unexpected change of ADC value with multiple voltage dividers (thermistors and pot)

Question

So this is my basic circuit. Three voltage dividers: one 10k pot, 2 thermistors. Fan driver circuit using TIP120 (currently just driving an LED from the 3.3v source). Board is a Leonardo clone, circuits.io only had the Uno in their library. Edit: circuit is now more simplified.

Schematic:

reality:

When I adjust the pot, the ADC value of airPinValue changes, regardless of which analog input it is connected to. meatPinValue does not change. Swapping the inputs that airTempPin and meatTempPin are assigned to has no effect, airTempPin changes the same. Physically swapping the thermistors has no effect, nor does switching which input they connect to.

Code:

#define potPin A0
#define airTempPin A5

int potValue = 0;
int airPinValue = 0;
int i = 0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  i++;
  potValue = analogRead(potPin);
  airPinValue = analogRead(airTempPin);
  if (i == 2000) {
    Serial.print(potValue);
    Serial.print(",");
    Serial.println(airPinValue);
    i = 0;
  }
}

EDIT: much simplified code

EDIT v2.0 : I have now swapped out the board for an official Uno, as well as swapped everything to a different breadboard and a different Leonardo, with no change in behavior.

EDIT v3 : Corrected code sample. I have also dropped my DMM onto this, reading from the 5v rail to the voltage going to A2. With the pot set to 0 I see 2.401v from the divider, pot to max, same voltage, 2.401v.

EDIT v4: with only the pot and airTemp hooked up, still happens. Further DMM probing actually shows that this only happens when the pot and thermistor are both hooked up to the arduino. If either one is disconnected, the voltage being read by airTempPin is stable/correct. Once both are hooked up to the analog pins on the arduino, that's when the voltage at airTempPin goes off...

Also update diagram, pic, and code to match what I'm currently testing with.

EDIT v5 : This is some sort of analog bleed between the analog input ports. Not really sure how I can combat this :\

Answer 1

#define meatTempPin A0
#define airTempPin A1

Your code says A0,A1 but the circuit and picture of the wires say A0,A2.

Since A1 doesn't appear to be connected, as you scan things, the ADC could be sampling and holding some voltage from the prior conversion of potPin.

Try this instead:

#define meatTempPin A0
#define airTempPin A2

Answer 2

Moral of the story, the multiplexer was not switching between inputs quickly enough causing the odd reading, looking very much like bleed between the pins. By simply reading each analog input twice and only storing the value of the second reading, I was able to get a valid, stable, reading as expected.

#define potPin A4
#define airTempPin A0


int potValue = 0;
int airPinValue = 0;
int i = 0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  i++;
  analogRead(potPin);
  potValue = analogRead(potPin);
  analogRead(airTempPin);
  airPinValue = analogRead(airTempPin);
  delay(10);
  if (i == 50) {
    Serial.print(potValue);
    Serial.print(",");
    Serial.println(airPinValue);
    i = 0;
  }
}

Answer 3

https://www.arduino.cc/en/main/arduinoBoardUno

Vin. The input voltage to the Arduino/Genuino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.

Thisi not to be used as powersource for motor/LED whatever, this is input.

Use 5V instead, maybe this would solve the problem.

Answer 4

You need to have a reference voltage for ADC the problem is that you are not setting the reference voltage for ADC.Change the code in setup where you add a analog reference voltage as internal or default according to your requirement.