Practical case: ESP32 Wi-Fi Freezer Monitor

#include <Arduino.h>
#include <WiFi.h>
#include <WebServer.h>

// ------------------------------------------------------------------
// Network Credentials
// ------------------------------------------------------------------
const char* WIFI_SSID = "LabNetwork";
const char* WIFI_PASSWORD = "LabPassword123";

// ------------------------------------------------------------------
// Pin Definitions
// ------------------------------------------------------------------
const int NTC_PIN = 34;      // ADC1_CH6 - Safe to use with Wi-Fi
const int BUZZER_PIN = 26;   // Output for active buzzer
const int LED_PIN = 27;      // Output for status LED

// ------------------------------------------------------------------
// Thermistor & Steinhart-Hart Parameters
// ------------------------------------------------------------------
const float SERIES_RESISTOR = 10000.0;    // 10k Ohm fixed resistor
const float NOMINAL_RESISTANCE = 10000.0; // 10k Ohm NTC at 25 degrees C
const float NOMINAL_TEMPERATURE = 25.0;   // Nominal temperature in Celsius
const float B_COEFFICIENT = 3950.0;       // Beta value of the thermistor
const float ALARM_THRESHOLD = -10.0;      // Alarm triggers if temp rises above -10.0 C

// ------------------------------------------------------------------
// Global Variables
// ------------------------------------------------------------------
WebServer server(80);
float currentTemperature = 0.0;
bool isAlarmActive = false;

// Timing variables for non-blocking alarm
unsigned long previousMillis = 0;
const long blinkInterval = 500; // 500ms toggle rate for buzzer/LED
bool toggleState = false;

// ------------------------------------------------------------------
// Function Prototypes
// ------------------------------------------------------------------
void connectWiFi();
void handleRoot();
float readTemperature();
void handleAlarmLogic();

// ------------------------------------------------------------------
// Setup
// ------------------------------------------------------------------
void setup() {
    Serial.begin(115200);
    delay(1000); // Allow serial monitor to stabilize

    // Initialize output pins
    pinMode(BUZZER_PIN, OUTPUT);
    pinMode(LED_PIN, OUTPUT);
    digitalWrite(BUZZER_PIN, LOW);
    digitalWrite(LED_PIN, LOW);

    // Initialize ADC
    analogReadResolution(12); // 12-bit ADC (0 - 4095)

    Serial.println("\n--- Freezer Temperature Alarm System ---");

    connectWiFi();

    // Setup Web Server Routes
    server.on("/", handleRoot);
    server.begin();
    Serial.println("Web server started.");
}

// ------------------------------------------------------------------
// Main Loop
// ------------------------------------------------------------------
void loop() {
    server.handleClient(); // Listen for incoming HTTP requests

    // Read temperature every cycle
    currentTemperature = readTemperature();

    // Evaluate and execute alarm logic
    handleAlarmLogic();

    // Small delay to stabilize ADC reads and prevent watchdog resets
    delay(50);
}

// ------------------------------------------------------------------
// Functions
// ------------------------------------------------------------------

void connectWiFi() {
    Serial.print("Connecting to Wi-Fi: ");
    Serial.println(WIFI_SSID);

    WiFi.mode(WIFI_STA);
    WiFi.begin(WIFI_SSID, WIFI_PASSWORD);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }

    Serial.println("\nWi-Fi Connected!");
    Serial.print("IP Address: ");
    Serial.println(WiFi.localIP());
}

float readTemperature() {
    // Read the analog value (0-4095)
    int adcValue = analogRead(NTC_PIN);

    // Prevent division by zero if pin is shorted to Ground or 3.3V
    if (adcValue == 0) return -99.0;
    if (adcValue >= 4095) return 99.0;

    // Calculate NTC Resistance
    // Based on Voltage Divider: Vout = Vcc * (R_NTC / (R_NTC + R_SERIES))
    // Derives to: R_NTC = R_SERIES * (ADC / (4095 - ADC))
    float ntcResistance = SERIES_RESISTOR * ((float)adcValue / (4095.0 - (float)adcValue));

    // Apply Steinhart-Hart equation
    float steinhart;
    steinhart = ntcResistance / NOMINAL_RESISTANCE;       // (R/Ro)
    steinhart = log(steinhart);                           // ln(R/Ro)
    steinhart /= B_COEFFICIENT;                           // 1/B * ln(R/Ro)
    steinhart += 1.0 / (NOMINAL_TEMPERATURE + 273.15);    // + (1/To)
    steinhart = 1.0 / steinhart;                          // Invert
    steinhart -= 273.15;                                  // Convert Kelvin to Celsius

    return steinhart;
}

void handleAlarmLogic() {
    if (currentTemperature > ALARM_THRESHOLD) {
        isAlarmActive = true;

        // Non-blocking toggle for LED and Buzzer
        unsigned long currentMillis = millis();
        if (currentMillis - previousMillis >= blinkInterval) {
            previousMillis = currentMillis;
            toggleState = !toggleState;
            digitalWrite(LED_PIN, toggleState ? HIGH : LOW);
            digitalWrite(BUZZER_PIN, toggleState ? HIGH : LOW);
        }
    } else {
        isAlarmActive = false;
        // Ensure outputs are turned off when temperature is safe
        digitalWrite(LED_PIN, LOW);
        digitalWrite(BUZZER_PIN, LOW);
        toggleState = false;
    }
}

void handleRoot() {
    // Construct a simple, auto-refreshing HTML dashboard
    String html = "<!DOCTYPE html><html><head>";
    html += "<meta name='viewport' content='width=device-width, initial-scale=1.0'>";
    html += "<meta http-equiv='refresh' content='5'>"; // Auto-refresh every 5 seconds
    html += "<style>";
    html += "body { font-family: Arial, sans-serif; text-align: center; margin-top: 50px; }";
    html += ".temp { font-size: 3em; font-weight: bold; }";
    html += ".status-ok { color: green; font-size: 2em; }";
    html += ".status-alarm { color: red; font-size: 2em; font-weight: bold; animation: blinker 1s linear infinite; }";
    html += "@keyframes blinker { 50% { opacity: 0; } }";
    html += "</style></head><body>";

    html += "<h1>Freezer Monitor Dashboard</h1>";
    html += "<div class='temp'>" + String(currentTemperature, 1) + " &deg;C</div>";

    if (isAlarmActive) {
        html += "<div class='status-alarm'>WARNING: TEMPERATURE HIGH</div>";
    } else {
        html += "<div class='status-ok'>STATUS: NORMAL</div>";
    }

    html += "</body></html>";

    // Send the response to the client
    server.send(200, "text/html", html);
}