I'm sure a nine stage comparator could be built. Just a little beyond my skill level. That's all.
You can tie the two comparator outputs to each other as a wired-or, because they are NPN open collector outputs. Then when both outputs are in the off state the green LED can light, but when either output is low the red LED will be brighter and the green LED will be off.Like I said, I can't figure out how to get three states (three LED's). I can (and have) draw a two state tester. In the circuit below Open or Short you get a red light. Only when the resistance is between the upper and lower limits will you get a green light. And yes, you can use two 10KΩ pots for the limit controls.
I used a 9V battery and calculated for a current of 10 mA through each LED. Resistor values shown are exact requirements. Use of very close standard resistance will yield good results. Instead of the 700Ω resistor, a 680Ω will produce nearly the same results.
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by Steve Arar
by Luke James
by Luke James