After much curve fitting and data analysis, I decided that the fluctuations in the readings were too much, and i had to find another way of fixing things.
I had previously made up my mind that the attiny board i made was the source of the error, not the scale itself. After working on it one evening, I saw the readings move quickly, then stabilize after i put the unit back in the garage (in the cold) from the nice warm house. The scale stayed out there the whole time.
So i figured it was the resistors changing and affecting the gain on the amplifier.
I ran some calculations and determined that the gain should not have changed anywhere near what it did in reaction to the temperature fluctuations, and the changes in resistance that might bring. So that leaves one thing: the ADC unit on the attiny85 itself.
I have been using the 1.1V reference voltage on the attiny as the basis for the readings. It’s a ‘bandgap’ based thing, which sounds fancy and accurate, but let’s give it a test. To test it, i got a 5k potentiometer and put the end pins on +5v and GND, and the wiper was set so it read about 1 volt. I connected the 1volt output to a spare pin on the attiny, and started taking readings to see if it would exhibit the same pattern as the scale.
About a day later, i’m pleased to report that the constant voltage varies EXACTLY the same way the scale does.
After some analysis and curve-fitting, the coefficient is -1.0, which means that the error in the voltage reading is exactly the same as the scale reading, just opposite (which is to be expected, since i am storing the scale reading as 1024 – raw_reading). So it’s basically a matter of adding them together then subtracting an offset.
As you can see, it works really well, and the total range is just 2 ticks!
Also note that I was able to use a potentiometer (could easily have been 2 resistors) as a MUCH more effective temperature sensor than the internal temperature sensor of the attiny itself. That is somewhat sad.
For extra accuracy, i may someday try using an external VREF input, and see if it’s the ADC itself or if it’s the bandgap voltage moving around. For now, this is good enough.