AC voltmeter on an Arduino Uno

// AC voltmeter.
//
// The ZMPT101B's OUT rests at half the supply, about 512 counts, with the
// mains sine on top of it, made small by the transformer and the gain the
// trimmer sets. So the sketch finds the middle first, the mean over whole
// mains cycles, then the RMS of the readings about it, then scales that up by
// the trimmer's ratio: how many mains volts make one volt at OUT.
//
// The ratio is found by calibrating: read the mains with a meter you trust,
// then RATIO = RATIO x (the meter's reading / this sketch's). Here the part's
// trimmer slider shows it.
//
// The mains is set on the part's slider. A ZMPT101B on real mains has line
// voltage on its screw terminals: only a qualified person should wire one.

const int SENSOR = A0;
const float RATIO = 600.0;             // mains volts per volt at OUT, rms
const unsigned long WINDOW_US = 100000; // five 50 Hz cycles

void setup() {
  Serial.begin(115200);
  Serial.println("AC voltmeter");
}

void loop() {
  // The middle of the wave: the mean over whole cycles.
  unsigned long start = micros();
  long sum = 0;
  unsigned int n = 0;
  while (micros() - start < WINDOW_US) {
    sum += analogRead(SENSOR);
    n++;
  }
  float middle = (float)sum / n;

  // The RMS about it.
  start = micros();
  float squares = 0;
  unsigned int m = 0;
  while (micros() - start < WINDOW_US) {
    float d = analogRead(SENSOR) - middle;
    squares += d * d;
    m++;
  }
  float rmsCounts = sqrt(squares / m);
  float outVolts = rmsCounts * 5.0 / 1023.0;
  float mains = outVolts * RATIO;

  Serial.print("middle ");
  Serial.print(middle, 1);
  Serial.print("  OUT ");
  Serial.print(outVolts, 3);
  Serial.print(" V rms  mains ");
  Serial.print(mains, 1);
  Serial.println(" V");
  delay(1000);
}