How do I hack a body weight scale?

[ygreq]

Hi guys!

I am trying to figure out how I can interact with 4 load sensors (like this http://manyyear.en.alibaba.com/product/359992876-210312211/MLC902_Micro_sensor.html. I think this might be similar to this one: http://www.sparkfun.com/products/10245

I am actually trying to hack a body weight scale which looks pretty similar to this one (see attached pic or link http://www.dietsinreview.com/diet_column/wp-content/uploads/2009/09/escali-track-and-target-scale.jpg)

I just want to use it with arduino so either just replacing the original board or trying to get the output from the original pcb to use as input to arduino will work for me. Specifically I want to output to serial from Arduino to the computer.

There are loads of info about the subject on the net, but none of it helps as they mostly relate to other prototyping boards.

Here are some links you can find on the net:

http://www.nerdkits.com/videos/weighscale/ this is not ok from the start as it uses a 1 load sensor instead of 4. Really interesting tutorial video, though!

http://cerulean.dk/words/?page_id=42 Another good tutorial! Unfortunately it uses another type of sensor and, again, it uses a single sensor.

http://scanlime.org/2010/01/hacking-a-digital-bathroom-scale/ Really nice hacking project! Unfortunately again, here she uses a different prototyping board and, even though she presents the hack as simple, I have a different body weight scale, so I am a bit confused.

http://code.google.com/p/casainho-projects/wiki/SdCardBathroomScale This body weight scale looks pretty similar to mine! And the hack is really good. From what I understand, he is hacking into the info goind to the original LCD. But again, using different board and lots of cables

Any thoughts? Anybody finished a project like this?

[robtillaart]

Generally there are two ways to hack such a device
1) find some meaningfull signal on the PCB, with a bit of luck such a signal can be found, might need some decode

2) reverse engineer the signals on the display, you probably need many lines to monitor, doable (with 4 shift registers you have enough lines)

Finally you need to find the signal lines that indicate [wakeup] [ready for measure] [measure done] These status lines indicates when the measurement-data makes sense.

[ygreq]

I found this schematic that uses a PIC MCU. It seems to be for a body scale as it uses CS3002 Amp to amplify the load cells' signal.

Can you tell if it is any good and maybe how to change the PIC for an Arduino in this schematic?

1.I don't get why the E- is not present (you can see E+, S-, S+), but instead it is GND (check bottom left corner). Is this normal?
2.Also, as Arduino has the ATmega built in, what do I need from this schematic and should I put aside? I am guessing I don't need the crystal, for example, as it is already integrated in the arduino.
3.What should the code on the arduino look for or measure?

Any help is truly appreciated!
Or if anyone has any other ideas?

[jsk2222]

hey everyone...
ygreq: i am trying to do the same thing but i never found an answer i took the sense wires out and connected them to an instrumentation  amplifier (AD620) and the E+ and E- are connected to a source (whatever the value) and then hooked them up to a multimeter but the value i got is never stable when applying weight and the scale sometimes measures wrong weight(gives higher value)...
Any Suggestions ??
any help is appretiated coz i have  my senior project due in less than a month and i really need this part working

[MarkT]

Can you provide more detail on the circuit with the AD620 you're using - circuit diagram, supply voltage, maybe a picture?

[Edimar]

Hi buddy?
Did you get your digital scale ?

I got a solution using 2 load cells and amplifying the signal (green+ white) in a INA125P.
I´m designing a circuit for 4 load cells.

Did you get or gave up ?

Thanks
Edi

[AboMan]

Hey, I work on scales professionally and I can tell you that  when a scale has more than one load cell, we use a summing card (junction box) to connect them all. There are typically 2 types of cards like this Signal trim, and Excitation trim; the trimming is just potentiometers used to balance the voltage from either the Sig or Exc bridge so that all of the load cells dead load output is within 0.1 or 0.2 mV of each other (rule of thumb, just get it as close as you can) you can then use a single interconnect cable from your summing card to your indicator to represent the weight no matter where it is on the scale.

--Also, a scale for weighing your self like that is typically done with just one load cell anyway, usually a 'single ended beam' cell or 's' type if it has a pedestal, that is attached to a firmament and the weighing surface (with shims at the physical contact points [<-important])

--Lastly, if its possible to check most load cells come with a rating like "3mV/V" etc.. the technical specification that comes with the SPECIFIC load cell usually is a bit more accurate it will say something like "3.23mV/V" or "2.98mV/V" (2.9-3.3,a consistency and closeness to 3mV in this rating cell to cell (if you have a bunch to compare) is a good quality indication of the manufacturers process) --Anyway-- so long as that value for all four are within that 0.1-0.2mV range you don't need anything more than a terminal strip to sum the cell outputs accurately--though it may be tricky getting a decent calibration if these values are too far apart, If they aren't close enough you'll have repeatability issues with your weighments and depending on the extent, a tough time getting a stable count for calibration.