Strain Gauge Scale: Instrumentation Amp Noise Help (Noob Q)

Hi, and thanks in advance for the help.
I'm an electronics noob who's getting a lot of noise in a circuit and would appreciate some help de-noisifying it, please :slight_smile:

I had the idea to build a web-enabled bathroom scale as an Arduino project. I ripped apart a cheap digital bathroom scale and gutted the electronics, leaving me with 4 pairs of strain gauges. I have taken 2 pairs of strain gauges and arranged them in a Wheatstone bridge, as per this diagram from http://www.allaboutcircuits.com/vol_1/chpt_9/7.html

Next, I built an instrumentation amplifier as per this circuit from

I used an LM324N quad op-amp with a single-sided supply for this. My R values are 200k?, and my gain resistor is 500?. Both the bridge circuit and the in-amp circuit are driven from the same regulated 5V supply.

I get the amp to do its thing in a useful output voltage range, but the signal never stabilizes to the point where I'd consider the information reliable input to the Arduino's analog input. I'm sure I need some filtering and/or shielding, but not sure how or where to put this.

All advice appreciated!
Thanks, Carl

LM324's are not exactly the cat's meow for this project -- instrumentation amplifiers generally carefully choose op-amps for low input noise, low input current, very low offset voltage, etc. You also have to carefully match your resistor values (are your 200k resistors within 0.1% of each other?) else you'll get a lot of common-mode amplification and other stuff.

Better to buy a dedicated instrumentation amplifier chip :slight_smile:

For now, you can try running your strain gauge leads over a twisted pair of wires to your circuit, keeping the twist as long as possible up to your op-amp inputs.

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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected