const byte pin_r1 = 4, pin_r2 = 3, pin_r3 = 2;
const float r1=220, r2=22e3, r3=2.2e6;

const float Uref = 5.0;

void setup() {
  Serial.begin(115200);
  pinMode(pin_r1, INPUT);
  pinMode(pin_r2, INPUT);
  pinMode(pin_r3, INPUT);
}

float messung(byte pin, float r)
{
  pinMode(pin, OUTPUT);
  digitalWrite(pin, HIGH);
  int rohwert = analogRead(A0);
  digitalWrite(pin, LOW);
  pinMode(pin, INPUT);
  //if(rohwert < 3 || rohwert > 1019)
  //  return -1.0;

  /*
  U2 / Uref = R2 / (R1+R2)
  Uref / U2 = (R1+R2)/R2 = R1/R2 + 1
  (Uref / U2 - 1 ) / R1 = 1/R2
  R1*U2 / (Uref-U2) = R2
  */
  float U0 = rohwert * Uref / 1023.0;
  float r0 = r * U0 / (Uref-U0); 

  Serial.println(rohwert);
  Serial.println(U0);
  Serial.print(r, 0);
  Serial.print(" -> ");
  Serial.println(r0, 1);
  return r0;
}

void loop()
{
  Serial.println("Messreihe:");
  messung(pin_r1, r1);
  messung(pin_r2, r2);
  messung(pin_r3, r3);
  delay(1000);
}