Practical case: Ultrasonic Distance Alarm with Arduino UNO

Validated Code

Create a new directory named ParkingAlarm and save the following code as ParkingAlarm.ino. The pin definitions and thresholds are included at the top of the file for easy tuning.

#include <LiquidCrystal.h>

// -----------------------------------------
// Pin Definitions
// -----------------------------------------
#define TRIG_PIN 9
#define ECHO_PIN 10
#define BUZZER_PIN 8

// HD44780 LCD (4-bit mode)
#define LCD_RS 12
#define LCD_EN 11
#define LCD_D4 5
#define LCD_D5 4
#define LCD_D6 3
#define LCD_D7 2

// -----------------------------------------
// System Thresholds (in centimeters)
// -----------------------------------------
#define DIST_MAX   200  // Maximum reliable distance to process
#define DIST_WARN  100  // Start warning (slow beep)
#define DIST_ALARM 50   // Start alarm (dynamic fast beep)
#define DIST_STOP  10   // Stop immediately (continuous tone)

// Initialize the LCD library with the interface pins
LiquidCrystal lcd(LCD_RS, LCD_EN, LCD_D4, LCD_D5, LCD_D6, LCD_D7);

// Variables for non-blocking buzzer timing
unsigned long previousBuzzerMillis = 0;
bool buzzerState = false;

void setup() {
    // Initialize Serial for debugging
    Serial.begin(115200);

    // Configure hardware pins
    pinMode(TRIG_PIN, OUTPUT);
    pinMode(ECHO_PIN, INPUT);
    pinMode(BUZZER_PIN, OUTPUT);

    // Initialize the LCD's columns and rows
    lcd.begin(16, 2);

    // Display a startup message
    lcd.setCursor(0, 0);
    lcd.print("Parking Assist");
    lcd.setCursor(0, 1);
    lcd.print("System Booting..");
    delay(2000); // 2-second boot delay is acceptable here
    lcd.clear();
}

// Function to trigger the HC-SR04 and calculate distance
long readDistance() {
    // Ensure the trigger pin is low before pulsing
    digitalWrite(TRIG_PIN, LOW);
    delayMicroseconds(2);

    // Send a 10 microsecond high pulse to trigger the sensor
    digitalWrite(TRIG_PIN, HIGH);
    delayMicroseconds(10);
    digitalWrite(TRIG_PIN, LOW);

    // Read the echo pin; timeout after 30,000 microseconds (approx 5 meters)
    long duration = pulseIn(ECHO_PIN, HIGH, 30000);

    // If timeout occurred, return -1 to indicate out of range
    if (duration == 0) {
        return -1;
    }

    // Calculate distance in centimeters
    // Speed of sound is 343 m/s or 0.0343 cm/microsecond
    // Divide by 2 to account for the round trip (ping and echo)
    return (duration * 0.0343) / 2;
}

void loop() {
    long distance = readDistance();
    unsigned long currentMillis = millis();

    // Debug output
    Serial.print("Distance: ");
    Serial.println(distance);

    // Top row: Display distance
    lcd.setCursor(0, 0);
    if (distance == -1 || distance > DIST_MAX) {
        lcd.print("Out of Range    "); // Pad with spaces to clear old chars
        noTone(BUZZER_PIN);

        lcd.setCursor(0, 1);
        lcd.print("Status: STANDBY ");
    } else {
        lcd.print("Dist: ");
        if (distance < 100) lcd.print(" "); // Alignment padding
        if (distance < 10)  lcd.print(" "); // Alignment padding
        lcd.print(distance);
        lcd.print(" cm      ");

        // Bottom row: Display status and handle buzzer logic
        lcd.setCursor(0, 1);

        if (distance > DIST_WARN) {
            // Safe zone
            lcd.print("Status: SAFE    ");
            noTone(BUZZER_PIN);

        } else if (distance > DIST_ALARM) {
            // Warning zone: Slow beep
            lcd.print("Status: SLOW    ");
            if (currentMillis - previousBuzzerMillis >= 500) { // 500ms interval
                previousBuzzerMillis = currentMillis;
                buzzerState = !buzzerState;
                if (buzzerState) {
                    tone(BUZZER_PIN, 1000); // 1000 Hz pitch
                } else {
                    noTone(BUZZER_PIN);
                }
            }

        } else if (distance > DIST_STOP) {
            // Alarm zone: Dynamic fast beep
            lcd.print("Status: ALARM   ");

            // Map the distance to a beep interval (closer = faster beep)
            // 50cm -> 400ms interval, 10cm -> 50ms interval
            int beepInterval = map(distance, DIST_STOP, DIST_ALARM, 50, 400);

            if (currentMillis - previousBuzzerMillis >= beepInterval) {
                previousBuzzerMillis = currentMillis;
                buzzerState = !buzzerState;
                if (buzzerState) {
                    tone(BUZZER_PIN, 1500); // 1500 Hz pitch
                } else {
                    noTone(BUZZER_PIN);
                }
            }

        } else {
            // Stop zone: Continuous tone
            lcd.print("Status: STOP!   ");
            tone(BUZZER_PIN, 2000); // 2000 Hz high pitch alert
        }
    }

    // Small delay to stabilize the loop and prevent LCD flickering
    delay(50);
}