Vehicle Speed Detector Using Arduino

Have you ever wondered how speed detection systems work on roads? In this project, we will build a Vehicle Speed Detector using Arduino UNO that calculates the speed of a moving object using two ultrasonic sensors.

This is a beginner-friendly yet powerful project that teaches:

Sensor interfacing
Real-time measurement
Embedded programming
Practical physics (Speed = Distance / Time)

By the end of this guide, you’ll be able to build and understand the system completely on your own.

How This Project Works :

This system uses two ultrasonic sensors placed at a fixed distance.

Working Flow :

A vehicle passes Sensor 1 → Timer starts
The same vehicle reaches Sensor 2 → Timer stops
Arduino calculates speed using:

Speed = Distance / Time

The result is displayed on an LCD
A buzzer alerts if speed exceeds the limit

Component List :

Name	Quantity	Component
PIEZO1	1	Piezo
DIST1 DIST2	2	Ultrasonic Distance Sensor (4-pin)
U1	1	Arduino Uno R3
U3	1	PCF8574-based, 39 (0x27) LCD 16 x 2 (I2C)

Complete Connections :

1. First Ultrasonic Sensor

HC-SR04	Arduino UNO
VCC	5V (recommended)
TRIG	D2
ECHO	D3
GND	GND

2. Second Ultrasonic Sensor

HC-SR04	Arduino UNO
VCC	5V (recommended)
TRIG	D4
ECHO	D5
GND	GND

3. Buzzer Connection

Buzzer	Arduino UNO
(+)	D6
(-)	GND

4. LCD I2C Connection

LCD I2C	Arduino UNO
VCC	5V
GND	GND
SDA	A4
SCL	A5

Arduino Code :

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

LiquidCrystal_I2C lcd(0x27, 16, 2);

// Pins
#define TRIG1 2
#define ECHO1 3
#define TRIG2 4
#define ECHO2 5
#define BUZZER 6

// Settings
#define DISTANCE 0.20 // distance between sensors in meters
#define THRESHOLD 40 // detection range in cm
#define SPEED_LIMIT 10.0 // km/h limit

bool started = false;
unsigned long t1 = 0;

// Measure distance from ultrasonic sensor
float getDistance(int trig, int echo) {
digitalWrite(trig, LOW);
delayMicroseconds(2);

digitalWrite(trig, HIGH);
delayMicroseconds(10);
digitalWrite(trig, LOW);

long duration = pulseIn(echo, HIGH, 30000);
if (duration == 0) return 999;

return duration * 0.0343 / 2;
}

// Stable detection to avoid false triggers
bool detectStable(int trig, int echo) {
int count = 0;
for (int i = 0; i < 5; i++) {
if (getDistance(trig, echo) < THRESHOLD) count++;
delay(5);
}
return (count >= 3);
}

// Buzzer alert
void beep(int t) {
digitalWrite(BUZZER, HIGH);
delay(t);
digitalWrite(BUZZER, LOW);
}

void setup() {
pinMode(TRIG1, OUTPUT);
pinMode(ECHO1, INPUT);
pinMode(TRIG2, OUTPUT);
pinMode(ECHO2, INPUT);
pinMode(BUZZER, OUTPUT);

lcd.init();
lcd.backlight();

lcd.setCursor(0, 0);
lcd.print("Vehicle Speed");
lcd.setCursor(0, 1);
lcd.print("Detector");
delay(2000);
lcd.clear();
}

void loop() {

lcd.setCursor(0, 0);
lcd.print("Waiting Vehicle ");
lcd.setCursor(0, 1);
lcd.print("Ready... ");

// Sensor 1 detection
if (!started && detectStable(TRIG1, ECHO1)) {
t1 = micros();
started = true;

lcd.clear();
lcd.print("Vehicle Found");
lcd.setCursor(0, 1);
lcd.print("Measuring...");
delay(300);
}

// Sensor 2 detection + speed calculation
if (started) {

// Timeout safety
if (micros() - t1 > 3000000) {
started = false;
lcd.clear();
lcd.print("Timeout Reset");
delay(1000);
lcd.clear();
return;
}

if (detectStable(TRIG2, ECHO2)) {

unsigned long t2 = micros();
float timeSec = (t2 - t1) / 1000000.0;

if (timeSec > 0) {
float speed = (DISTANCE / timeSec) * 3.6;

lcd.clear();
lcd.setCursor(0, 0);
lcd.print("Speed:");
lcd.setCursor(0, 1);
lcd.print(speed, 2);
lcd.print(" km/h");

if (speed > SPEED_LIMIT) {
beep(200);
} else {
beep(80);
}
}

delay(3000);
started = false;
lcd.clear();
}
}
}