Multisensor Home Safety Alert System

#include <Arduino.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// ----- Pin assignments -----

// Forward declarations
void showStatus(const String &line1, const String &line2);

const byte FLAME_PIN = 2;       // YL-38 DO: LOW means flame detected
const byte PIR_PIN = 3;         // HC-SR501 OUT: HIGH means motion
const byte BUZZER_PIN = 4;      // Small active 5 V piezo buzzer
const byte GAS_LED_PIN = 5;
const byte FIRE_LED_PIN = 6;
const byte TEMP_LED_PIN = 7;
const byte MOTION_LED_PIN = 8;
const byte MQ2_PIN = A0;        // MQ-2 AO
const byte LM35_PIN = A1;       // LM35 SIG

// ----- Adjustable demonstration thresholds -----
const int GAS_THRESHOLD = 350;              // Raw ADC level (0-1023), tune after observing clean-air readings
const float TEMPERATURE_THRESHOLD = 35.0;   // Degrees C; a configurable classroom demonstration warning level
const unsigned long MQ2_WARMUP_MS = 60000UL;
const unsigned long PIR_STABILIZE_MS = 30000UL;
const unsigned long BUZZER_ON_MS = 180UL;
const unsigned long BUZZER_PERIOD_MS = 700UL;

// Most common 16x2 I2C backpack address. Change to 0x3F only if your module uses that address.
LiquidCrystal_I2C lcd(0x27, 16, 2);

unsigned long startTime;
unsigned long lastDisplayUpdate = 0;
String lastLine1 = "";
String lastLine2 = "";

void showStatus(const String &line1, const String &line2) {
  // The LCD is changed only when its visible words need to change.
  if (line1 != lastLine1 || line2 != lastLine2) {
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print(line1);
    lcd.setCursor(0, 1);
    lcd.print(line2);
    lastLine1 = line1;
    lastLine2 = line2;
  }
}

void setup() {
  pinMode(FLAME_PIN, INPUT);
  pinMode(PIR_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  pinMode(GAS_LED_PIN, OUTPUT);
  pinMode(FIRE_LED_PIN, OUTPUT);
  pinMode(TEMP_LED_PIN, OUTPUT);
  pinMode(MOTION_LED_PIN, OUTPUT);

  digitalWrite(BUZZER_PIN, LOW);
  digitalWrite(GAS_LED_PIN, LOW);
  digitalWrite(FIRE_LED_PIN, LOW);
  digitalWrite(TEMP_LED_PIN, LOW);
  digitalWrite(MOTION_LED_PIN, LOW);

  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  startTime = millis();
  showStatus("Safety system", "Starting up...");

  Serial.println(F("Multisensor Home Safety Alert System"));
  Serial.println(F("MQ-2 gas readings are relative values, not calibrated ppm."));
  Serial.println(F("Wait for MQ-2 warm-up and PIR stabilization before testing."));
}

void loop() {
  const unsigned long now = millis();
  const bool mq2Ready = (now - startTime >= MQ2_WARMUP_MS);
  const bool pirReady = (now - startTime >= PIR_STABILIZE_MS);

  // 1. Read every sensor first.
  const int gasRaw = analogRead(MQ2_PIN);
  const int lm35Raw = analogRead(LM35_PIN);
  const float temperatureC = (lm35Raw * 5.0 * 100.0) / 1023.0;
  const bool flameDetected = (digitalRead(FLAME_PIN) == LOW);
  const bool motionDetected = pirReady && (digitalRead(PIR_PIN) == HIGH);

  // 2. Decide each hazard independently. MQ-2 and PIR are ignored only during their normal startup settling time.
  const bool gasDanger = mq2Ready && (gasRaw >= GAS_THRESHOLD);
  const bool fireDanger = flameDetected;
  const bool temperatureDanger = (temperatureC >= TEMPERATURE_THRESHOLD);
  const bool motionDanger = motionDetected;
  const bool anyDanger = gasDanger || fireDanger || temperatureDanger || motionDanger;

  // 3. Each LED directly follows its own hazard condition.
  digitalWrite(GAS_LED_PIN, gasDanger ? HIGH : LOW);
  digitalWrite(FIRE_LED_PIN, fireDanger ? HIGH : LOW);
  digitalWrite(TEMP_LED_PIN, temperatureDanger ? HIGH : LOW);
  digitalWrite(MOTION_LED_PIN, motionDanger ? HIGH : LOW);

  // 4. A repeating short beep avoids a continuous tone and never blocks sensor monitoring.
  const bool buzzerOn = anyDanger && ((now % BUZZER_PERIOD_MS) < BUZZER_ON_MS);
  digitalWrite(BUZZER_PIN, buzzerOn ? HIGH : LOW);

  // 5. Show the highest-priority active condition, or startup progress, on the LCD.
  if (fireDanger) {
    showStatus("FIRE ALERT!", "Check safely");
  } else if (gasDanger) {
    showStatus("GAS ALERT!", "Ventilate area");
  } else if (temperatureDanger) {
    showStatus("TEMP ALERT!", "High temperature");
  } else if (motionDanger) {
    showStatus("MOTION ALERT!", "Movement seen");
  } else if (!mq2Ready) {
    unsigned long secondsLeft = (MQ2_WARMUP_MS - (now - startTime) + 999UL) / 1000UL;
    showStatus("MQ-2 warming", String(secondsLeft) + " sec remaining");
  } else if (!pirReady) {
    unsigned long secondsLeft = (PIR_STABILIZE_MS - (now - startTime) + 999UL) / 1000UL;
    showStatus("PIR settling", String(secondsLeft) + " sec remaining");
  } else {
    showStatus("System Safe", "Monitoring...");
  }

  // Print calibration/debug readings twice each second without stopping the system.
  if (now - lastDisplayUpdate >= 500UL) {
    lastDisplayUpdate = now;
    Serial.print(F("GasRaw="));
    Serial.print(gasRaw);
    Serial.print(F(" TempC="));
    Serial.print(temperatureC, 1);
    Serial.print(F(" FlameDO="));
    Serial.print(digitalRead(FLAME_PIN));
    Serial.print(F(" PIR="));
    Serial.print(digitalRead(PIR_PIN));
    Serial.print(F(" Hazards G/F/T/M="));
    Serial.print(gasDanger);
    Serial.print('/');
    Serial.print(fireDanger);
    Serial.print('/');
    Serial.print(temperatureDanger);
    Serial.print('/');
    Serial.println(motionDanger);
  }
}