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Automatic Shutdown on Bathroom Scales

Discussion in 'Electronic Basics' started by Tony, Jan 4, 2004.

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  1. Tony

    Tony Guest

    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.
     
  2. Jerry G.

    Jerry G. Guest

    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
    =========================================
    WebPage http://www.zoom-one.com
    Electronics http://www.zoom-one.com/electron.htm
    =========================================


    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.
     
  3. 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
     
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