Instructables

I would like to know how much force should be applied to get and output of 8v on a piezo electric ring.

I would like to make a mobile phone charger using a piezo electric crystal which can be placed inside our shoe, so that when we walk, we step on the piezo electric crystal and thus we can charge our mobile. for that i would like to kno more about piezo electric crystals, and how much force is to applied for getting an output of around 8 v and type of crystal to be used for this purpose.



Thanks in advance.


Picture of I would like to know how much force should be applied to get and output of 8v on a piezo electric ring.
Pace, Frollard, BUT Linear Technology have a very pretty "energy harvesting" chip DESIGNED to scavenge from sources like piezos.

Note to AllSpark - that does NOT take anything away from Frollard's comments on energy extraction from your gait. It is IMPOSSIBLE toextract much without impacting on your walk !

Steve
That's pretty neat -- elsewhere in the wiki they suggest using piezo to extract energy from vibrating machinery -- there are some useful applications for LOTS of technologies - agreed that the human body is not a good one for this setup.
allsparkunleashed (author)  frollard2 years ago
so .. is ther any other way to charge our device from human body?
There are a lot of ways. Turn a hand-crank generator, use a peltier effect device to use body heat temperature differential...

Therein lies the problem.

The body has only so much energy, and any time you extract a single joule of it, the body works HARDER to replenish it.
Interesting conceptually, because at rest, we are dissipating ~60W a piece anyway.

Riffing on the theme, if one shoved a thermopile.....up somewhere warm, shall we say.....and put the other junctions on the outside of ones clothing....how much power could we extract.

Peltier cells....where I was thinking of....might be a bit too sharp......
Yeah, probably not ideal;

Those 60 watts are enticing, but the Entropy simply isn't there. If we only needed to run a chocolate factory getting the candy just up above 35C we would have it made - scores of people in chocolate farms, 'the matrix' style keeping it warm. Alas, alack.
Just for information. Neat link.
http://www.edn.com/design/components-and-packaging/4374932/Coupling-a-supercapacitor-with-a-small-energy-harvesting-source-
allsparkunleashed (author)  frollard2 years ago
thanks.. i like the idea of peltier effect device.. can u give more info on that??
Can you not do your own research?
bwrussell2 years ago
As far as harvesting off the body goes, I have seen some induction based systems. Essentially a magnet is captured in a torus which is wound with wire. The movement of the magnetic field through the coils create a current. You could also do a linear version. I'm not sure what kind of out they got, it would need to be normalized, and it will add mass meaning you exert more energy but that could be seen as a good thing if you are trying to get/stay in shape or loose weight.
frollard2 years ago
Piezo is really a poor technology to charge your device.

1) ALL energy extracted from your running steps is taken from the spring of your shoes, and must be accounted for, making walking proportionally harder to do -- It's not like your shoes heat up when you run, so there isn't a lot of energy being dissipated by your shoes.  If you started extracting power from your stride, you'd find it more difficult to run.  Shoe companies go to GREAT lengths to determine the best way to keep the 'spring in your step' so to speak, allowing you to run faster, further, etc.
"A similar idea is being researched by DARPA in the United States in a project called Energy Harvesting, which includes an attempt to power battlefield equipment by piezoelectric generators embedded in soldiers' boots. However, these energy harvesting sources by association have an impact on the body. DARPA's effort to harness 1–2 watts from continuous shoe impact while walking were abandoned due to the impracticality and the discomfort from the additional energy expended by a person wearing the shoes."


2) You don't get a constant voltage or current from a piezo - you get vast spikes.  From wikipedia: "For example, a 1 cm3 cube of quartz with 2 kN (500 lbf) of correctly applied force can produce a voltage of 12500 V.[11]"  It's easy to imagine 500pounds-force during the peak of the bottom of your tread when sprinting, That kind of voltage discharge is HUGE, and hard for electronics to compensate for (convert to something useful).  Charging a mobile device is best done with constant 5 volts at 2-10 watts depending on the device.

3) Piezo develops thousands of volts at nearly no current.  That's how they can make sparks jump to start barbeques, but its not useful to store that energy.
edit: source:
http://en.wikipedia.org/wiki/Piezoelectricity