const int analogPin = A0;
const int ledPin = 4;

const float R1 = 47000.0; // ohms
const float R2 = 33000.0; // ohms
const float dividerRatio = R2 / (R1 + R2); // 0.4125

const float VREF = 5.00;
const int ADC_MAX = 1023;
const float thresholdv = 7.20; // V, turn LED ON when below this
const float hysteresis = 0.10; // V of hysteresis (prevents chattering)

float measuredVbat = 0.0;
bool lowState = false; // true when battery is "low" and LED should be ON

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);
  Serial.begin(9600);
  delay(50);
}

float readBatteryVoltage(int samples = 10) {
  long sum = 0;
  for (int i = 0; i < samples; ++i) {
    sum += analogRead(analogPin);
    delay(5);
  }
  float avg = float(sum) / samples;
  float vAtPin = (avg / ADC_MAX) * VREF;
  float vBat = vAtPin / dividerRatio;
  return vBat;
}

void loop() {
  measuredVbat = readBatteryVoltage(12); // average 12 samples
  // hysteresis: switch ON when below (threshold - h/2), switch OFF when above (threshold + h/2)
  float lower = thresholdv - (hysteresis / 2.0);
  float upper = thresholdv + (hysteresis / 2.0);

  if (!lowState && measuredVbat < lower) {
    lowState = true;
    digitalWrite(ledPin, HIGH); // battery low -> LED ON
  } else if (lowState && measuredVbat > upper) {
    lowState = false;
    digitalWrite(ledPin, LOW); // battery recovered -> LED OFF
  }

  // debug print
  Serial.print("VBAT = ");
  Serial.print(measuredVbat, 3);
  Serial.print(" V  state=");
  Serial.println(lowState ? "LOW" : "OK");
  delay(1000);
}