Practical case: solar night light with Arduino UNO

/*
 * Solar Night Light Controller
 * Target: Arduino UNO R3 (ATmega328P)
 * Purpose: Controls a 12V LED via MOSFET based on LDR light levels,
 *          incorporating Low Voltage Disconnect (LVD) for battery protection.
 */

// --- Hardware Pins ---
const int PIN_BATTERY_SENSE = A0;
const int PIN_LIGHT_SENSE   = A1;
const int PIN_LED_DRIVE     = 9;

// --- Calibration & Thresholds ---
// Voltage Divider: R1 = 30k, R2 = 10k. Ratio = (30+10)/10 = 4.0
const float VOLTAGE_DIVIDER_RATIO = 4.0;
const float ADC_REFERENCE_VOLTAGE = 5.0;
const float ADC_MAX_VALUE = 1023.0;

// Battery Thresholds (12V Lead-Acid / 3S Li-ion example)
const float BATT_LVD_DISCONNECT = 10.5; // Turn off below 10.5V
const float BATT_LVD_RECONNECT  = 11.5; // Require 11.5V to turn back on

// Light Thresholds (0-1023). Higher value = brighter ambient light.
const int LIGHT_THRESHOLD_DARK  = 300;  // Below this is considered night
const int LIGHT_THRESHOLD_LIGHT = 450;  // Above this is considered day

// --- System State ---
enum SystemState {
  STATE_DAYTIME,
  STATE_NIGHT_ON,
  STATE_LOW_BATTERY
};

SystemState currentState = STATE_DAYTIME;

// --- Timing ---
unsigned long lastLogTime = 0;
const unsigned long LOG_INTERVAL_MS = 1000;

void setup() {
  Serial.begin(115200);

  pinMode(PIN_BATTERY_SENSE, INPUT);
  pinMode(PIN_LIGHT_SENSE, INPUT);

  // Initialize MOSFET drive pin
  pinMode(PIN_LED_DRIVE, OUTPUT);
  analogWrite(PIN_LED_DRIVE, 0); // Ensure LED is OFF at boot

  Serial.println("Solar Night Light Controller Initialized.");
  Serial.println("Timestamp_ms,Battery_V,Light_ADC,State,PWM_Out");
}

void loop() {
  // 1. Read Sensors
  int rawBatteryADC = analogRead(PIN_BATTERY_SENSE);
  int rawLightADC = analogRead(PIN_LIGHT_SENSE);

  // 2. Calculate Real Voltage
  float pinVoltage = (rawBatteryADC / ADC_MAX_VALUE) * ADC_REFERENCE_VOLTAGE;
  float batteryVoltage = pinVoltage * VOLTAGE_DIVIDER_RATIO;

  // 3. State Machine Logic
  switch (currentState) {

    case STATE_DAYTIME:
      // Transition to NIGHT if it gets dark AND battery is healthy
      if (rawLightADC < LIGHT_THRESHOLD_DARK) {
        if (batteryVoltage > BATT_LVD_RECONNECT) {
          currentState = STATE_NIGHT_ON;
        } else {
          currentState = STATE_LOW_BATTERY; // Too dark, but battery is dead
        }
      }
      // Transition to LOW_BATTERY if voltage drops even during day
      if (batteryVoltage < BATT_LVD_DISCONNECT) {
        currentState = STATE_LOW_BATTERY;
      }
      break;

    case STATE_NIGHT_ON:
      // Transition back to DAYTIME if light returns
      if (rawLightADC > LIGHT_THRESHOLD_LIGHT) {
        currentState = STATE_DAYTIME;
      }
      // Transition to LOW_BATTERY if voltage drops below safe limit
      if (batteryVoltage < BATT_LVD_DISCONNECT) {
        currentState = STATE_LOW_BATTERY;
      }
      break;

    case STATE_LOW_BATTERY:
      // Only recover from Low Battery if voltage rises above reconnect threshold
      if (batteryVoltage > BATT_LVD_RECONNECT) {
        if (rawLightADC > LIGHT_THRESHOLD_LIGHT) {
          currentState = STATE_DAYTIME;
        } else {
          currentState = STATE_NIGHT_ON;
        }
      }
      break;
  }

  // 4. Actuate Outputs
  int pwmValue = 0;
  if (currentState == STATE_NIGHT_ON) {
    pwmValue = 255; // 100% duty cycle (Full brightness)
  }
  analogWrite(PIN_LED_DRIVE, pwmValue);

  // 5. Telemetry Logging (Non-blocking)
  unsigned long currentMillis = millis();
  if (currentMillis - lastLogTime >= LOG_INTERVAL_MS) {
    lastLogTime = currentMillis;

    Serial.print(currentMillis);
    Serial.print(",");
    Serial.print(batteryVoltage, 2);
    Serial.print(",");
    Serial.print(rawLightADC);
    Serial.print(",");

    if (currentState == STATE_DAYTIME) Serial.print("DAYTIME");
    else if (currentState == STATE_NIGHT_ON) Serial.print("NIGHT_ON");
    else if (currentState == STATE_LOW_BATTERY) Serial.print("LOW_BATT");

    Serial.print(",");
    Serial.println(pwmValue);
  }
}