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: hardware setup

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?

• Edgar Bonet: 2.5 V is what you expect. Why would you read 4.5 V?
• betion: so you think my arduino nano is not working properly?
• Edgar Bonet: Or you used different resistors, or you used different code.

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
}

enter image description here

• Edgar Bonet: Don't see mistakes, but here are a few suggestions: 1) Use higher valued resistors (but no more than 10 kΩ for R2) to avoid excessive current draw. Otherwise a 50 V source would blow up your 1/4 W resistors. 2) Declare the resistor values as float. 3) Write V = analogRead(sensor) * Vmax / 1024 instead of using map(). 4) Why divide the voltage by 100?
• KIIV: That float int resistor1 and float int resistor2 works? If yes, what type is it in the end? float or int?
• Dat Ha: opps, its just float, my bad!