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Auto-Ranging Ohmmeter

Arduino auto-ranging ohmmeter using Pro-Mini & Oled display. Live Test: 56K ohm resistor shown.

I wanted to design and build a simple ohmmeter for my electronic test bench instead of using a multimeter. I just needed to quickly measure new and used resistors to sort them for bin storage or to match certain values. The accuracy would not be that important as long as I could guess a standard value, as I quickly tested them.

Then I came across Mirko Pavleski’s Nano unit plus Simple Circuit project These were just what I was looking for.

However, after I built the prototype, a problem showed up while testing known resistor values. It worked accurately enough to sort resistors from 10 ohms to 1 Meg ohms, but for some reason did not work between 120k-190k ohms range?? 100k and 220k read fine. I double checked all the calibration resistor values and wiring and found no issues?

However this problem was resolved on the Arduino Forum by user BOFFIN. Many thanks to him!

After changing the 2MΩ resistor to 1MΩ and the constant table from 2000000ul to 1000000ul in the program the unit performed A-OK

I settled on building with a small case with Oled display and bolt terminals.”

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