HC-SR04 Based Door Unlock Detection System

Lately I have replaced my smoke alarm, the buzzer still operates (at least it makes enough noise even though there is no smoke… haha). I decided to adapted it so it can be a door opening detector. Well it could be a cheap alternative to all the expensive door opening alarm system on the market... you can change the buzzer with a regular one, the architecture and the program will stay the same, even though the one i used is noisy enough to make any intruder changed his mind. Using an Ultrasonic Ranging Module HC-R04 I was able to detect the presence of the door doorknob.

To build this project you need:

• An Arduino UNO or compatible.
• 1 buzzer in my case i have used a smoke alarm buzzer.
• Ultrasonic Ranging Module HC-SR04
• A mini breadboard.
• Some male to male jumper wires

I removed the battery of the smoke detector and replace it with DIY 9 volt battery connector, attached to two male jumper wires, as shown in the picture

Warning: some of the smoke alarm contains radioactive material such as the AMERICIUM 241. Precaution is needed when manipulating them.

the HC-SR04 connection is given in the pictures above.

Past the following program into your Android software, and transfer it to your Arduino UNO

/*

HC-SR04 Ping distance sensor:

VCC to arduino 5v

GND to arduino GND

Echo to Arduino pin 7

Trig to Arduino pin 8

*/

#define echoPin 7 // Echo Pin

#define trigPin 8 // Trigger Pin

#define BuzzerPin 13 // Onboard LED, Buzzer

int maximumRange = 200; // Maximum range needed

int minimumRange = 0; // Minimum range needed

int secumaximumRange = 10; // secure Maximum range needed

int secminimumRange = 0; // secure Minimum range needed

long duration, distance; // Duration used to calculate distance

void setup() {

Serial.begin (9600);

pinMode(trigPin, OUTPUT);

pinMode(echoPin, INPUT);

pinMode(LEDPin, OUTPUT); // Use LED indicator (if required)

}

void loop() {

/* The following trigPin/echoPin cycle is used to determine the

distance of the nearest object by bouncing soundwaves off of it. */

digitalWrite(trigPin, LOW);

delayMicroseconds(2);

digitalWrite(trigPin, HIGH);

delayMicroseconds(10);

digitalWrite(trigPin, LOW);

duration = pulseIn(echoPin, HIGH);

//Calculate the distance (in cm) based on the speed of sound.

distance = duration/58.2;

if (distance >= maximumRange || distance <= minimumRange){

/* Send a negative number to computer using Serial protocol.*/

Serial.println("-1");

}

else {

/* Send the distance to the computer using Serial protocol. */

Serial.println(distance);

}

if (distance >= secumaximumRange || distance <= secminimumRange){

digitalWrite(BuzzerPin, HIGH);

}

else {

digitalWrite(BuzzerPin, LOW);

}

//Delay 50ms before next reading.

delay(50);

}

Place the HC-SR04 near your doorknob, it should measure a distance lower than 10cm, this distance can be read in the serial monitor of the Arduino software. you can change this distance in the program if you need to.