Simple resistor network measurement

A product has a 9 way d type, on the end pins 1-2 = 100k ohms, pins 2-3 = 110k ohms, pins 1-3 = 10k ohms. I needed a solution I could give to an operator to accurately determine that these resistances are correctly wired between pins 1,2 and 3 (long story) and call an engineer (probably me) if incorrect for further investigation as to how it is being incorrectly wired as it is not clear where in the process it is happening.

I thought initially I could design some hardware to do this, then I thought how easily could I do this with an arduino. It sounds easy to do however due to the resistors and the way they are configured as a resistor network you need to switch out one pin while measuring the other 2 pins.

I used 3 DIO outputs of an UNO connected to 3 x 50k resistors which feed back to 3 x analogue inputs, that’s it. I create a 3 resistor potential divider between each of the pins by controlling the 3 DIO outputs setting them either high or low between pins 1,2 and 3, for example pins 1-2 = 100k I calculate the current flowing through the bottom resistor of the potential divider by measuring the voltage across this resistor using an analogue input. Then with this current I can calculate accurately the resistance between pins 1-2 using the voltage difference across the middle resistor and ohms law. See the attached as I probably haven’t explained it well, then repeat for the other pins If any of the resistors are incorrect the LED on the box continues to blink, if the resistors are all correct the LED on the box doesn’t blink and stays active, so the operator can tell if there is a problem or not then call an engineer. I have to set one of the DIOs as an input when setting the other 2 output pins high and low as otherwise this creates a difficult to calculate resistor network.

I love the simplicity of this project, although it’s just measuring resistances to do this with pure hardware isn’t as simple as it first seems, it is easy to change tolerances in the sketch and really quick and easy to build compared with a veroboard design (arduino uno + 3 resistors). Also there is a diode across the 10k resistor and I can check this diode is the correct way around if I need to.

sketch.ino (3.46 KB)