> but I'm not really that knowledgeable on the subject to be honest.
> You can charge cell phones and toothbrushes (and power LEDs) using 
> inductive charging, but that's a bit different and it requires pretty 
> close proximity to the charger in order to be efficient.  If you are 
> talking about wearing a watch around all day and having it powered 
> from the stray radio waves then I'm not sure.
> On Fri, Jan 24, 2014 at 4:15 PM, Elizabeth Phelps <lizphelps18 at aol.com 
> <mailto:lizphelps18 at aol.com>> wrote:
>     Hello,
>     I was wondering, if you can power an LED with radio waves, do you
>     think you could power a watch that way?
>     Thanks,
>     Joshua
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