A lot has happened since the last log. I've completed the giant meter, made a 3D printed circuit holder, written some code, and thoroughly tested the unit. I'll be posting the files in a git repo soon, but for now enjoy this video of the project in action.
I decided that it would be nice to have a large analog meter to show deformation of the rock as well as a graph. Sadly, I got a bad screen from SparkFun. (I did try re-flashing the serial backpack and even that didn't fix it.) In the mean time I used a servo and make a proof of concept demo. See below!
Today I hooked up an old granite cube that is about 5 cm on a side with 4 strain gauges to the HX711 board. I modified the example sketches to just output the raw ADC readings (since I am not trying to weigh something right now). I squeezed the rock as hard as I could 4 times and recorded the data at about 4Hz. Graph below.
I did notice that when I moved the cube around, the DC offset of the signal would change, but that's because it has a 1.5+ meter long cable with no shielding. I think I'll build another block with 8 strain gauges (to double the sensitivity) and a shorter cable. I think I'll just have 4 binding posts on the case to connect any block/load cell. Should also have knobs for DC offset and scale unless I can think of ways to have the Arduino cleverly scale the graph that will be displayed on the LCD.