Let's say I want to read the value of a resistor of X ohms (more accurately, voltage drop?). In my application I am using a 2 wire RTD, but it is essentially a variable resistor. According to my understanding the +5v pin is a +5v voltage source, it always supplies +5volts relative to GND. If this is the case wouldn't the voltage difference ACROSS the resistor of x ohms always be 5volts?
Sure the current would change based on the resistance, but the voltage would not. From my understanding Analog Read gives the VOLTAGE at the pin. See the image on the left. Just for the heck of it, assume my resistor was 10ohms. What would analog read output?
Now for my second n00b question. Is the image on the right a valid circuit? Basically can I use an external battery to send power through the resistor and read the value, or if I intend to do analog reads must power come from the +5v pin? Assuming the battery is 12V and the resistor is 10 ohms, what would analog 0 read?
Would I have to connect arduino's GND pin to the common ground that this battery uses