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I connected a HC-SR04 Ultrasonic Sensor to Arduino Uno and built the circuit as shown in the link below. I also used the code below the link. Together, this code and the circuit detect any motion that occurs in front of the ultrasonic sensor. When the sensor detects motion, the console in the Arduino IDE prints that it detects motion and how far away the moving object is. Otherwise, nothing is printed.

I created an app using MIT App Inventor. Every time the Arduino detects motion, I want a sound to be played on the phone of the person with the app. In order to do this, I plan on using a HC-05 Bluetooth Module.

Does anyone have any suggestions for how I should build the circuit and what code I should use?

arduinobasics.blogspot.de/2012/11/arduinobasics-hc-sr04-ultrasonic-sensor.html

#define echoPin 7 // Echo Pin
#define trigPin 8 // Trigger Pin
#define LEDPin 13 // Onboard LED

int maximumRange = 200; // Maximum range needed
int minimumRange = 0; // 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)
}

int prevdist = 0;

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){
 //Print "Out of range when there is no object within the range of the ultrasonic sensor.
 Serial.println("Out of range.");
 digitalWrite(LEDPin, HIGH); 

 }
 else {

 /* Send the distance to the computer using Serial protocol, and
 turn LED OFF to indicate successful reading. */

   if((prevdist - distance) > 10 || (prevdist - distance) < -10)
   {
     prevdist = distance;
     Serial.println("[OBJECT MOVED]");
     Serial.print("Detecting object ");
     Serial.print(distance);
     Serial.println(" cm away");
     Serial.println();
     digitalWrite(LEDPin, LOW);
   }  
 }

 //Delay 1 millisecond before next reading.
 delay(1);
}
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Every time the Arduino detects motion, I want a sound to be played on the phone of the person with the app. In order to do this, I plan on using a HC-05 Bluetooth Module.

Does anyone have any suggestions for how I should build the circuit and what code I should use?

The Arduino sketch above is a good start.

The first step is to connect the Bluetooth module so that the Serial output can be sent. The circuit is simply connecting the Bluetooth module to RX/TX on the Arduino. I am assuming that you have written an app that can read from Bluetooth on the phone and write to the screen.

The second step is to parse the output and play the sound. That is also part of the app and not the code on the Arduino. To make this easier the sketch output statements can be changed to something easier to parse (detect).

All and all the work is mainly app programming.

Cheers!

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