I was wondering if I could get a little electronic help with load cell type proj

Using Arduino General Electronics
1

Hi, I must really apologize in advance here, I have very limited knowledge of electronics. I would like some help if possible.

So I have a project I am working on that uses Arduino to measure the relative stance of a user standing on a platform.

I took apart a scale they were selling as an open box item for like $10 and this yielded 4 load cells, or should I say half load cells. They look a lot like the one here: Load Sensor - 50kg (Generic) - SEN-10245 - SparkFun Electronics. They have 3 wires and not the standard 4.

So, I want to be able to use them to get some input. Accuracy is not as important. I plan on putting one load cell on one side of the platform and one on the other. I just want to be able to detect if the user is leaning forward on it or backward really.

So from what I have gathered:

The "load cell"s I have are actually half wheatstone bridges and I need to complete them somehow. How would I go about doing that?

I am guessing but not sure that their input is about 5v Is there any way to find this out with a multimeter? The scale I got it from was running on 2 3-volt solar panels but I really don't know.

So ok I've got 2 barriers I think.

  1. I probably need to complete the wheatstone bridges for these and I don't know what resistors I would need. How would I find this out and how would I set it up?

  2. As far as I know there are 2 options for me - I could go with an instrument amplifier chip or I could somehow reduce the analog input reference to 1v. If I did that I would need probably a resistor for this. How would I know how many ohms I would need?

I would be definitely be grateful for any kind of insight into any of this. :slight_smile:

Thanks...

2

Just make a wheatstone bridge from the two halfs - then the output voltage polarity
will directly encode the forwards/backwards difference.

As with all load cells you need a good instrumentation-amplifier chip to boost the
voltages from microvolts/millivolts up to sensible levels. Typical gains needed are
like 500 or 2000, no way will using a 1.1V reference be enough!

3
  1. If you look at a picture of a wheatstone bridge and then chop it in half, your strain gauges represent one half. You basically have three wires separated by two resistors. Use an ohmmeter and compare the resistances between the wires -- that will give you the answers you need.

See http://kegmonitor.webs.com/letsbuildit.htm for one example of how to combine two of the gauges into one full bridge. No guarantees your wire colors are the same.

  1. If the full bridge is in balance your signal wires will each read 1/2 your excitation voltage. So if you use a 5V excitation voltage they will read 2.5V, or over your 1V analog reference. You wouldn't want to do that :wink:

The ADS1231 is a popular choice for load cells, but in your case you might want the ADS1232 since you'll have two bridges. It's relatively simple to interface and you can find code examples posted on the forum, or here's a library for the ADS1231. These amplifiers have a digital interface and provide 24 bit resolution -- much higher than the 10 bits of the Arduino.

4

One of the sides is the strain gauge, the other side is identical but does not change, it is there for temperature compensation.

http://www.proto-pic.com/Resources/Application%20Example%20of%20using%20the%2050kg%20Load%20Sensor%20PPSEN-10245.pdf

Also see the half-bridge here:

You will want to put a trimmer potentiometer between R1 and R2, with the wiper connected to VCH(-) to trim it to zero. Then you'll need an Op Amp wired up as a differential amplifier, or an Instrumentation Amplifier, which is an Op Amp wired as a precision, trimmed differential amplifier.