Practical case: 12V protector for Arduino UNO

#include <LiquidCrystal.h>

// LCD pins: RS, E, D4, D5, D6, D7
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

// Pin assignments
const int PIN_BATTERY = A0;
const int PIN_RELAY = 8;

// Relay module behavior:
// Many modules are ACTIVE LOW.
// Set RELAY_ON_LEVEL and RELAY_OFF_LEVEL to match your hardware.
const int RELAY_ON_LEVEL = LOW;
const int RELAY_OFF_LEVEL = HIGH;

// Voltage divider values in ohms
const float R1 = 30000.0;  // battery+ to A0
const float R2 = 7500.0;   // A0 to GND

// ADC reference for standard UNO powered by USB
const float ADC_REF_VOLTAGE = 5.0;
const int ADC_MAX = 1023;

// Battery thresholds
const float DISCONNECT_VOLTAGE = 11.80;
const float RECONNECT_VOLTAGE  = 12.40;

// Timing
const unsigned long SAMPLE_INTERVAL_MS = 250;
const unsigned long LCD_INTERVAL_MS = 500;
const unsigned long SERIAL_INTERVAL_MS = 1000;
const unsigned long LOW_VOLTAGE_DELAY_MS = 5000;

// Averaging
const int NUM_SAMPLES = 20;

// State variables
bool loadConnected = true;
bool lowVoltagePending = false;
unsigned long lowVoltageStartMs = 0;
unsigned long lastSampleMs = 0;
unsigned long lastLcdMs = 0;
unsigned long lastSerialMs = 0;
float filteredBatteryVoltage = 0.0;

// Read battery voltage using averaging
float readBatteryVoltage() {
  long total = 0;

  for (int i = 0; i < NUM_SAMPLES; i++) {
    total += analogRead(PIN_BATTERY);
    delay(2);
  }

  float adc = total / (float)NUM_SAMPLES;
  float vA0 = adc * ADC_REF_VOLTAGE / ADC_MAX;
  float batteryVoltage = vA0 * ((R1 + R2) / R2);

  return batteryVoltage;
}

void setLoadConnected(bool enabled) {
  loadConnected = enabled;
  digitalWrite(PIN_RELAY, enabled ? RELAY_ON_LEVEL : RELAY_OFF_LEVEL);
}

void updateControlLogic(float vbat, unsigned long nowMs) {
  if (loadConnected) {
    if (vbat < DISCONNECT_VOLTAGE) {
      if (!lowVoltagePending) {
        lowVoltagePending = true;
        lowVoltageStartMs = nowMs;
      } else if ((nowMs - lowVoltageStartMs) >= LOW_VOLTAGE_DELAY_MS) {
        setLoadConnected(false);
        lowVoltagePending = false;
      }
    } else {
      lowVoltagePending = false;
    }
  } else {
    // Load is disconnected; reconnect only after voltage rises enough
    if (vbat > RECONNECT_VOLTAGE) {
      setLoadConnected(true);
      lowVoltagePending = false;
    }
  }
}

void updateLcd(float vbat, unsigned long nowMs) {
  if (nowMs - lastLcdMs < LCD_INTERVAL_MS) {
    return;
  }
  lastLcdMs = nowMs;

  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Bat:");
  lcd.print(vbat, 2);
  lcd.print("V");

  lcd.setCursor(0, 1);

  if (loadConnected) {
    if (lowVoltagePending) {
      unsigned long elapsed = nowMs - lowVoltageStartMs;
      unsigned long remain = 0;
      if (elapsed < LOW_VOLTAGE_DELAY_MS) {
        remain = (LOW_VOLTAGE_DELAY_MS - elapsed + 999) / 1000;
      }
      lcd.print("LOW WAIT ");
      lcd.print(remain);
      lcd.print("s");
    } else {
      lcd.print("LOAD ON");
    }
  } else {
    lcd.print("CUT OFF");
  }
}

void printSerialStatus(float vbat, unsigned long nowMs) {
  if (nowMs - lastSerialMs < SERIAL_INTERVAL_MS) {
    return;
  }
  lastSerialMs = nowMs;

  Serial.print("Vbat=");
  Serial.print(vbat, 3);
  Serial.print(" V, Load=");
  Serial.print(loadConnected ? "ON" : "OFF");
  Serial.print(", Pending=");
  Serial.println(lowVoltagePending ? "YES" : "NO");
}

void setup() {
  pinMode(PIN_RELAY, OUTPUT);
  setLoadConnected(true);

  lcd.begin(16, 2);
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Battery Guard");
  lcd.setCursor(0, 1);
  lcd.print("Starting...");

  Serial.begin(9600);
  delay(1500);

  filteredBatteryVoltage = readBatteryVoltage();
}

void loop() {
  unsigned long nowMs = millis();

  if (nowMs - lastSampleMs >= SAMPLE_INTERVAL_MS) {
    lastSampleMs = nowMs;

    float rawV = readBatteryVoltage();

    // Light smoothing filter
    filteredBatteryVoltage = 0.8 * filteredBatteryVoltage + 0.2 * rawV;

    updateControlLogic(filteredBatteryVoltage, nowMs);
    updateLcd(filteredBatteryVoltage, nowMs);
    printSerialStatus(filteredBatteryVoltage, nowMs);
  }
}