Measuring low resistances with Arduino

Question

Hi,

I want to measure a resistance of about 0.4-1.6 ohms with an Arduino. I'm guessing this can't be done by a simple voltage divider because of the low resistances.

How could I achieve this? The measurement doesn't have to be very accurate. The question is pretty much about "is it less than or more than 1 ohm?".

Thanks

Robert Peter Oakes · Accepted Answer

The arduino UNO already has an analogue comparator that can be used for this, use one end connected to a set point (Perhaps from a PWM output with an RC to emulate an 8bit DAC or simply a resistor divider, then the comparator will trip once the threshold is reached, knowing the value of the DAC will indicate if the resistor is above or below the threshold to a roughly 1% accuracy. selecting the right resistors to produce say 0 - 1V output and then the 1R + some other value to limit the current can be used for the other input No external op amp needed in this case alternate and probably easier is to use a resistor divider with the target resistor in question as part of this. Set the VREF to the ADC to be the 1.1V reference, not the 5V reference (Default). this will give you far better resolution at the low voltage end. the arduino has a 10bit ADC so 0 - 1023 value which equates to roughly 1mV per count (Assuming the 1.1V VREF) so a 100R in series with 1R and 5V across the two will result in 5v/101R = 50mA, this across the1R gives 50mV a 0.5R will give about 25mV 1.5R will give 75mV the mV equate to the rough count given from the ADC Yes your working in the low end of the ADC but given your not interested in accurate measurement of the resistor, just a rough go/no go test, this should be more than enough if you want a better resolution then you could drop the resistor to say 50ohms which will give roughly 100mA through the divider and so 1R = 100mV (Roughly), 150mV = 1R5, 50mV = 0R5 Not bad for just the addition of a single resistor

Former Member · Answer

The reason I ask is that in the following circuit, the resistor in the middle is the resistance you are measuring from whatever source it may be. The other 2 allow us to create potential dividers with 2 seperate outputs. The output between the middle resistor and the lower resistor can be seen as a virtual ground that we can reference as a ground. The output between the middle resistor and the top resistor can be seen as a potential difference.

If we measure the voltage between this new ground reference and the potential difference that we created, it should change whenever the resistance in the center changes, in this configuration the potential between those two points becomes 0.13V at 0.4ohm and 0.48V at 1.6ohm, a very much increased range and easier to measure.

Former Member · Answer

Hi, I did consider this approach too, boosting the voltage with an op-amp is also a potential.. There are often many ways to skin a cat!! Have you tried changing the 220 ohm resistor for a lower value? That would initially increase the range of the voltage maybe even bring it into range to be measured by an analog to digital converter built into your microcontroller (a higher sample range on the adc would be beneficial. If you can choose a 10bit option over an 8bit this would be better as would 12bit. Im not certain what range the Arduino has?). Changing the 220ohm resistor for a lower value seems a good idea to start with, this will increase the "baseline" voltage, choosing right the baseline can be increased from 0.022v to 0.74v(or within reasonable parameters.) This leaves the differential range the same so there would still only be a very slight change when the variable resistors value changes. What is the nature of the variable resistance? is it an actual variable resistor or something else? maybe there's a bit of trickery we can do without needing op-amps or transistors..

Measuring low resistances with Arduino

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