digital scale modification

Discussion in 'General Electronics Chat' started by fangsandthings, Oct 24, 2011.

  1. fangsandthings

    Thread Starter New Member

    Oct 24, 2011
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    This is my first post, and I'm not an electronics person, but I do own a well-used pencil-type soldering iron...quit laughing!

    Here's my problem. I would like to modify a hanging scale that has a really annoying and useless HOLD feature.
    AWS model SR-5.

    This scale locks onto a "favorite" weight after some indeterminate length of time or some other unfathomable (to me at least) electronic variable.
    It just fails to get reliable and repeatable measurements.

    I want to use the scale to get field weight measurements on snakes, venomous snakes, which are most safely weighed after being secured inside a bag.
    Thus, a hanging scale.

    The AWS SR-5 would be perfect for my application, with a range up to 11# by .01# increments (metric mode also)

    I have explored the scale market...and this is the best I have seen for my purposes.
    I have corresponded with their customer service, and yes, they have a new model in planning stage which will include an on/off option for the HOLD feature.

    After opening the case, and exploring the internet to see what kind of circuit diagrams etc that I could find, I arrived here at allaboutcircuits.

    I have also opened the case of an AWS model SR-1, which does not have the HOLD function, but has a maximum capacity of 1Kilo.
    I observe only 2 visual differences between the 2 models.

    1-the load cell mechanical structure is different, but hey, one unit has a kilo max, and the other ~5K, so that seems predictable.

    2-I see a little extra resistor? on the top of the SR-5 circuit board, which is absent on the SR-1; otherwise the tops look identical.
    (Except for the area under the LCD, which I cannot see very well)

    The bottoms of the circuit boards will reveal themselves only at further risk of disturbing solder connections adjacent to the mount screws.

    Sorry if this is tedious, but I am seeking expert feedback, and am grateful for any guidance.

    Is there any chance that I might be able to bypass the HOLD circuitry on the SR-5?
    Or, is this function programmed like firmware?

    And, is there any chance that someone might answer these questions based on photos of the hardware?

    Or, do you need circuit diagrams?

    Thank you so much already for the time spent reading my post!
     
  2. chrisw1990

    Active Member

    Oct 22, 2011
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    if you have already been in contact with their customer services, say your an electrical engineer or some other fancy business and request the circuit diagrams for personal use, then more help might be available.. either way, can you upload the pics?
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    Wouldn't the feature be a function of the programming within the device rather than the hardware?
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    I did a google search. And the almost all that showed up was about Toyota cars. I think the mods need to close this thread:p The last thing was a joke. But do you have any links to info about this weight.
     
  5. chrisw1990

    Active Member

    Oct 22, 2011
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    @BillB3857,
    the hold could be programming, unless they did it passively.. it could just be a passive hold to save money in development etc.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    As in a peak/hold circuit?

    To the OP- How is the reading cleared? If by pressing a button, does the reading follow applied weights without latching if the button is held down?

    Never mind. I went here..... http://www.americanweigh.com/product_info.php?cPath=40&products_id=1230 and looked at the data sheet. Button is only for resetting to zero, tare weighing or power functions
     
    Last edited: Oct 24, 2011
  7. chrisw1990

    Active Member

    Oct 22, 2011
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    that sorta thing ya
     
  8. fangsandthings

    Thread Starter New Member

    Oct 24, 2011
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    Here is the manufacturer site:
    http://www.awscales.com/hanging-scales/158-sr-5-digital-hanging-scale

    I have requested a circuit diagram, straight up, I told the rep that I want to hack the scale.
    We'll see if I get it.

    I also suspect that the HOLD feature is programmed.

    @ t06afre, if you put "scale" in your search query, you might find it.

    I will make photos unless mods think I should go away.

    Thanks again for looking into this!
     
  9. chrisw1990

    Active Member

    Oct 22, 2011
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    oh yeh, probably programming, it looks neat!
     
  10. colinb

    Active Member

    Jun 15, 2011
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    Did you get a look at the circuit board inside? You might (or might not) be able to discern how the hold feature is implemented. The scale probably is driven by some sort of microcontroller where the hold feature is implemented. But it it's possible that it's all done in discrete devices without any programmable device.
     
  11. fangsandthings

    Thread Starter New Member

    Oct 24, 2011
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  12. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
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    What exactly happens when you use the SR-5 that is giving you trouble?

    According to the manual, which covers both the SR-1 and SR-5, "hold" is displayed once the scale has settled on what the weight is, much like the beeping from a personal thermometer once it has determined the correct (steady) temperature. I don't see any mention of a favorite weight or a reason the scale would save a measurement and compare it to another one (unless it is not mentioned in the manual or the tare button is somehow shorting out intermittently).

    Is "hold" not displayed anywhere on the screen of the SR-1 when you weigh something?

    So, step-by-step, what happens from the time you turn the unit on until a weight is displayed?

    Based solely on your desciption and the manual, it almost sounds like you have a faulty unit.

    Also, looking at the pictures, there appears to be a short caused by excessive solder across R16 and R17 on the SR-5 board.
     
  13. colinb

    Active Member

    Jun 15, 2011
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    The flat black “blob” with the “RHOS” sticker (LOL, obviously misspelling of ROHS) is probably the microcontroller. The microcontroller IC is attached to the PCB using chip-on-board (COB) attachment. It makes the device much harder to hack if you want to play with it.

    However, I think that, as exciting as it sounds, hacking the firmware is not the answer to your problems in this case.
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    That is "Chinese honesty"... An expression that has become something of an oxymoron.

    If they spell ROHS wrong, it is no longer legally prosecutable as a ROHS fraud, they can just claim "RHOS" means something else. Of course it's a deliberate half-truth as the purpose of the "RHOS" tag is to fool the customer into thinking the product complies with ROHS regulation.

    Another example I saw recently was some Chinese power tools that look almost exactly like a popular big brand, and the Chinese brand name was "Bocsh".

    And another example; advertised "German bearings" where "German" was one registered business name of the Chinese bearing manufacturer.
     
  15. someonesdad

    Senior Member

    Jul 7, 2009
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    Interesting; as always, caveat emptor.
     
  16. fangsandthings

    Thread Starter New Member

    Oct 24, 2011
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    @ elec_mech

    Thanks for looking!
    I agree, R16 and R17 area in general looks messy.
    Moo Goo Gai Pan lunch at the workbench that day?

    But, the scale seems to work OK, so, probably doesn't have a short.
    It is just a little finicky for field work.
    It would be better without the HOLD feature.

    To answer your questions:

    Q- "What exactly happens when you use the SR-5 that is giving you trouble?"

    A- Under the circumstances of my desired application, which is FIELD use, the HOLD feature takes effect too quickly, freezing the result before the full item weight has been engaged by the scale.
    I put a snake in a bag, in the field, and then engage a prepared catch point along the edge of the bag with the scale hook.
    Then, lift the bag for a weight reading.
    Sometimes before the snake bag has cleared the ground, or before the item has been completely released from my hand after engaging the hook, the HOLD feature freezes the weight, resulting in a premature estimation of the maximum weight.

    Under LAB conditions, with the scale mounted on a hanger, and the item to be weighed fitted with an easily engaged hanger loop, I can get satisfactory and repeatable weight results as long as I follow a rigorous and deliberate protocol.

    Under LAB conditions I can see that the scale is probably performing to production expectations.

    But in the FIELD, things are different.
    Especially when handling a venomous snake.
    The apparent swiftness of the HOLD feature to freeze a weight is less than satisfactory.

    So, I would like to defeat the HOLD function for my own purposes.

    Q-"According to the manual, which covers both the SR-1 and SR-5, "hold" is displayed once the scale has settled on what the weight is, much like the beeping from a personal thermometer once it has determined the correct (steady) temperature. I don't see any mention of a favorite weight or a reason the scale would save a measurement and compare it to another one (unless it is not mentioned in the manual or the tare button is somehow shorting out intermittently)."

    A- See above.
    I have no idea how the HOLD feature actually works, and little idea about electronics in general. The term "favorite" weight is my own, not from the manual, just a literary effort to describe the apparent "choosing" process of this scale when I use it.

    ("Favorite weight" = "once the scale has settled on what the weight is")

    Q-"Is "hold" not displayed anywhere on the screen of the SR-1 when you weigh something?"

    A- No, the SR-1 does not have the same HOLD feature as the SR-5.
    The 2 scales share an information document, but I also use an SR-1, with its 1K maximum weight range, and it does not have the HOLD feature.

    That is why I opened the SR-1 plastic case.
    The SR-1 does not have the annoying HOLD feature.
    I wanted a visual check to see if I could discern a physical difference between the circuit boards, and thus possibly defeat the HOLD circuit on the SR-5 scale.

    There are only 2 only visual differences I can see between the 2 scales.
    I think the photos make these 2 points clear.

    1-They have a different mechanical beam structure.
    2-The SR-5 has an extra resistor or something, right in the (messy) R16 circuit, along with a lot of sloppy Moo Goo Gai Pan, or whatever...

    Q-"So, step-by-step, what happens from the time you turn the unit on until a weight is displayed?"

    A- RE: the SR-5 scale:
    When the power button is pressed, the unit begins to power up.
    If I press again briefly during power-up, I can change the units mode between lbs and kilos.
    After units mode is determined, the scale zeros and is ready for weighing.
    After the scale decides how much an item weighs, it freezes the weight and displays the word HOLD on the LCD.
    After that, I can TARE the scale by pressing the power button briefly, which returns the displayed weight to zeros.

    Q-"Based solely on your desciption and the manual, it almost sounds like you have a faulty unit."

    A- I do not believe that the unit is faulty.
    I think that the HOLD function is ill-designed, but is working, as-designed.

    It is possible however, that this unit is "freezing" the HOLD weight way too fast.
    This could be a defect, possibly related to a solder short at R16 R17 as you observed.
    Or, it could be a bad design, based on the code written into a microprocessor/microcontroller.

    Q-"Also, looking at the pictures, there appears to be a short caused by excessive solder across R16 and R17 on the SR-5 board."

    A-I think it looks like Moo Goo Gai Pan spilled on the board during lunch break! Some subsequent efforts with small tools minimized the effect on the electronics, and somehow, the board passed quality control and managed to be installed into a distributable product, cross the ocean, and now it is in my house/backpack.

    Thanks!
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
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    Is it possible that during field operations that the weight applied is giving the appearance of increasing, then, momentarily, decreasing? Try lifting a weight from a table and before full load is applied, slightly lower the scale and see if it goes into hold mode.
     
  18. fangsandthings

    Thread Starter New Member

    Oct 24, 2011
    5
    0
    Yes BillB3857
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    That is exactly what is happening.

    Perhaps this gets passed along to the AWS company, (and perhaps I get an R&D compensation, since they goofed up the product from the get-go.)

    This product, a "hanging scale", is cheap, and pocket-sized.
    It appears to be intended for a "field-use" market, or "customer-base" etc.

    The scale should NOT be so finicky on the timing of the HOLD feature.

    My theory is this:

    The SR-1 with a 1 kilo maximum range does not need a HOLD feature, since a "wandering" readout at under 1K would be acceptable and legible to the user.

    But, during R&D, the AWS company recognized that the "wandering" readout would be confounding and confusing to a user when weights above 1K were measured.

    Especially when weights were significantly above 1K...as in the ranges of the next 2 scales in the line...SR-5 and SR-20.

    So, the solution was to create a "freeze" or HOLD feature, perhaps programmed into whatever the resident microprocessor device might be...

    Again, sorry, I am not an electronic engineer...

    All 3 models are [stupid] pocket scales.
    No need for the HOLD feature, unless the designer is trying to avoid a consumer issue.
    And if so, the feature should be an option, not a fail-safe.

    My 2 cents.
     
  19. colinb

    Active Member

    Jun 15, 2011
    351
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    That sounds like a “max hold” feature. If I understand correctly, the scale is latching onto the greatest instantaneous force applied as the weight to record. I would say this is never the right thing to do, as even the most careful placement of the object to measure on a tabletop scale may result in extra momentary force due to the deceleration as the object meets the scale.
     
  20. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    I looked at the solder blob between R16 and R17. There is a trace connecting those two connections on the board, so you're right, it shouldn't affect operation.

    First, I suggest you get some rubbing or isopropol alcohol and a toothbrush and clean off all the brown gunk (burnt rosin from the solder). Next, take a piece of copper braid and use a soldering iron to briefly heat the board and get the access solder (silver stuff) off.

    This should not hurt the device, but proceed at your own risk: Desolder and remove R16 and test unit. Worst case should be that the scale is off, in which case you can solder R16 back on. Best case you get rid of the hold feature or the rapid response anyway. R16 might be used to help calibrate the strain gage or, if we're lucky, enable/disable the hold feature. I'd have to see the board and make a diagram of the traces before I could more accurately "guess" its function.
     
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