Load Cells - Help :(

Other Hardware Sensors
1

Hey, my names is Alberto.
I’m currently working on my high school thesis. My deadline is in 2 weeks and i’m having some trouble.
I’ ve bought this scale (http://www.amazon.it/gp/product/B0033XCC34?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00) and the INA125P needed. I’ ve disassembled my scale and i’ ve found 4 load cells, each one with 3 wires in output (VCC, GND and data, i think). I think each one has its job: E+, E-, S+, S-. We have 2 more wires for power and 2 more for switching from kilos to libre.

I need to understand how to connect, or eventually, disconnect the cables from the load cells and decide the gain of the IC. I have to read the value from my Arduino pin, and then calibrate it via software.

Could you help me?

Thank you in advance for your attention. The attachments explains better.

Best regards,

Alberto

2

Try to figure out the internals of the load cells, they may be half bridges (as you think) or quarter bridges + shield. Then wire each one to an INA125, get and sum up the analogous values.

3

Have you tried Google?

With three wire load cell there are about 1,000,000 hits to choose from.

Include Arduino in the search, and the number drops to a manageable 57,000.

4

Don't use an INA125P; use an HX711 module -- it's cheaper and much more precise. One module (~$2) will handle two load cells.

5

I don't know about your load cell, but for general info, a while back I bought this scale at Walmart and recently took it apart, to find exactly the same load cell as shown on the phidgets page - and where you will find a lot of technical info.

http://www.amazon.com/gp/product/B0049I8UEC/

Looks like all you need is a fairly simple bridge circuit, don't really need their $90 board,

6

This is my situation: http://i59.tinypic.com/24q67hj.jpg

Am i wiring all right? The orange cable is the one which is going to Arduino's A0; the greens are from the 2 load cells (They are soldered inversely: First + soldered with second GND and viceversa), right?

  • How to decide the gain?

  • How should i power the cells? The cables are soldered together, no GND and VCC...

7

I have some answers:

In the picture, we have 2 capacitors. Should i put them? Or i can do without?

Wiring all, the DATA cables returns 1 mV. If i go on the scale, i have 5 mV (for 70 kilos, it is nice that we have that shift, right? This if i only connect the VCC of the load cells on the + 5 V of Arduino

How much gain?

Why i should take the supply from the IC’s pin 4 - 15? Doing this (and not with 5V of Arduino), the DATA cables return bad signals.

The output of the IC (INA145P), we have big swings (for the reading with a multimeter, + on the cables and - on GND, right?)

Photos: Imgur: The magic of the Internet

Thank you again in advance,

Alberto

8

No one?

9

A schematic would be much simpler to decipher.

No, each one does not do a different job. All have Vcc and Ground, then analog voltage out. Connect them all in parallel, then it averages the outputs.

I agree with Chagrin, get an HX711. It has programmable gain amps and a 24 bit ADC in it. Made just for this. It isn't really made for two load cells, as the two inputs can't be programmed for the same gain.

Use the output of the load cells for S+, then you'll need a voltage divider that you can trim to provide S- since your load cells aren't 4 wire.

It is not a trivial task using an INA125P. And the Arduino's ADC is only 10 bit or about 8 or 9 ENOB, plus you will be biasing it at 1/2 of Vcc so you'll get half that resolution. Better to use the HX711.