Science Friday: power from gravity, deciwatt

scott at scott at
Sun Jan 20 10:46:47 EST 2013

Hi All,

If light is the goal, they can accomplish this with an earth battery. 
This is essentially a chunk of magnesium ( I use thermite ribbon, wound as 
a coil on a wood former, but commercial fire starter blocks will work too) 
buried in the ground, with a chunk of copper or carbon buried north of the 
magnesium.  The potential difference between the metals is created due to 
telluric currents.. about 1.2V or so with current delivery being 
proportional to surface area of your electrodes.  I know, not much juice, 
but enough to trigger a silicon NPN bipolar transistor.  What good is 
that?  In a blocking oscillator circuit, you can use that low potential to 
charge and abruptly shut off an inductor, which gives you  higher 
potential transients at relativley high frequencies.  These transients are 
collected off the coil into the LED(s) to deliver light.


On Sun, 20 Jan 2013, smithbone at wrote:

> On 01/19/2013 02:39 AM, Hal Murray wrote:
>> Their target is light in Africa.   They are only getting 30 mW to 1/2 W so
>> the name is appropriate.  That's enough for illumination from LEDs.
>> It's not likely to be useful for OLPC any time soon.  It might be something
>> to keep an eye on.
> The crowd sourcing link was posted on in the IRC devel channel a few months 
> ago and we were debating how much power you could really extract out of it. 
> We came up with some really low numbers.  Low enough that I was questioning 
> the usefulness but it seems that we were right on.  Just enough to light a 
> low power LED.  Which he said was possible due to todays really efficient 
> LEDs.
> The 1/2W upper end required an adjustment in the descend rate for a much 
> faster drop and he didn't give the time or weight used so difficult to make a 
> guess at the actual energy available.
> I think the most interesting thing was said just at the end of the interview 
> where they were going to use some of the money they raised to research a very 
> low power satellite receiver coupled with some sort of low power display to 
> try and be able do download a wikipedia page.
> -- 
> Richard A. Smith
> smithbone at
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