I had a $20 bathroom scale apart a few/5 years ago for the same reasons.
What is found is likely to be a nearly flat hump of epoxy covering a
leadless ADC IC on the module, and only about a dozen external components
(no board markings for components or module identity).

The trouble with throw-away products is that there is rarely any
documentation available.. no FCC approval or any other regulations wrt
standards or specs, so no need to submit any design specs before production.

I posted some questions in a newsgroup (maybe sci.electronics.repair) asking
about DIY adaptations of similar scale modules, and was lucky that a guy who
had some experience in designing and/or working background with similar
products, replied with the limited info I've included here wrt capacity and
span.

There are selected component values to meet the specific cell/sensor (also
unmarked), capacity of the scale, and span values, IIRC.
So as the Chinese are famously known for, they'll replace a 3 cent pot with
a .003 cent fixed resistor every time it's possible.

BTW, the 3 digit display wasn't even the usual 7-segment LED digits, but
instead, the actual tiny individual chips without encapsulization (you can
see 'em gazing into a clear acrylic LED), placed in an arrangement of 7 per
digit.. and the materials that made up the 7 "bars" of the 3 digits was
actually a colored cellophane-like film over a plastic box with partitions
in it to simulate a 3 digit LED display.
When seen thru the front panel red plastic display window, it's unlikely
that anyone would have guessed what the actual internal display components
were.

The individual LED chips had the miniscule gage gold bonding wires attached
to gold "plated" dots at the ends of the board traces.

So.. the the printed retail product box, molded battery compartment and 9V
clip lead were relatively expensive parts in the manufacture of that scale.

It looks as though at least several others would also be interested in what
you find out if you try experimenting with some inexpensive
off-the-retail-shelf items.

--
WB
..........


"RogerN" wrote in message
...

I'm wanting to make a display for load cells, good looking ones on eBay
seem to start around $150. This is a bit hard to justify for tinkering
around, especially since the load cell I want also costs around $150. As
I understand it, most digital scales work with some sort of strain gauge
load cells. This leads to the idea of getting a cheap scale (bathroom,
fishing scale, food scale) for $20 or so and using the display and
electronics to read whatever load cell I want. In order to do this I
would need to be able to recalibrate the scale. I don't mind simple X10
or X100 display (example, display shows 200 for 2000 or 20,000 lbs) but I
don't want to need a calculator to scale and offset the reading for units
conversion.

Anyone here done anything like this? Know of any cheapie scales that can
be recalibrated to different weights with other load cells? If this isn't
easy to do I'll probably just get a load cell amplifier board and use the
scale and offset pots to get the reading on my voltmeter, maybe something
like 1.000 Volts = 10,000 lbs or whatever depending on the load cell.

My most immediate use will be to tension the guy wires on the antenna
tower I'm preparing to put up and later hopefully to do tests on
construction of a homemade crane. I'd like to verify/test my force
calculations to keep the design factor correct.

RogerN