THE INSPIRATION

One day I decided to make Krepak's ultrasonic security system. I followed the instructions, used MaddMatt's modifications to the code, tested it, puzzled over inaccurate measurement, found the calculation error, fixed it, and then got bored. Because I have a LCD, I wanted it to also display the distance read by the ultrasonic sensor. In the following parts of this tutorial, I will explain how the main two components work.

The Ultrasonic Sensor

So, I thought I would build this. Wait, this tutorial is confusing. He keeps on changing which wire goes to where. I must reverse-engineer the code.

Some Time Later

Finally. I reverse-engineered that, and then had to check if MaddMatt's modifications to the code changed anything. I'll just upload...

Okay, that is done. Wait, what? That is definitely not 5 centimetres. You've messed up your maths, mate. I'll have a look at this.

Now I have the proper equation. For more information on how to use the HC-SR04, (and a more accurate equation than the one I used, which is still pretty accurate), you can look at this tutorial by Isaac100. Now, you might want the wiring diagram. It is at the bottom, where the schematics go. Note that this is of the finished device. So that is the Ultrasonic Sensor done, now for:

The LCD

So I have finished the ultrasonic sensor, now I need to wire up the LCD. Fortunately there is a good tutorial on how to do such a thing, you can find it here. But I have a problem. Some of the pins that are used in this tutorial are already used by the ultrasonic sensor. I will have to use different ones.

Some time goes by...

It is time for you to build it, using the schematic at the bottom, if you haven't already.

Now that you have wired it up, let's get it working! Insert this code into your Arduino IDE of choice and upload it to the board:

const int trigPin = 2;
const int echoPin = 3;
const int LEDlampRed = 4;
const int LEDlampYellow = 5;
const int LEDlampGreen = 6;
const int buzzer = 7;
int sound = 500;
void setup() {
Serial.begin (9600);
pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
pinMode(LEDlampRed, OUTPUT);
pinMode(LEDlampYellow, OUTPUT);
pinMode(LEDlampGreen, OUTPUT);
pinMode(buzzer, OUTPUT);
}
void loop() {
long durationindigit, distanceincm;
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
durationindigit = pulseIn(echoPin, HIGH);
distanceincm = (durationindigit*0.034) / 2;
if (distanceincm > 50) {
digitalWrite(LEDlampGreen, LOW);
digitalWrite(LEDlampYellow, LOW);
digitalWrite(LEDlampRed,LOW);
noTone(buzzer);
}
else if (distanceincm <= 50 && distanceincm > 20) {
digitalWrite(LEDlampGreen, HIGH);
digitalWrite(LEDlampYellow, LOW);
digitalWrite(LEDlampRed,LOW);
noTone(buzzer);
}
else if (distanceincm <= 20 && distanceincm > 5) {
digitalWrite(LEDlampYellow, HIGH);
digitalWrite(LEDlampGreen, HIGH);
digitalWrite(LEDlampRed,LOW);
tone(buzzer, 500);
}
else if (distanceincm <= 0) {
digitalWrite(LEDlampGreen, LOW);
digitalWrite(LEDlampYellow, HIGH);
digitalWrite(LEDlampRed, LOW);
noTone(buzzer);
}
else {
digitalWrite(LEDlampGreen, HIGH);
digitalWrite(LEDlampYellow, HIGH);
tone(buzzer, 1000);
digitalWrite(LEDlampRed, HIGH);
delay(300);
digitalWrite(LEDlampRed, LOW);
}
Serial.print(distanceincm);
Serial.println(" cm");
delay(300);
} 

Thank you for building!

EXPANDED TECHNICAL DETAILS

Spatial Intrusion Detection

This security system creates an invisible boundary using ultrasonic sound waves.

• HC-SR04 Sensor: The Arduino triggers a 40kHz sonic pulse and measures the echo return time. The distance is calculated in centimeters by multiplying the time by 0.034.
• Variable Distance Trigger: Unlike a simple PIR sensor, the user can set a specific "Zone." For example, the alarm only triggers if an object moves within 50cm, ignoring movement further away.

Interface & Alerts

• LCD Visualization: A 16x2 LCD or OLED displays the live distance of any object in the field of view.
• Alarm Sequence: Upon violation of the zone, the Arduino triggers a high-pitched Passive Buzzer and a blinking red LED. The system can be armed/disarmed using a keypad or a simple hidden switch.