arduinovoltmetercode

float vout = 0.0;
float vin = 0.0;
float R1 = 20000.0;
float R2 = 3300.0;
int value = 0;
void setup()
{
   pinMode(A2, INPUT);
   Serial.begin(115200); //I use 115200 because it's fast ;) but you can use 9600 if you want. You'd have to change the baud rate in the c# program to 9600 if you do as serial ports' baud rates need to be the same
}
void loop()
{
   value = analogRead(A2);             //read the analogue voltage
   vout = (value * 5.0) / 1024.0;      //math
   vin = (vout / (R2 / (R1+R2)));      //more math, this also uses the resistor values too.
   String msg = String(vin + 1) + "v"; //creates a string with the end voltage (e.g, 11 + 1 as it's a bit inaccurate) and adds "v" for volts
   Serial.println(msg);                //sends it

   delay(250);
   //this delay is used to avoid flooding the serial port. if you want faster updates, it'd change this to a minimum of 25. Put at 100 for updates every 1/10th of a second, or every 0.1 seconds
}