Nano Aref Voltmeter problem

Projects General Guidance
1

Hey,
I have small problem with my arduino nano (I'm new to this topic).

Vm --> 470kOhm -> pin A0 --> 24,7kOhm --> GND
Vm - measured voltage
For test
Nano 5V (measured ~4,2V) -> R1 -> pin A0 (~0,203V) -> R2 --> GND
But my nano measure this strange, like 6V, after few minutes 11V, it looks randomly.
Even if i connect Vm to GND it still "measure" ~5-6V
Voltage on Aref ~0,7V (i use internal analog ref)

Part of code:

#define R1 475000.00  
#define R2 24700.00 
#define samples 100

float V_check()
{
  float Vread = 0;
  for(int i=0; i <samples; i++)
  {
  Vread += analogRead(VoltINPUT);  
  delay(5);
  }
  Vread = Vread / samples;
  return (Vread * (1.10 / 1023.00)) / (R2/(R1+R2));
           //    1 * 1,1   / 1023     /  0,05 = 0,02
           //  511 * 1,1   / 1023     /  0,05 =  10,99
           //  512 * 1,1   / 1023     /  0,05 =  11,01
           // 1023 * 1,1   / 1023     /  0,05 =  22
}
void setup() {
    Serial.begin(57600);
    analogReference(INTERNAL);
    pinMode(A0, INPUT);}
void loop() {
float V = V_check();
Serial.println(V);
}

Any solution?

2

After setting the internal reference, the AREF should show that reference voltage (or maybe after the first analogRead). It should be between 1.0 and 1.2V. Is something connected to the AREF pin ?

Do you switch between external and internal voltage reference ? Or between DEFAULT and INTERNAL ?

It is important for us that you show a full working (minimal) sketch that shows the problem. Can you make such a sketch and test the voltages and result ?
There is a website for that: https://snippets-r-us.com.
The problem is often in the part that we don't know :wink:

The normal way to make a project, is to make a few small test-sketches. Only beginners rush towards the end and try to fix the complete sketch.

To reduce high frequency electronic noise, the delay(5) between the samples it not needed.

The number should be 1024.0 instead of 1023.0 :crazy_face:

For a accurate analog measurement, the impedance of the circuit should be below 10k.
You have 470k parallel with 24.7k. The circuit impedance at A0 is:
470k // 24.7k = 23k (calculation).
Others might tell you to lower the resistor values, but I don't mind. I think that the difference between 23k and 10k is hardly noticeable.

It could be a faulty Nano board after all. Can you check the board with a magnifier if there is something wrong near the AREF pin ? For example a solder blob or shortcut.

3

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