Arduino Vs Strain gauge

Community General Discussions
1

Hi all, thanks for looking!

Im building a project (not a robot sorry) which is using strain gauges. I have found this EXCELLENT guide online:

http://www.deferredprocrastination.co.uk/blog/2013/reading-strain-gauge-scales-with-arduino/

Ive built my project up on a breadboard, to see if my shiny new INA125PA works! It appears to, but i have a couple of potentially noobish questions! Ive set my sketch up to read A2 only currently. It is serialprint-ing analogue reads from the Pin, and not doing any fancy pants processing yet.

  1. I cant see in the example sketch what the A5 connection does?! any ideas?
  2. I have pulled the strain gauge out of a set of cheapo scales, therefore have no datasheet. Is there any other more intelligent way i can choose a resistor, other than just trying one out to see? (i realise i need to pick one to provide amplification to suit the weights i will be dealing with)
  3. The RAW analogue read on A2 out sits at around "70". It increases by loading the strain gauge (therefore decreasing the resistance?). Is there a maximum limit to what it should be? i presume 1024 is 5V? can i overload the arduino via this pin and damage it? is there any protection i can put in or is just a case of sensible resistor selection on the amp?

I look forward to getting this project built up and posted for y'all to admire ;-)

2

suggestion for question

suggestion for question 1:

having had a look and a think, A5 is connected to the reference voltage, therefore it can be used to check that the reference voltage hasnt drifted perhaps? (reading it alongside A2, it gives a steady 1023…

3

Since you didn’t mention which example code you are using,

telling you what it is doing is difficult.

https://github.com/DefProc/ArduinoInstAmp/blob/master/examples/ReadScalesToDial/ReadScalesToDial.ino

has no mention of A5. The other sample code only mentions A2 and A3.

As to overloading, don’t connect the pin to -V or anything over 5v+. I am pretty sure either one of those would damage the pin, if not the arduino. 

4

Hey Ladvien. I think ive got

Hey Ladvien. I think ive got it going! I bought a new set of scales, one with a “half bridge” on each corner (which annoyingly added up to two bridges).

As you suggested Oddbot, ia have wired two of the four half bridges “backwards”, so one pair increase, the other decrease with load, and then wired it up accordingly.

I cut up the scales (the strain gauges in the feet had ready moulded little holders, which i have just tacked on with double sided stickers), and have spent the evening trying to reverse engineer the wires. Ive got it reliably delivering an accurate value, which doesnt vary whether the load is in the middle of the scales, or over one corner!

Also, overloading it just pushes the Analogue read right up to 1023, so i think it must be limited by the ref voltage! (so when powered by arduino, it cant damage itself)

not pretty, but the extended wires should hold on pretty well under all that packingtape, and that due to be face down anyway!

My setup is for a max of 3.5kg currently, though i may raise that to 4kg  (im having to use a whole stack of resistors in series to get it where i want it.

really pleased with this evenings progress on the mechanical side of things, its onto code next, and possibly moving my breadboard build over to the protoshield (it is destined to be a data logger after all)

 

Once ive got something to look at, ill do a full “something else” write up.

 

Thanks for your help guys