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How to interface external load cell with ATMega256rfr2 evaluation board

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Prashant_2102

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Hello,

I am sorry if this is a very basic question...

I am using the ATMega256RFR2 evaluation board for prototyping one of my projects..

What I want to do is to transmit a weight captured via a load cell over the zigbee protocol to the coordinator.

What I have done so far is below:
1) I have two evaluation boards
2) One I have configured as coordinator and one as device
3) I can transmit the temperature using the inbuilt temperature sensor in the board from the device to the coordinator successfully

Now what I want to do is this:
1) Interface a load cell with one of the ADC pins in the board and transmit the weight (like temperature) to the coordinator

My questions are this:
1) How do I interface a load cell with the evaluation board ADC. Can the load cell be connected directly or do I need to create some circuitry for doing that.
2) Do I need an external amplifier or not? (There seems to be one inbuilt if differential input is used)
3) Which load cell should I choose (are there any voltage/current specifications).

Any help on this will be highly appreciated...!

Thanks,
Prashant
 

KlausST

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Hi,

3) Which load cell should I choose

This should be your first question.
It depends on your needs. Load cell range, excitation voltage, output voltage, linearity, resolution....

There are several schematics on how to do the signal conditioning for load cells.
Check wich one best fits to your needs.

Klaus
 

Prashant_2102

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Hi,



This should be your first question.
It depends on your needs. Load cell range, excitation voltage, output voltage, linearity, resolution....

There are several schematics on how to do the signal conditioning for load cells.
Check wich one best fits to your needs.

Klaus

Hi Klaus,

Thanks for the reply..!

I am a newbie so having trouble figuring out these details..

Basically I am building a home automation system... I am interested in making a wireless kitchen scale sort of thing..

so, my load cell range is 0-5/10 kg...

I am not exactly sure how to pick what should be the excitation voltage, output voltage, linearity etc..

Basically I have to connect this load cell to a 10 bit ADC in my evaluation board with reference voltage of 0/1.5/1.6 V with 1.6V being the most accurate (as mentioned in the datasheet).

Based on some internet research I think I want the 4 wire load cell which I believe has the wheatstone bridge in-built, and to connect this to an instrument amplifier which will then be wired to ADC..

So, if you could help answer my questions about which load cell/ amplifier to use I will be very grateful..

BR,
Prashant
 

KlausST

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Hi,

i´d go the oher way round. You need a load cell for 5...10kg.
Look what´s available in that range.

I there are a lot to choose from,i´d choos the one that is cheap, is good to applicable, good precision, has the highest output signal...
What ever is more important to you.

After choosing the load cell i´d design the amplifier.

*********
I´d go this way, because i think it is more difficult to find the right load cell, than to design the amplifier.

Klaus
 

Prashant_2102

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Hi Klaus,

I figured the load cell.. It is 3133_0 - Micro Load Cell (0-5kg) - CZL635
https://www.phidgets.com/products.php?product_id=3133

It has 1mv/V rated output..

Now I need to amp this up before plugging to my ADC.. I want it to be in range of 0-5V..

Now I found two amps which should work for me..
TL072 op Amp
LM324 Quad Op Amp

But I am not sure whether these will work with my load sensor or not... or how to choose between them..

BR,
Prashant
 

KlausST

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Hi,

well done.

****
with 5V excitation you get a maxoutput voltage of 5mV.
If you want it to be amplified to 5V then you need a gain of 1000.

for good quality (with a single stage amplifier) you need a DC gain of 100 x 1000 = 100 000.
And you need low temperature drift.

... To your application: what precision do you need, what resolution do you need, how often do you need a new value?

This give the new parameters for the amplifer.

***
TL072 is a standard amplifier with FET input stage.
But it has relatively high voltage noise, low bandwith, and worst of all a lot of offset voltage and temperature drift.

LM324 is also a standard OPAMP, but with bjt input stage. I sould have less noise, but it is not given in the ti datasheet.
Offset voltage, drift and gain are not good for load cells.

It depends on your needs if you are staisfied with the results.

*******
load cell:
it has 5mV for 5000g, --> 1uV/g (nom. @ 5V excitation)
zero balance: 75g --> 75uV
drift: 0,5g/°C --> 0.5uV/°C

--> OPAMP
if you want the amplifier to meet this specification you need a more precise OPAMP, maybe use a two stage (gain of 30 each).
for extremely low ofsset and drift look for "chopper stabilized" OPAMPs.

*****
another point is thermocouple effect.
At each joint of different materials (wire, connector solder...) there will be a small DC voltage. It increases with temperature.
This causes an error. If this error is too high for your needed precision consider AC excitation. But it is way more difficult ...


Klaus
 
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