I'm workin' on a automatic powder dispenser. Gunpowder, that is. Yes, I know such things are available as from RCBS. That isn't the point at all, not at all. It's an interesting "senior project".
I want to use a powder balance, not an electronic straingage loadcell scale. Very good balances are available from several sources (RCBS, Dillon) for about $50. They are based on gravity, not electronics. Gravity tends to be steady over time. These balances may or may not be dead nuts accurate thruout their ranges, but they are definitely repeatable day to day and month to month.
I fitted a $4.95 surplus stepper motor to a Redding powder trickler, making it a power powder trickler, and found that it could dispense a variety of powders, extruded and spheroidal, at rates from zero to about 1 grain per second -- and probably more if I could keep it from overshooting the pan. At low speed, it can tick out individual granules, or maybe clumps of 1 to 3 granules of extruded powder. There are 9 or 10 granules of IMR4350 (extruded) to the tenth grain. OK, got powder feed, how to sense the scale's beam position without attaching anything to any moving parts so the scale remains unmodified in function or performance? Optics, maybe?
I found some 64 x 1 and 128 x 1 linear photo CCD arrays at Mouser, new parts, current production, about 8 bux ea. in onesies. I ordered one of each. I programmed a $2 microcontroller (PIC12F675) to generate the clock and synch signals necessary to drive the sensor. Set them up in a bench kludge to see how they might sense the shadow of the powder balance's beam if I put an LED opposite.
They work very well. The 128 x 1 even senses the effect of random air currents on the balance, though I can't see the beam moving. I figure it's delivering resolution of about .008 grain, so getting repeatable powder dispensing to well within +/- 0.1 grain *every time* might be possible. No commercial automatic powder dispenser under $1000 does that every time with a variety of powders, though the RCBS Chargemaster comes close -- for $400 or so.
There's a little movie of the sensor's response to changes in powder balance beam position at