 # Hooking up a load cell.

Having not much luck in project guidance after creating a thread on this subject, I figured I bring it toward this sub-forum. The following question continues from my question at the end of the thread. Hope someone can help!

I have looked at the pdf and just would like to know how the mapping of the amp translates better and how it actually applies with hooking up my one load cell to the Arduino. To my understanding of the diagram, +V is the gnd line coming from load cell and -V is the continuation of that to the Arduino. +In and -In are the inputs, but my load cell only has one ongoing instead of most load cells two. Output, goes to analog pin on Arduino. Should REF just be grounded? Should a 300 ohm resistor be fine between the gain (RG on the pdf)?

You need to post all relevant information here, in this thread.

For a start: The rating is 1kg = 1mV/V. The no-load voltage is 1/2 of the driving voltage. So if you feed it 5V, the output is 2.5V offset by 5mV for every kg of force.

stef100111: Having not much luck in project guidance after creating a thread on this subject, I figured I bring it toward this sub-forum. The following question continues from my question at the end of the thread. Hope someone can help!

I have looked at the pdf and just would like to know how the mapping of the amp translates better and how it actually applies with hooking up my one load cell to the Arduino. To my understanding of the diagram, +V is the gnd line coming from load cell and -V is the continuation of that to the Arduino. +In and -In are the inputs, but my load cell only has one ongoing instead of most load cells two. Output, goes to analog pin on Arduino. Should REF just be grounded? Should a 300 ohm resistor be fine between the gain (RG on the pdf)?

If you have a three-wire strain guage then you will need a high accuracy resistor divider to provide the other arm of the bridge. That means 0.1% or better metal oxide resistors I suspect, or a trimmable network of high quality resistors. Then you route the strain-guage to one input and the mid-point of divider to the other. A separate, less critical, divider is needed to drive the reference input.

The output value is the voltage difference between amp's output and its reference.

Read up about instrumentation amplifiers for more insights. Search for examples of the chips use.

I need to ask stef10011: do you understand how load cells work?

Please describe to me how you understand them. What do you expect to get directly out of a load cell?

And what is the maximum weight you'll put on this?

My bad. I am using one of these to have a weight activated alarm. Apparently the AD620 is best for this. I am using it for a school project due soon, I chose this topic without much background at all in electronics but have tinkered with an Arduino in a club at school. I just would like to know how to set this up, as I have seen tutorials for many 4 wire strain gauges but not the three wired one like I have. It would be great to be able to see a diagram of how I can relate the load cell, amp, and Arduino together, I can understand the mapping of circuit diagrams.

My understanding is the while under strain, the load cell measures the change in resistance. This measurement is initially very small to my understanding and that is why an amp is needed.

MarkT: If you have a three-wire strain guage then you will need a high accuracy resistor divider to provide the other arm of the bridge. That means 0.1% or better metal oxide resistors I suspect, or a trimmable network of high quality resistors. Then you route the strain-guage to one input and the mid-point of divider to the other. A separate, less critical, divider is needed to drive the reference input.

The output value is the voltage difference between amp's output and its reference.

Read up about instrumentation amplifiers for more insights. Search for examples of the chips use.

Since I am very much a visual person, I would assume you mean something like this when you speak of a resistor divider. Yet how will this relate to the amp, would this just be set up before feeding it into it? Even a crude drawing in mspaint would help, things are much better explained visually for me.

I am sorry but as more of a beginner on this side of things visuals do really help, I know I am asking for a lot. However I appreciate any guidance given on this project.

Do you know what a Wheatstone Bridge is? In this diagram, R2 represents a sensor of some kind that changes resistance in response to whatever it is meant to measure. The amount of change is often only a tiny fraction of the total resistance and may be difficult to measure. So a second voltage divider is set up and the difference in voltage between the two voltage dividers is measured, thereby subtracting the offset created by the total resistance. A differential aka instrumentation Op Amp is connected in place of the voltmeter. In this way, the DC offset is removed.

What you have there is known as a half-bridge, it has two identical strain gauges inside. Typically, they are connected internally one of two ways:

1. One gauge is compressed while the other is stretched when weight is applied. 2. One gauge is unstressed and the other is stretched when weight is applied. The reason for two identical gauges is to compensate for resistance changes due to temperature.

So you need to add two resistors to make another voltage divider. Preferably with a trimpot in between to adjust the voltage to match the voltage from the load cell when it is at 0kg stress.

You don't need 0.1% resistors or low thermal drift resistors. They should just be the same kind of resistor, equal value, with a much lower value trimpot between them, with the wiper of the trimpot going back to the instrumentation Op Amp.

I prefer not to make one resistor variable, as then its temperature drift won't match the resistor. With the trimpot, the resistors will swamp any changes in the trimpot, and both halves of the trimpot will match the temperature drift of the other half.

I have seen the Wheatstone bridge before, yes. Thank you for the pictures, some things are clearer. However, using the information given though I would like to see the bigger picture of how I can connect my load cell to this amp to get a readable measurement. Sure, I could set up the resistors and make a voltage divider to go back to the Op Amp, but into which pins? (Pdf for reference: http://www.analog.com/static/imported-files/data_sheets/AD620.pdf). Also since my load cell has 3 wires, in many tutorials for hooking them up to amps aren't useful because I am not sure if there is both a sense and excite +/- or just one of them, since 4 wires are normally required for such a transfer. One of the comments is "White to 5v, Black to gnd, Red to Input on OpAmp".

Essentially, what is the big picture of this, with the one load cell, resistors, op amp (AD620), Arduino? For example, where would the placement of a said voltage divider go (sorry if this is obvious)?

Thanks for all input so far, as a noob I appreciate it. I just am really wanting to know how it all comes together. :)

Right. White to 5V, Black to ground. Now you'll see 2.5V on the Red wire, which will vary with the pressure.

With the other parts I described, you are turning a 3 wire load cell into a 4 wire load cell.

So now you need to figure out how much gain you'll need.

The datasheet says 1mV/V per kg. Meaning that for each kg of pressure, 1mV of change in voltage per V applied between the White and Black grounds. Therefore for 5V, that is 5mV per kg. For full scale 50kg, that is therefore 250mV total change.

So you want to amplify that to 5V/250mV = 20.

The trimpot I described above will serve to zero the output of the Op Amp.

You can simply buy an instrumentation Op Amp, and choose Rg for a gain of 20.

Or build one from 3 Op Amps: It is -much- simpler to just use an instrumentation Op Amp like the AD623: AD623

polymorph: Right. White to 5V, Black to ground. Now you'll see 2.5V on the Red wire, which will vary with the pressure.

With the other parts I described, you are turning a 3 wire load cell into a 4 wire load cell.

So now you need to figure out how much gain you'll need.

The datasheet says 1mV/V per kg. Meaning that for each kg of pressure, 1mV of change in voltage per V applied between the White and Black grounds. Therefore for 5V, that is 5mV per kg. For full scale 50kg, that is therefore 250mV total change.

So you want to amplify that to 5V/250mV = 20.

The trimpot I described above will serve to zero the output of the Op Amp.

You can simply buy an instrumentation Op Amp, and choose Rg for a gain of 20.

Or build one from 3 Op Amps: It is -much- simpler to just use an instrumentation Op Amp like the AD623: AD623

Using the AD623, would it be a lot more simple to hook up the load cell? Will I still have to create the whole setup previously stated?

I am looking for the most simple way to do this, basically as close as I can get to Load cell to amp to Arduino without too much addition.

Yes, the AD623 will make it much simpler, because it has all that built-in. You just need to figure out how much gain you need, and select Rg based on the requirements of the AD623.

AD623 Datasheet

polymorph: Yes, the AD623 will make it much simpler, because it has all that built-in. You just need to figure out how much gain you need, and select Rg based on the requirements of the AD623.

AD623 Datasheet

That helps a lot. So if I go the AD623 it would just be knowing what ends of the load cell to put where, what gain to set, and then knowing what goes out to the Arudino then? That sounds a lot better, thanks so much. While looking at the AD620's datasheet and this one, I was wondering, what exactly does Rg stand for?

Rg is the gain set resistor. Read that data sheet carefully… a copy of Lancasters OP-Amp Cookbook although dated… is a most handy reference.
This from Analog Devices is also a must: AD623 App notes From Analog devices
IHTH

Doc

Docedison: Rg is the gain set resistor. Read that data sheet carefully.. a copy of Lancasters OP-Amp Cookbook although dated.. is a most handy reference. This from Analog Devices is also a must: AD623 App notes From Analog devices .. IHTH

Doc

Thanks very much. If I can know exactly how to piece this together it would be great. Call me stupid and say I can't read a diagram, but the letters don't help to much when knowing what to do with them as a beginner. -VS? +VS? -I? Etc. Explaining what these stand for would be a good start.

-Vs is a negative supply voltage, +Vs is the positive supply voltage.

I can't teach you all that you need to know in a few messages in a forum.

Do you know Kirchoff's voltage and current laws? Ohm's law? Do you know how Op Amps work? The difference between linear and nonlinear, active and passive? Do you know what a rail to rail Op Amp is, or what the difference is between RRI, RRO, and RRIO? Or why rail to rail Op Amps are not always the correct solution?

If all you care about is one class project then using an all-in-one amp is a good solution, but if the reason you are taking the class is to learn electronics then sooner or later you need to become acquainted with the op amp. There are so many applications for the op amp I couldn't begin to touch on them but as an example, I bought 166 LM-307 op amps (normally \$3.95 at that time) for 25c each on sale. Thirty years later I still had 60 left (which I gave away to an electronics student). It would take too long to list all the different circuits I built with them. Currently, my preference is the LT1215 for arduino circuits because it is designed especially for 5V circuits but can still be used for other applications and voltages.

raschemmel: If all you care about is one class project then using an all-in-one amp is a good solution, but if the reason you are taking the class is to learn electronics then sooner or later you need to become acquainted with the op amp. There are so many applications for the op amp I couldn't begin to touch on them but as an example, I bought 166 LM-307 op amps (normally \$3.95 at that time) for 25c each on sale. Thirty years later I still had 60 left (which I gave away to an electronics student). It would take too long to list all the different circuits I built with them. Currently, my preference is the LT1215 for arduino circuits because it is designed especially for 5V circuits but can still be used for other applications and voltages.

I just need it for this one project and how to correctly use it with the load cell and Arduino.

Can anybody help? I have been trying different ways, and I do not know how to get the Arduino to read the 3-wire load cell. Which wires would go where, I have tried everything? Once again, here is the load cell: https://www.sparkfun.com/products/10245

I asked a few questions:

http://forum.arduino.cc/index.php?topic=280109.msg1980821#msg1980821

Here is a video telling you how to use this sensor. They use a second sensor to get a voltage divider, you can just use resistors and a trimpot as I suggested if you don't want to buy a second sensor:

http://youtu.be/Bm6ANv93YjM

What different ways have you tried? How can we help you if we don't know what you've done and the results you've gotten? Which IC are you using? What gain resistor did you select? How did you wire it up?

Schematic, please.

I don't answer questions like this in private email.

Yes, I am only using one load cell. No, I do not know the laws and other questions you had asked. I have three AD623s, as suggested for use.

I don't know what program I can draw a schematic with, but I can write it. I tried all three wires of the load cell as -IN and +IN... I have two 10K resistors for Rg, and REF is grounded. +Vs is from the 5v on the Arduino analog side, and -Vs is grounded. Output going to Analog Pin 3 on the Arduino and I used the open-source serial read code for testing. What is the deal with the trimpot? Using the AD623 I thought you had stated I did not need to use it.

Since you only have one load cell, you only have one half of a Wheatstone bridge. The two resistors and trimpot are to match the output to the load cell when it has no weight on it.

Well, you -should- know Ohm's Law (for a linear resistance, current is proportional to voltage and inversely proportional to resistance, ie, I = V/R) and Kirchoff's Laws of current (the sum of all currents in a node is zero, ie, as much comes out as you put in) and voltage (the sum of all currents around a loop is zero, ie, for every voltage adding as you go around a loop, you have the same voltage subtracting).

Better to watch some Youtube videos on that.

As for schematics, draw them with a pencil on paper.

What class is this that has you building a digital scale but you don't know the most basic laws of electronics, or how to draw a schematic?