hey everyone here's
the very sensitive
balance that I built that's capable of
measuring down to about 10 or 20
micrograms so this project was based on
a design demonstrated and built by Paul
Gro Hey and I'll put a link in the
description I started by taking apart an
old panel meter that came out of a
broken vacuum gauge one thing to keep in
mind is that most panel meters are one
milliamp full scale and the DC
resistance of these is often about 50
ohms so despite having meters for
different voltages in different current
ranges generally it's just the resistors
that are changed and the actual movement
is almost always 1 milliamp although for
really sensitive current meters you can
get hundred micro amp or even even more
sensitive ones so I took the just the
guts of the meter out and mounted them
on this acrylic block and then added to
it one of these opto interrupters a slit
type opto interrupter so with three
wires there is a common a power supply
for an LED an infrared LED shining on
one side and there's a photodiode on the
other which is the third lead so when
when the path is clear the infrared
shines on the photodiode and we get some
current through the diode here's what
the circuit looks like we've got the
photodiode here so that when light is
shining on it it will pass some current
in the reverse direction and the output
of this op-amp circuit is driving the
meter and so what we have here is a
servo loop so that when the light beam
is blocked it sends more current into
the meter and pulls the needle up and
then when there's light hitting this it
tries to pull the meter back down so
it's constantly serving so that the
photodiode receives just the right
amount of light and that just a right
amount is set by this pot over here this
is the gain of the op-amp circuit and it
doesn't seem to have a huge effect on
this thing mainly because the photodiode
turns on and off very quickly just the
physical arrangement of this opto
isolator means that as soon as the beam
brake comes down the whole circuit
either goes off to on very suddenly and
that's bad because the circuit does tend
to oscillate
very easily so to make this circuit at
all stable I have to tune this pot
really carefully to get it into a stable
area and I tried adding filters and that
sort of thing and it really doesn't work
what it actually needs is either a more
gradual sensor here so that we get some
analog values before the thing suddenly
cuts on or off or and or more physical
damping in the meter but I couldn't
figure out how to do damping without
also affecting the accuracy of this
thing so for a quick little project I
didn't worry about it too much the idea
is that the voltage at this node is
going to change based on how we load the
meter so if this thing is constantly
trying to servo to the same exact
physical point in space if we put some
maths on the tip of the needle the meter
needle then this thing is going to have
to give the current or give the meter
more current to pull it back up into
that spot and since this thing is so
sensitive it's only 1 milliamp full
scale if the meter were mounted in a
panel then very very small current
changes here will change the position of
the needle and then to get even more
gain I take the the voltage at this node
and multiply it by negative 5 here and
then also get rid of the offset so when
the thing is sitting there it's this
that one happens to run at about
negative 0.8 volts so with nothing on
the balance this node is about negative
0.8 and just to make it easier to read
that voltage and to get it closer to
zero this offset will take care of that
and get another 5x gain so if you see I
can perturb it here and it will actually
servo itself back and it's not
temperature stable so I've just recently
adjusted the pot that controls how
snappy this thing is but it's currently
in a pretty good spot where it's not
prone to oscillation it will servo
itself back to calibrate the balance and
also to check what the precision
actually is I took a piece of paper and
cut out a 50 by 50 millimeter square of
it and then measured the mass of that
square of paper with my milligram
balance and this came out to be about
190 milligrams
so then to check a really small mass
what I did was cut a 1 by 1 millimeter
section of that so a 25 hundredths of
that and that comes out to be about 76
micrograms so to check the scale just
quickly to see you know approximately
what the precision of this thing would
be I'll put the the 1 by one millimeter
piece of paper on there so currently
I've got the the plastic bin over the
top just to keep air currents down and
it's coming in at about 48 millivolts
let's say there's quite a bit of noise
and I haven't really done a whole lot to
reduce the noise okay so lift the
plastic cover up and place the little
measured the one by one millimeter piece
of paper here
okay
okay so now we're reading about 70
millivolts so the Delta there may be 22
millivolts corresponds to 76 micrograms
I've been having problems with the
plastic cover I think the static charge
on it is actually making the thing even
worse than keeping the air currents away
so I've cut a two by two millimeter
piece of paper to see to test the
linearity of the system so 76 micrograms
corresponding to about 20 millivolts or
22 so this should be four times that and
so we're reading about forty-five
millivolts it is kind of wandering quite
a bit okay okay so this is really good
so if the reading here is 125 and we had
45 that's 80 which is exactly 4 times 20
so the system appears to be pretty
linear okay so we're back to about 45
millivolts and I'll try putting a single
eyelash on the platform there I'd say
that's reading about 55 so 10 millivolts
my measurement of a single one of my
eyelashes is probably more like 35 to 40
micrograms it's entirely possible the
eyelash is way different amounts to okay
we're still hovering about that 45
millivolt background so if I place a
single grain of sugar
that's really close to 70 so a single
grain of sugar is probably about 75
micrograms or thereabout with better
construction technique to eliminate some
more of the noise and also really high
low-pass filtering so that the thing
will sample over the course of you know
five or ten seconds to generate a
reading it should be possible to extract
a fair bit more precision out of this so
getting down to single-digit micro grams
shouldn't be too hard I tried extending
the lever arm here further out you know
to make the thing more sensitive
basically but that didn't work very well
because it added so much mass to the to
the needle that avoiding oscillation was
even more difficult than it already is
okay hope you found that interesting see
you next time bye
s
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