Step 1: Requirements for the Arduino Alarm

Step 2: Build and Wire the Alarm

Step 3: Programming an Arduino alarm 

#define trigPin 7 //#define gives a name to the connection points we use.
#define echoPin 6
#define greenLed 9
#define yellowLed 10
#define redLed 11
#define buzzer 3

int sound = 500; //Here we create a variable for the Hertz (hz) that the buzzer will sound.

void setup ()
{
Serial.begin(9600); //Here we prepare the connection points and the serial monitor
pinMode (trigPin, OUTPUT);
pinMode (echoPin, INPUT);
pinMode(greenLed,OUTPUT);
pinMode(yellowLed, OUTPUT);
pinMode(redLed, OUTPUT);
pinMode (buzzer, OUTPUT);
}

void loop ()
{
long duration; //Here we create two more variables for the distance and the duration that the signal from the sensor is on the way.
long distance;

digitalWrite(trigPin, LOW); //Here we calculate with the duration of the signal what the distance to the object is.
delayMicroseconds (2);
digitalWrite (trigPin, HIGH);
delayMicroseconds (10);
digitalWrite (trigPin, LOW);
duration = pulseIn(echoPin, HIGH);
distance = (duration / 2) / 29.1;

Serial.print(distance); //Here we write the distance on the serial monitor
Serial.println ("cm");

if(distance > 50) //If the distance is large then 50cm:
{
digitalWrite(greenLed, HIGH);
digitalWrite(yellowLed, LOW);
digitalWrite(redLed, LOW);
}

else if(distance < 50 || distance > 10) //If the distance is between 50 and 10 cm:
{
digitalWrite(greenLed, LOW);
digitalWrite(yellowLed, HIGH);
digitalWrite(redLed, LOW);
}

else if(distance < 10) //If the distance is less than 10cm:
{
digitalWrite(greenLed, LOW);
digitalWrite(yellowLed, LOW);
digitalWrite(redLed, HIGH);
tone(buzzer, sound);
delay (100);
noTone (buzzer);
}

else if(distance <=0) //If distance is less than or equal to 0: (error message)
{
Serial.println("Out of range");
tone(buzzer, sound);
delay (500);
noTone (buzzer);
}
delay (300);
}