Why would a non-technical person want a stable voltage reference?
I can think of a few cases, neither of which probably apply to this
case though: 1) A child growing up in a very poor family, but
learning, and wanting to actually _do_ some science on the cheap or
just learn. 2) Someone living in circumstances where there is no
infrastructure but only primitive raw materials and physical theory to
go on. This case, of course, presumes a meter in hand and that
generally means that there either is infrastructure, money, or else
it's a one-off that just happened to fall into poor hands (given,
perhaps?)
BTW, voltage references aren't complex - one chip with 3 pins (supply,
gnd, ref out), that's it no "circuit" required. Shunt types might need
an additional resistor, but that it.
Local electronics stores carry basic types.
If they exist. Sometimes it's just fun to answer the general question
from physical theory -- for example, what kinds of materials that can
be readily found and with low technology means turned into a
reasonable voltage reference that provides _any_ valid approximation.
Think of this in terms of what can be described on paper in words in
sufficient detail that they can be replicated reasonably well, say, 2
millennia ago. Would be interesting, no?
Some time ago, someone I knew currently living among native indians in
South America asked me about the possibility of arranging a way they
could make their own AM radios. Batteries were impossible to be had,
so of course the idea of a "crystal set" came to mind. I set about to
describe how to make an AM receiver. Galena was locally available,
luckily, so that was a good starting point. Needles of metal (steel)
were also locally available. Together, with a little bit of molten
lead, that was sufficient to make a diode detector. The result was
that I gave them instructions for fabricating circular permanent
magnets, coupled with flat bits of metal for diaphragms, wound wire,
etc, to make their own higher impedance earpiece (which was the harder
part of this), along with the rest (tuning coil, capacitor) to make an
AM radio from what could be accessed in what was then rather a remote
area.
[All kinds of wonderful things can be done with simple items. You can
fabricate a microbarometer capable of detecting an elevation change of
less than a meter using DOT brake fluid from a car, some glass tubing,
two LEDs, some nichrome wire from a toaster, and a few other bits.]
I think it's interesting to think about how to develop a voltage
reference from common materials available in different locals.
Sometimes, one might want to make one and doesn't have access to high
tech resources for the purpose.
What would you do if you were making such a voltage reference and
lived, say, 100 years ago and there were no ICs, transistors, bandgap
references, and so on? Yet you wanted to "do science" all the same?
What about 200 years ago?
I haven't done the experiments to see, but I wonder about using two
dissimilar metals, freshly sanded with emery paper and placed in a
lemon or lime for a short time (the reactions will soon block the EMF,
but fresh metal surfaces placed a consistent distance apart may
provide a reasonably repeatable result. Unfortunately perhaps not a
_known_ result from theory, though. So accuracy would still be in
question.
Still, copper sulfate pentahydrate (simple, hydrated copper sulfate)
is available as root killer at garden supply stores. Which is a
copper salt. That, with copper wire can easily provide one half of a
wet cell with known (predictable) voltage. The only need would then
be to find another metal and an associated salt for the solution. One
could then use either a saline bridge between (traditional method) or
else just use sausage casings (squish out the sausage first and wash
thoroughly!) filled with one of the salts and the metal electrode and
immerse it directly in the other salt solution.
Anyway, the electric potentials from the reactions are readily
available for calculation -- for example, the standard Cu/Zn combo
yields a predicted (+0.342 - -0.762) = 1.104V of potential difference.
I'm going to set about the local store today and see what I can easily
find to test this out. (1) copper sulfate (root killer), (2) sausages
for their casings to use as a porous membrane, (3) copper wire, (4)
angle iron, (5) ferrous or ferric sulfate (etchant or moss kill) to go
with the iron, (6) aluminum, sanded, from pop cans, (7) aluminum
sulfate from "slug kill" supplies. Might be fun. That's three metals
and their salts to play with and a porous membrane.
Jon
