Practical case: fridge door alarm with Arduino UNO

/*
 * Fridge Door and Temperature Alarm
 * Architecture: Arduino UNO R3 (ATmega328P)
 * Sensors: Reed Switch (Pin 2), LM35 (Pin A0)
 * Actuator: Piezo Buzzer (Pin 8)
 */

// Hardware Pin Definitions
const int REED_PIN = 2;
const int BUZZER_PIN = 8;
const int LM35_PIN = A0;

// Configuration Thresholds
const float TEMP_THRESHOLD_C = 8.0;          // Alarm triggers if temperature exceeds 8.0 Celsius
const unsigned long DOOR_TIMEOUT_MS = 20000; // Alarm triggers if door is open for 20,000 ms (20 seconds)
const unsigned long TEMP_READ_INTERVAL = 2000; // Read temperature every 2,000 ms (2 seconds)

// State Tracking Variables
unsigned long doorOpenedAt = 0;
unsigned long lastTempRead = 0;

bool isDoorOpen = false;
bool tempAlarmActive = false;
bool doorAlarmActive = false;
float currentTemp = 0.0;

void setup() {
    // Initialize serial communication for data logging
    Serial.begin(115200);

    // Configure pins
    // INPUT_PULLUP applies an internal 20k-50k ohm resistor to 5V.
    pinMode(REED_PIN, INPUT_PULLUP);
    pinMode(BUZZER_PIN, OUTPUT);

    Serial.println("--- Fridge Monitor Initialized ---");
    Serial.print("Temp Threshold: ");
    Serial.print(TEMP_THRESHOLD_C);
    Serial.println(" C");
    Serial.print("Door Timeout: ");
    Serial.print(DOOR_TIMEOUT_MS / 1000);
    Serial.println(" seconds");
}

void loop() {
    // Capture current time once per loop iteration
    unsigned long currentMillis = millis();

    // ---------------------------------------------------------
    // 1. Evaluate Door State
    // ---------------------------------------------------------
    // Switch is closed (LOW) when magnet is present (door closed).
    // Switch is open (HIGH) when magnet is removed (door open).
    bool currentDoorState = (digitalRead(REED_PIN) == HIGH);

    // Detect state change: Door just opened
    if (currentDoorState && !isDoorOpen) {
        isDoorOpen = true;
        doorOpenedAt = currentMillis;
        Serial.println("[EVENT] Door Opened.");
    } 
    // Detect state change: Door just closed
    else if (!currentDoorState && isDoorOpen) {
        isDoorOpen = false;
        doorAlarmActive = false;
        Serial.println("[EVENT] Door Closed.");
        noTone(BUZZER_PIN); // Silence any active door alarms
    }

    // Check if door has been open longer than the timeout
    if (isDoorOpen && (currentMillis - doorOpenedAt >= DOOR_TIMEOUT_MS)) {
        if (!doorAlarmActive) {
            Serial.println("[ALARM] Door Open Timeout Exceeded!");
            doorAlarmActive = true;
        }
    }

    // ---------------------------------------------------------
    // 2. Evaluate Temperature
    // ---------------------------------------------------------
    if (currentMillis - lastTempRead >= TEMP_READ_INTERVAL) {
        lastTempRead = currentMillis;

        // Read 10-bit ADC value (0-1023)
        int adcValue = analogRead(LM35_PIN);

        // Convert ADC value to voltage (5V reference)
        // Note: 1024.0 is the correct divisor for a 10-bit AVR ADC
        float voltage = (adcValue / 1024.0) * 5.0;

        // LM35 outputs 10mV (0.01V) per degree Celsius
        currentTemp = voltage / 0.01;

        Serial.print("[DATA] Current Temp: ");
        Serial.print(currentTemp);
        Serial.println(" C");

        // Trigger temperature alarm if threshold is exceeded
        if (currentTemp > TEMP_THRESHOLD_C) {
            if (!tempAlarmActive) {
                Serial.println("[ALARM] Temperature Critical!");
                tempAlarmActive = true;
            }
        } else {
            if (tempAlarmActive) {
                Serial.println("[RESOLVED] Temperature returned to normal.");
                tempAlarmActive = false;
                noTone(BUZZER_PIN); // Silence temp alarm
            }
        }
    }

    // ---------------------------------------------------------
    // 3. Actuate Alarms (Non-blocking Buzzer Logic)
    // ---------------------------------------------------------
    // Priority 1: Door Alarm (Fast high-pitched beep)
    if (doorAlarmActive) {
        // Toggle tone every 250ms
        if ((currentMillis / 250) % 2 == 0) {
            tone(BUZZER_PIN, 1200); // 1200 Hz
        } else {
            noTone(BUZZER_PIN);
        }
    } 
    // Priority 2: Temperature Alarm (Slow lower-pitched beep)
    // Only sound if the door alarm isn't already sounding
    else if (tempAlarmActive && !doorAlarmActive) {
        // Toggle tone every 1000ms
        if ((currentMillis / 1000) % 2 == 0) {
            tone(BUZZER_PIN, 600); // 600 Hz
        } else {
            noTone(BUZZER_PIN);
        }
    }
    // No alarms active
    else if (!doorAlarmActive && !tempAlarmActive) {
        noTone(BUZZER_PIN);
    }
}