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Digital Weight Scale Design Ideas

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Johnson777717

New Member
Hey folks.
I've been wanting to build a digital weight scale for use with my Reloading Hobby. In case that you are not familiar with "reloading" it's basically reloading brass casings for firearms. In simpler terms, it's making bullet cartridges (Adding the primer, propellant and the bullet).

During the reloading process, the powder must be accurately weighed in GRAINS (1 US pound = 7000 grains, 1 US Gram = 15.43 grains). The accuracy should be around +- 1 grain.

I've been weighing the powder using a counter-weight scale, which isn't all that accurate. Plus, using this scale takes about 3 times longer than using a digital weight scale. These digital weight scales usually sell for over $150 :shock:

I'm wondering if anyone has any ideas about how to build a digital weight scale. The accuracy doesn't have to be within 1 grain for now. I just want to get the basic knowledge and concept down first, before developing something that I would actually use for reloading. Rest assured, when I build the actual scale that I would use for reloading, I would certainly perform a great deal of statistical analysis to gain an understaning of the performance specifications and tolerances.

Thank you for your support!
 

Johnson777717

New Member
Thanks for your reply Joe. I don't have any experience with Load Cells. Is that the actual name of the device? Do you know of someone who sells these so I can take a look at the datasheets?

Thanks bud.
 

joe_1

Member
Load cells are cells that can measure pressure or force (weight). There are two wires in (V+, GND), and two wires out with voltage. This output voltage will vary based on pressure applied on the cell. From that you can calculate accurately the weight. Most digital scales use load cells. They come in all shapes, even with display. You do not need the pressure type since that is for fluid pressure.
You can use google search for load cells, but here is one:
https://www.transducertechniques.com/

joe,
 

Klaus

New Member
Perhaps it also should be mentioned that accurate load cells are not cheap! :shock:

The basic load cell has strain gauges inside, arranged in a bridge. When a load is applied one of the bridge elements gets unbalanced, resulting in an output voltage. This output is very small (mV) and requires a very good, linear and temperature compensated, amplifier to get it to the range suitable to drive a LCD display. That amplifier is also rather expensive! :eek:

You then need a dual power supply for the amplifier. The load cell gets excited from this as well, via the appropriate trip pot and drive circuit.
The exitation voltage is set to suit the expected output range. Set it too high and the bridge output swing becomes too great for the amplifier gain
(~1000) , clipping it. Set it too low and the sensitivity suffers.

I have played about with home made strain gauge load cells and commercial amplifiers, in the kilo range. For gram (or grain) ranges I would imagine the home construction of a load cell is too tricky, leaving you the expensive option to buy one :oops:

I would imagine a purely mechanical jeweller's balance scale to be very accurate for your usage. Its not an electronic solution to your problem but perhaps you could pick up a used set of scales, with the appropriate weight set, for a reasonable price.
Good luck
 

Johnson777717

New Member
Thanks for the replies folks, and the information on load cells.

Do you think that manufacturers are using pressure sensors rather than load cells in this circumstance?
 

laroche73

New Member
accurate weight measurement

I recall reading a brilliant article by Jim Williams (now a Linear Tech guru) on designing accurate analog scales circa 1980/81 in EDN magazine. I don't remember what the sensing element was, but the article's worth reading just for the concepts, if you can track it down.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
pramodram2k6 said:
how to make calibration in weighing scale

Perhaps not by posting in a three and a half year old thread?.

But all you need to do is use known weights, and calibrate using those - just as you would anything else!. Plot a graph using the known weights you have, and use the resultant curve to calibrate the scale.
 

Leftyretro

New Member
pramodram2k6 said:
how to make calibration in weighing scale

I played around with a very small load cell from IOMEGA once, seemed to be in the 0-5 OZ range. I used freshly minted US quarters (coins) to check calibration and linearity. Worked very well but I never bothered to put the circuit into a finished product. Now with cheap PIC processors with onboard A/D convertors I might try and locate and start over :p

Lefty
 

rajat

New Member
hi every one i am doing a mini project based on the same weight cells accuracy is not a constraint
this is my first project so can any one suggest me any load cells? please
 
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