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Re: [ProgSoc] Beer voltages



I can't believe I'm answering this...

On Tue, May 30, 2000 at 06:31:59PM +1000, Christian Kent wrote:
> 
> Okay, ProgSoc got the Funway book for $5.  Quote from page 44:
> 
> 	NOTE:  Do NOT drink the beer after use.  Apart from the fact
> 	that it will taste really horrible, it could be poisonous
> 	because it has been in contact with copper.  Throw the beer
> 	out immediately you have finished using it!
> 
> Anyway, it's for the beer-powered radio, which uses a ZN414 IC, and they
> say that if you use more than five beakers of beer, you will overload the
> IC.  They also say that you can substitute the beer for a 9V battery plus
> a 33Kohm resistor.
> 
> So what does this mean?
> 
> I tried applying V=IR but got stuck.
> 
> 	Battery:    9 = I * 33000
> 		    I = 9/33000 A
> 		      = 3/11 mA
> 		      = approx .25 mA

Probably about right, the ZN414 uses very little power and I believe
it was only driving an earphone.

> 	Beer:       V = I * R	where R = 0?
> 		   (V = beaker voltage * 5)

R depends on the surface area of the plates, the distance between the
plates and the available ions in beer.

beaker voltage is available by subtracting the redox potential of the
two metals (usually copper and zinc) from each other. If you use a carbon
electrode you get more resistance from the carbon but the redox potential
of carbon is pretty close to zero (i.e. it doesn't redox). You can also
use hydrogen as one terminal which gives you a nice high redox potential
but hydrogen is not very conductive (unless you freeze it :) so you must
use a ``metal hydride'' (thus you end up with a metal hydride battery).

Most of the useful metal hydride formulas are patent protected at the
moment but this doesn't stop you making prototypes for your own testing.

> Without the battery + 300Kohm resistor, there's a 100Kohm resistor in
> there, plus two capacitors (0.1uF and 0.01uF), a var capacitor with range
> 60-160pF, the IC and, finally, the earphone.

The ZN414 is very special-purpose for radios and only a few circuits
are really possible, you can probably download the data sheets from the web.

	- Tel
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