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Guys can you help me how to use a load cell with Arduino. I'm not that new in using Arduino, i already use stepper motor, LCD, RTC in my project. And now i'm about to use Load Cell which is my first time. Can you guys help me.

What IC, resistor or any other materials i will use to use Load Cell in Arduino?
Do load cell measure in kg? Because i need the weight of something.
Example.
example variable weight is the measurement from loadcell

if(weight = 2) //2 is in kg
{
myStepper.step(50);
}

is this code applicable in using Loadcell. Because the function of my load cell is when it detect a certain weight. The stepper motor will move.

I already have LCD, Stepper Motor, RTC in my project. Is LoadCell possible to use when i'm using the Menu Library.?

Can you give me the connections? Thanks.
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Holland MI
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Load cells are typically Wheatstone bridges of 350 Ohm strain gage resistors. So you will need to power the bridge itself with between between 2 and 10 volts. It will use a fair amount of power, more than a battery will give for long, because of the bridge resistance being only 700 Ohms. The output will be in the millivolt range. So you'll need to amplify it by up to 500X, probably with a differential input op-amp.

I'm no Arduino expert, being just gotten into it as an interest. But I've used load cells in my profession for 20-some years. Tell me the make and model of your load cell and I could possibly calculate your requirements.
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Load cells are typically Wheatstone bridges of 350 Ohm strain gage resistors. So you will need to power the bridge itself with between between 2 and 10 volts. It will use a fair amount of power, more than a battery will give for long, because of the bridge resistance being only 700 Ohms. The output will be in the millivolt range. So you'll need to amplify it by up to 500X, probably with a differential input op-amp.

I'm no Arduino expert, being just gotten into it as an interest. But I've used load cells in my profession for 20-some years. Tell me the make and model of your load cell and I could possibly calculate your requirements.

I don't have yet any idea what model of load cell i'm going to use. But i need to measure a weight of 0kg to 0.5kg. Maybe thats the range. Is it easy to use? Because i need to make my stepper move dependent on the weight.
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In your application, will the load cell be subjected to any bending or torsional input? Or will the mechanical arrangement insure that it only sees compression/tension forces?

I ask because you can make your own load cell by super-gluing strain gages onto to a piece of metal in various configurations. Whether or not it will see force in ONLY one direction makes a difference on how to wire it. I have done this many a time. I have made my own torque meter, load cell, etc for the automotive industry. For instance, when you simply CANNOT install a real load cell into the front suspension of a mule car for the test track (either because it won't fit or else because its mass would distort events you want to measure) you strain gage the control arms instead.

Commercial load cells have as many as eight strain gages so arranged as to cancel out torsion and/or bending. But if the design shields (or decouples) extraneous forces from the strained element (beam, tube, whatever) on which the gages are applied, you can get by with four, two or even just a single strain gage.

Then you just determine your calibration factor by simple experimentation.
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Let us take the simplest case, bending beam. Think of it like a diving board. You step out onto it, the top stretches and the bottom gets scrunched. A gage glued to the top would go up in R (from 350 Ohms to 351). A gage on the bottom would go down in R (from 350 Ohms to 349). Wired as opposite legs of a Wheatstone bridge (with 350 Ohm 1% dummy resistors in the other two) the 2-Ohm difference would show a fair difference of Volts. This you measure and amplify.

That assumes you never PULL on the end of the board. That would give a signal too. And the amplifier would not know the difference. Using four active strain gages (with no dummies) you can enhance bending and cancel out tension (or vice versa).

So the question is, what physical arrangement do you want to employ for your sensing element? A beam you can bend? A rod you scrunch lengthwise? Commercial load cells use four beams that bend (microscopically) and eight gages (even though only 4 wires come out). These eight gages are so arranged as to cancel out both bending and torque, seeing only compression and tension. That way they are multi-purpose.

But with clever design for a single purpose, you need not use so many. Just two, or sometimes even one, does the trick. I always used four, myself. But then I was never paying for gages out of pocket.
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There is a store here at our place that sell load cell. So I don't need to make it.

http://www.e-gizmo.com/PRODUCT/ELECTRO/SENSOR.HTM
at the bottom part.

Do you know how to use this? And do you have sample code for this?
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Do you know what IC i will use. Because some people says that i will use this ina125P.

Or do you have any suggestion on other weight sensor that i much easier to use with Arduino.
« Last Edit: November 02, 2011, 06:59:02 pm by janno0718 » Logged

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In the photos of load cells on your link I see four wires: Red, Black, Green & White. Refer to this Wiki on Wheatstone bridges...

http://en.wikipedia.org/wiki/Wheatstone_bridge

Red = Power, Pos (A on the Wiki)
Black = Power, Neg (C on the Wiki)
White & Green = Differential Volts Output (B & D on the Wiki)

Supply your power to Red & Black. Connect White & Green to the + & - inputs of ANY differential input operational amplifier IC (or multi-chip IC circuit). You will get amplified volts out. Get any OpAmp cookbook for examples in differential amp circuits. They are simple enough: one chip, four resistors.

I am not the one to ask (yet) about Arduino code. I know only LabVIEW, Perl & PostScript having forgotten all my Forth and Fortran. Others here will have to answer your code requirements.
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In the photos of load cells on your link I see four wires: Red, Black, Green & White. Refer to this Wiki on Wheatstone bridges...

http://en.wikipedia.org/wiki/Wheatstone_bridge

Red = Power, Pos (A on the Wiki)
Black = Power, Neg (C on the Wiki)
White & Green = Differential Volts Output (B & D on the Wiki)

Supply your power to Red & Black. Connect White & Green to the + & - inputs of ANY differential input operational amplifier IC (or multi-chip IC circuit). You will get amplified volts out. Get any OpAmp cookbook for examples in differential amp circuits. They are simple enough: one chip, four resistors.

I am not the one to ask (yet) about Arduino code. I know only LabVIEW, Perl & PostScript having forgotten all my Forth and Fortran. Others here will have to answer your code requirements.

What value will i get in this load cell? Is it direct weight value? Or like a frequency or waveform that is hard to understand? smiley-sad
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You will get a voltage that varies with the weight on the load cell.

Here's a complete example: http://cerulean.dk/words/?page_id=42
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You will get a voltage that varies with the weight on the load cell.

Here's a complete example: http://cerulean.dk/words/?page_id=42

Sir. can i ask. What variable in the sample program in that site do i need to use if i will compare the weight measurement into certain value.

Example

if(weightmeasurement == 2) 2 is kg
{
myStepper.step(50)
}
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You would need to test your load cell with a known 2kg weight to see what measurement you get and then use that measurement in your code.

The full calibration process would typically entail testing the load cell with a number of different known weights and then building a graph of weight vs measurement -- and then building a function to calculate the weight for any given measurement. I would recommend using a spreadsheet (Excel, etc.) to record those values and build the graph because then you can also use an Excel feature to create a "line of best fit" which will give you your function.
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You would need to test your load cell with a known 2kg weight to see what measurement you get and then use that measurement in your code.

The full calibration process would typically entail testing the load cell with a number of different known weights and then building a graph of weight vs measurement -- and then building a function to calculate the weight for any given measurement. I would recommend using a spreadsheet (Excel, etc.) to record those values and build the graph because then you can also use an Excel feature to create a "line of best fit" which will give you your function.

So its different function for every load cell? smiley-sad
Can I ask a question. Is it easy? Or hard?
Do you have any tutorials how to do it? And where can i view the graph of weight vs measurement?
After I get the function. Its like a formula right? That I will just write in my arduino code to get the amount of weight.

Then the example code that i wrote in my past comment is possible? Right? I hope that its not hard.

Sorry for all the questions. But i'm really confuse and don't know what to do.
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Help please.

I need a load cell even its not calibrated to be accurate.
But i really need to get a weight measurement. That is not in a graph.
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Quote
But i really need to get a weight measurement. That is not in a graph.

First "catch your rabbit".

When you have your load cell and op-amp, etc working nicely, producing a voltage that swings reasonably well, but stays between 0 and 5v (or 0 and 3.3v, if you have a 3.3v Arduino), you're virtually done. (Feed that into an Arduino analog input, display the reading. You can then, with no programming skills, make up an ink-on-paper conversion chart by testing your tool with some known weights. It takes very little programming to put that chart and the look up "inside" the Arduino.

But first you need that "rabbit".
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