Automatic Shutdown on Bathroom Scales

[Tony]

I have recently purchased a set of Digital Bathroom Scales with 4
wires from the strain gauge to the circuit board. I have removed the
gauge and board to make a scale, with 0.5, 1, 2, 5 & 10 times
mechanical linkage giving a range of from 67.5kg to 1350kg full scale
deflection (Scale weighs to 135kg).

The big problem is that the circuit automatically switches off after
30 seconds and resets to zero(even with load on), if I switch it back
on.

The board has a 14 pin Quad Op Amp HA17324/A and 8 pin EEPROM ATMEL
93C46. I have a pin-out for both, but being a Mechanical Engineer, I
am at a bit of a loss as to where to go from here.

 

[Jerry G.]

Your problem is the internal software in the EPROM. You would have to find
a way to do a rewrite of the software to the way you want it. Then it would
be the matter of reburning a new EPROM. This would not be practical for
you, unless you would have a way to read out the original software, have
someone that can modify the software, and reburn a new EPROM. You would
also require the proper EPROM programming and burning hardware for the type
of EPROM that they are using, and the proper soldering set-up if it is a
surface mount device.

Consumer scales are mostly battery operated, and require auto zeroing. The
user simply presses a button or stands temporarily on the scale, then goes
back on, and gets his weight reading. After a number of seconds, the scale
will shut off to conserve the battery power. The scale will do a zero reset
each time the power cycle is re-initialized.

I would think that the cost involved for what you want to do would be
greater than purchasing the proper scale in the first place. You must have
spent a lot of time alone to do the modifications that you did. There are
many used industrial scales on the market that can be purchased for your
purpose, and can be modified, and then be certified for trade under
contract.

From a legal point of view, if you are using these scales for trade, the
grade of strain gauge, and class of design for reading the gauge that you
are trying to use is not legal for trade. That is if you are going to use
your scale for weighing goods to be sold, or move by carrier. All scales
used for trade must pass a registered certification from the manufacture,
and have the calibration checked according to a specified period of time
depending on the laws of where the trade is to be located. If you are
exporting goods, the scale certification must comply to the destination
regulations for goods being sold by weight (there are international
standards). If not, the receiver will have to re-weigh the goods to verify
accuracy.



--

Greetings,

Jerry Greenberg GLG Technologies GLG
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"Tony" <alr@senet.com.au> wrote in message
news:8296bafd.0401040443.1d532bf1@posting.google.com...
I have recently purchased a set of Digital Bathroom Scales with 4
wires from the strain gauge to the circuit board. I have removed the
gauge and board to make a scale, with 0.5, 1, 2, 5 & 10 times
mechanical linkage giving a range of from 67.5kg to 1350kg full scale
deflection (Scale weighs to 135kg).

The big problem is that the circuit automatically switches off after
30 seconds and resets to zero(even with load on), if I switch it back
on.

The board has a 14 pin Quad Op Amp HA17324/A and 8 pin EEPROM ATMEL
93C46. I have a pin-out for both, but being a Mechanical Engineer, I
am at a bit of a loss as to where to go from here.

 

[Spehro Pefhany]

On 4 Jan 2004 04:43:04 -0800, the renowned alr@senet.com.au (Tony)
wrote:

I have recently purchased a set of Digital Bathroom Scales with 4
wires from the strain gauge to the circuit board. I have removed the
gauge and board to make a scale, with 0.5, 1, 2, 5 & 10 times
mechanical linkage giving a range of from 67.5kg to 1350kg full scale
deflection (Scale weighs to 135kg).

The big problem is that the circuit automatically switches off after
30 seconds and resets to zero(even with load on), if I switch it back
on.

The board has a 14 pin Quad Op Amp HA17324/A and 8 pin EEPROM ATMEL
93C46. I have a pin-out for both, but being a Mechanical Engineer, I
am at a bit of a loss as to where to go from here.

It's almost surely got a microcontroller (say, mostly connected to the
display) that runs things. If that's programmed for auto-shutoff and
the designer didn't see fit to allow for an always-on option, you're
pretty much SOL. One could replace the entire section after signal
conditioning, but that might be non-trivial for you.

If you want to pursue it, and assuming you have no programming
experience, you could look at using a 4-1/2 digit DVM with a good
reference and a trimpot to offset the zero. If you've got a good DVM
now, you can try probing the op-amp outputs to see what you get with
varying scale readings. You'll likely see a relatively large voltage
with zero weight (which the pot/reference would be used to cancel
out).

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com