int agg = 0;
long int result;    //voltage number reading in counted loop; also used later for calculation of voltage
int nsamples = 64;  //number of samples to aggregate

// code for secret voltmeter adapted from a program by JRemington
void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);
}
long readVcc() {
  // Read 1.1V reference against AVcc
  // set the reference to Vcc and the measurement to the internal 1.1V reference
  //choose the right register settings for the processor in use
    #if defined(__AVR_ATmega32U4__) || defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
    ADMUX = _BV(REFS0) | _BV(MUX4) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
    #elif defined (__AVR_ATtiny24__) || defined(__AVR_ATtiny44__) || defined(__AVR_ATtiny84__)
    ADMUX = _BV(MUX5) | _BV(MUX0);
    #elif defined (__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || defined(__AVR_ATtiny85__)
    ADMUX = _BV(MUX3) | _BV(MUX2);
    #else
    ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
  #endif  

for (int count=0; count < nsamples; count++){
  delay(2); // Wait for Vref to settle
  ADCSRA |= _BV(ADSC); // Start conversion
  while (bit_is_set(ADCSRA,ADSC)); // measuring
  // read it a second time
 ADCSRA |= _BV(ADSC); // Start conversion
  while (bit_is_set(ADCSRA,ADSC)); // measuring

  uint8_t low  = ADCL; // must read ADCL first - it then locks ADCH  
  uint8_t high = ADCH; // unlocks both

  result = (high<<8) | low;
  agg = agg + result;  //collect and add up nsamples readings
  delay(10);
}

// we dont find the average here, it gives better resolution to multiply by the calibration constant first
// result = agg / nsamples  

  result = 1115702L * nsamples / agg; // 1115702 = 1.0896 * 1024 * 1000  - calibration constant, different for each processor
  return result; // Vcc in millivolts
}
void loop() {
  Serial.print("Vcc is ");
  Serial.print(readVcc());
  Serial.println(" mV");
  agg=0;  // reset aggreagate ready for next reading

  }