arduino pro mini analog pins read different values

Question

i need to read a voltage with an arduino pro mini using the analog pins.

I created this simple test code:

int sensorValue;
float voltage;

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

void loop() {
  sensorValue = analogRead(A0);
  voltage= sensorValue * (5.0 / 1023.0);
  Serial.println(voltage);
}

my setup looks like this:

everything works so far. I get about 4,5v as a result. Now i want to port everything to an arduino pro mini. After i did that i only got 2,5v as a result. I also tried it with different pins and with an other arduino pro mini.

So what did i do wrong? Do i need an other pull-up resistor?

Answer 1

I made a circuit bellow for you. Note that the top resistor is 100 ohm and the bottom one is only 10 ohm. What I did is called a voltage divider. This way you can read voltages up to 50V. Simply connect the positive of your power source that you want to measure to the red wire and the ground of that same source to the black wire.

I also included a code that you could use:

Please notify me if any mistakes were spotted.

#define sensor A0 //defining sensor pin

float resistor1 = 100; //you can change it if you want, just make sure you also do it on the circuit
float resistor2 = 10;

float Vmax = (5*resistor1 + 5* resistor2)/resistor2;

void setup() {
  pinMode(sensor, INPUT); //declaring sensor as an output
  Serial.begin(9600);

  Serial.print("Do not input more then (V) : "); // print max input voltage
  Serial.print(Vmax);

  Serial.println(); //skip line
}

void loop() {
  float V = analogRead(sensor); //measuring analog values

  V = analogRead(sensor) * Vmax / 1024

  Serial.println(V); //printing voltage
}