For the conductivity meter shown in the photo below, it is a non-NPN sensor for measuring conductivity / impedance.

I am trying to figure out the best method for measuring the output of the sensor using an Arduino.

I am told from the supplier that there is no polarity. I was planning to connect one of the leads through a 10k resistor to an analog input (A0) on my Arduino; and the other lead to 5V?

Without much other information on the sensor, is a 10k resistor a good place to start?

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  • what does "non-NPN" mean? ... the picture shows what looks like two metal prods ...there may be a component inside the long tube ... do you have a link to the spec sheet for the sensor? ... if it is just two metal prods, then connect +5V -> 10k -> prod#1 -> A0 and prod#2 -> GND (0V) ... if the long tube has a resistor, then connect +5V to long prod without using a 10k resistor
    – jsotola
    Feb 20, 2018 at 4:00
  • I think the npn dictates the type of output (logarithmic vs. linear). I am using a 10k resistor. What would the effect of adding a higher resistance or lower resistance be? Would it change the resolution of the sensor output? It looks like, for example when I use a 10k resistor, my numbers are much higher and change quicker than when I use a 1k resistor. It appears that there is higher resolution?
    – Gary
    Feb 20, 2018 at 17:13

1 Answer 1


It depends on the conductivity of the liquid which it is going to be placed/submerged to. For example if the liquid is highly conductive(ex. saltwater), then a 10k Ohms resistor should be fine. But if the liquid conductivity is low(ex. tap water), then its best to measure the output voltage of one lead with respect to the Arduino's ground to be sure what value of resistor to apply to or if there is no resistor needed at all.

But generally if you are going to supply it with 5 Volts, it should be fine to connect it with the Arduino's Analog inputs directly as the operating voltage of the Arduino itself is also 5 Volts.


  • Thanks for your comment! Could you just elaborate on what the resistor is doing in this case? For example, if I am using a 10k versus a 1k, does the conductivity sensor with the 10k keep it from getting too saturated from the high conductivity of the salt water?
    – Gary
    Feb 21, 2018 at 13:29
  • See photo that I posted in the original post. I have a probe in tap water, and used different resistor values between GND and A0, as per my Fritzing schematic also shown. My question is; why does the non-resistor value have a higher value than the 300k resistor value?
    – Gary
    Feb 21, 2018 at 14:10
  • I see, from the picture you posted that is a pull-down resistor. Why pull-down, because it is connected between the input(A0) and ground(GND), so when a positive signal comes in, the resistor acts as a "pull-down" voltage. For additional info: On the contrary there is "pull-up" resistor when the resistor is connected between the positive supply(5V) and the input pin(A0). this type of setup is used when the output of a sensor for example is going from high to low, so to determine there is a change the default state is "pulled-up" by the resistor to 5V so the sensor's output can be read.
    – EngrAbbas
    Feb 21, 2018 at 14:58
  • By the way, I think why they used a 10k ohms value of resistor is because probably the output of the sensor is 12Volts, so that it won't damage the Arduino, there is a 10k value of pull-down resistor. But if the output of the sensor is 5V, there should be no resistor needed at all.
    – EngrAbbas
    Feb 21, 2018 at 15:03

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