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Topic: Signal amplifier (Read 197 times) previous topic - next topic

tommynego95

Hello all,

I' m working on a project which involves a load cell.
I do own an HX-711 amplifier but for a firmware purpose i need an analog output instead of the digital one supplied by the board mentioned before.
I googled some signal amplifiers but i'm not really sure they will work with the load cell i have.
Can anybody suggest me a good and cheap breakout board or any other solution to amplify the signal from the load cell and have an analog output as result?

alto777

#1
Feb 22, 2021, 02:52 pm Last Edit: Feb 22, 2021, 02:54 pm by alto777
You need an instrumentation op amp.

"using instrumentation amplifiers like AD623 is a good choice which is not complex in design"

someone once said.

Ggoogle

instrumentation op amp module

I found several breakout boards available.

But it looks hard to beat just using the hx711 as intended, why can you not?

a7

tommynego95

I 'm using an .hex file loaded into Arduino and i have no access to the source code hence i cannot modify the program input pins that are set as  the Analog pins.

Thanks for your help!

alto777

OK, I see your circumstances. So what I said before, google is your friend. Instrumentation opamp, be it on a breakout or roll your own.

https://forum.arduino.cc/index.php?topic=280109.0

might be of interest to you.

HTH

a7

johnerrington

If you know the range of output voltage from your load cell its very easy to build a simple amplifier.


I understand you are not able to use a different interface eg i2c or spi, you need an analog voltage in.
I'm not aware of any ready made modules that do that.
I'm trying to help. If I find your question interesting I'll give you karma. If you find my input useful please give me karma (I need it)

raschemmel

Quote
I 'm using an .hex file loaded into Arduino
How does that work ? Why aren't you using a .ino file ? Do you mean you don't have the source code ?
Sorry for the dumb questions but I've never heard of someone using a hex file for arduino but I think I
once read about someone using a decompiler to convert a .hex file to arduino source code. Not sure if
that's what you are doing.

tommynego95

Thank you for your replies!

I think i will go with the "DIY" version of the amplifier.
I attach the electric scheme down below, could you please check if it is correct?
Mind that i would like to use INA122PA, an half-bridge load cell as input (A+ is the central wire, E- is the negative strain, E+ is the positive one), a potentiometer to set the gain and the capacitor to reduce oscillation.
I' m not really sure about the load cell connections to INA122PA tho.

@raschemmel it' s an already compiled code that can be loaded directly to Arduino using XLoader for example. I downloaded the file as .hex from a Youtube guy. I read something about decompiling an hex file and they said that was really difficult and time consuming hence I didn't really investigate further. I' ll go check out if i can find something about it.


JohnRob

If you change your mind you might look at this board  Instrument amp board
Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

johnerrington

#8
Feb 24, 2021, 07:32 am Last Edit: Feb 24, 2021, 07:33 am by johnerrington
I believe what you have is a half bridge load cell.

In which case E+ and E- are the EXCITATION voltages so this

Quote
(A+ is the central wire, E- is the negative strain, E+ is the positive one)
would not be correct.



E+ (normally white) goes to +5V; E- (normally black) goes to 0V.

A goes to Vin+

Vin- AND Vref go to 2.5V (resistive divider 1k to +5, 1k to GND.)

and on the IA +Vs goes to +5 and -Vs goes to GND.

I'd use an AD623 Instrumentation amplifier; the gain is quite critically dependent on Rg so you will need to play about with that value to get the range and sensitivity you want.

I'm trying to help. If I find your question interesting I'll give you karma. If you find my input useful please give me karma (I need it)

tommynego95

#9
Feb 24, 2021, 08:58 am Last Edit: Feb 24, 2021, 10:13 am by tommynego95
Maybe the circuit diagram image was not really clear but the connections are the same as you said.
As central wire I meant the wire which has the highest resistance value, in my case 2kohm, usually colored in red.
So the central wire, orange in the image, is connected to Vin+, red wire that rappresent E+ (White in a real life environment as you said) is connected to Arduino 5v while the E- is connected to GND.
I could switch to AD623 if you recommend it, the price tag seems the same. Could i ask why tho? Reading the datasheet seems that AD623 has a worst gain range (1-1000) than INA122 (5-10k).
Thank you for the suggestion

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