I am trying to read an NTC thermistor (for which I know the Steinhart coefficients) with an Arduino UNO. I've checked multiple tutorials and in most of them the code is quite similar, except for a notable difference: in certain tutorials (such as this one from Circuit Basics) to obtain the resistance value of the termistor the analog input is multiplied by the reference resistor value, while in other ones the reference resistor value is divided by the thermistor analog input (such as this one from Adafruit).
What's the difference? What is the correct way of reading an NTC thermistor?
Thanks @DVDdoug for your quick reply. I am fairly new to this and I still have trouble understanding even the basics of electronics. The schematics I was planning to use was the one from Adafruit as the idea of using the 3.3V pin to reduce noise seemed a smart idea.
If I understand correctly there the Vin is applied to the reference resistor, making the thermistor the "bottom" resistor, and, therefore, dividing reference by the thermistor would be correct. Did I get it?
Seems like a smart idea, but it's far from that.
You must power the voltage divider from the same point that the reference voltage is powered from, to keep ratiometric behaviour (supply voltage variation compensation).
Since Aref of the Uno is 5volt by default, you also should connect the voltage divider to 5volt.
You can power the thermistor from 3.3volt, as long as you also change Aref to 3.3volt, which can be deadly to the Uno if you do it wrong.
Leo..