Scale hack with LittleBits Arduino, INA128 amplifier, firmata reading?

Hi, I am using the strain gauge from a kitchen scale and amplifying the signal by using INA128P amplifier and breadboard set up. (Using these instructions). I am using Arduino 1.05 version, and uploaded standard firmata. When I run the firmata test program and then put a load on the sensor, my analog input A0 does not vary. Presuming I have this wired correctly to the breadboard, can you help me understand why I am not seeing the A0 input vary when I put a weight on the scale or how I might troubleshoot? I have hacked the WireBit In/Out connector and used that schematic as a reference.

This is what I see:

Besides testing another load cell, any suggestions?
Here is the set up:

Breadboard from scale to Arduino board ampifying signal with INA128P:

I am thinking this ultimately has to do with having a 3V scale talking to 5V little bits but am unsure how to solve that. Thanks

Hi hilary,

It looks like you have the instrumentation amplifier connected properly.

I guess the first thing to do would be to confirm that the output of your instrumentation amplifer is producing voltage. Can you place a number, led, or bargraph module between your hacked wire module and the arduino module? This will give you instant confirmation of whether the analog circuit is working.

Are the wires on your load cell labeled the same as in the link to the instructions that you posted?

Best,

Geof

Hi Geof! Thanks for getting back to me. Yes, I have taken the number module and inserted in between the wire module and the arduino module. It shows .3 volts (sometimes .2 and .1). Do you think that I need to calibrate the arduino module now to accurately represent the weight? The wires on my load cell differ from the posted instructions. Green is S+. White is S-. Black is V-. Red is V+.

And to confirm, I am sending 3 volts through the system using a dimmer set at 3v. The scale is operated by 2 1.5v batteries (3v). Anything come to mind knowing that too?

Thanks for the help!

Hilary

Hmm, ok. So does this mean that when there is no
weight on the scale, the number module say 0.3V, and when you put weight
on the scale, the number goes down to 0.1V?

If yes, then I recommend exchanging the two signal wires from the load cell to the instrumentation amp. Hopefully that will drive the output in the positive direction under load.

If no, then I guess I would expect that when there is no weight on the
load cell the number would be small, and when there is weight on the
load cell the number would get bigger, or vice-versa. In any event, if
putting weight on the scale does not change the display of the number
module in a predictable way, then either the load cell or the
instrumentation amp is not working as expected.

If on the other hand the number is predictable but only moves a very small amount, then probably you can adjust for that in software.

Oh wait! I see it now! You are using a 1meg resistor. I didn’t see that in the photos before. The gain of the instrumentation amp is inversely proportional to the value of this resistor. Try something much smaller, like 100 ohms or 10 ohms. 10 ohms is recommended on the web page you referenced.

Geof

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OK, perfect. I will try this shortly!! Hilary

Geof, I changed the resistor to a 10 ohm 1/4 watt resistor and now it works! I will get something smaller and hope that will give me better sensitivity. Thank you!!

Hilary

wow, i want to do such thing to my digital kitchen scale according to your method.