title: Multiple Ultrasonic sensors with Arduino
body: <p>I'm trying to create a robot using three HC-SR04 ultrasonic sensors and my Arduino Pro Mini but I've run into a few problems.  In short the robot's function is as follows: </p>
  
  <ol>
  <li>The robot is dual wheeled, with an H-bridge (SN754410) driving each wheel.</li>
  <li>There's one HC-SR04 sensor on each side of the robot, the left one activates the left wheel motor when it detects a hand in front of it, vice versa for the right side.</li>
  </ol>
  
  <p>i.e. To make the robot go forward, we place our hands near the left and right side of the robot, to make it turn right, we remove the right hand and keep the left one in place, vice versa for turning left, etc.  </p>
  
  <ol start="3">
  <li>A third HC-SR04 is located the top of the robot, such that it activates a third motor when the user's hand is hovering above the robot.    </li>
  </ol>
  
  <p>My test code is as follows:</p>
  
  <pre><code>#include &lt;NewPing.h&gt;
  
  #define SONAR_NUM 3      // Number of sensors.
  #define MAX_DISTANCE 20 // Maximum distance (in cm) to ping.
  
  NewPing sonar[SONAR_NUM] = {   // Sensor object array.
    NewPing(4, 5, MAX_DISTANCE), // Each sensor's trigger pin, echo pin, and max distance to ping. 
    NewPing(6, 7, MAX_DISTANCE), 
    NewPing(8, 9, MAX_DISTANCE)
  };
  
  #define ena1 10   //trigger for left motor H-bridge
  //#define ena2 11 //trigger for right motor
  //#define ena3 12  //for top motor
  
  long sensors[3]; //array to store sensor distances 
  
  void setup() {
   Serial.begin (115200);
  
   pinMode(ena1, OUTPUT);
   //pinMode(ena2, OUTPUT);
   //pinMode(ena3, OUTPUT);
  }
  
  void loop() {
  
   for (uint8_t i = 0; i &lt; SONAR_NUM; i++) { // Loop through each sensor and display results.
      delay(50); // Wait 50ms between pings (about 20 pings/sec). 29ms should be the shortest delay between pings.
      sensors[i] = sonar[i].ping_cm(); 
    }
  
    Serial.println(sensors[0]);
    if (sensors[0] &gt; 0 &amp;&amp; sensors[0] &lt;= 20){
    Serial.println("detected");
    digitalWrite(ena1, HIGH);
    }else{
    Serial.println("NA");
    digitalWrite(ena1,LOW);
    }
   }
  </code></pre>
  
  <p>As you can see, I'm using the NewPing.h library to collect the sensor data.  After each iteration of the for loop, the distances detected by the sensors are stored in a sensor array.  When a hand is placed about 15-20 cm away from a sensor, the arduino sends a digital "HIGH" trigger signal to the respective H-bridge, activating the respective motor (I only have one of these pins, "ena1", enabled in my code, the other two are commented for the test).  </p>
  
  <p>To test my code, I simply connected the H-bridge trigger pin "ena1" to an LED, this pin is activated by the sensor whose distance data is stored in variable "sensors[0]".  However, after I compile and upload my code, I notice that the LED simply flickers faintly as I put my hand in front of the sensor.  As if the LED is being turned on and off very fast.  </p>
  
  <p>The output from the serial monitor is as follows:</p>
  
  <pre><code>15
  detected 
  0
  NA
  16
  detected
  0
  NA
  14
  detected
  </code></pre>
  
  <p>As you can see,  by putting my hand about ~15cm in front of the sensor, the sensor returns the correct distance and the "ena1" pin is set to high (as evidenced by "detected" being printed to the screen).  </p>
  
  <p>However, the sensor always returns a "0" value at the next iteration of the main loop (while my hand is still in front of the sensor), subsequently setting the "ena1" pin to LOW again, which might explain why the LED is being turned on and off so fast.  </p>
  
  <p>I'm not sure why this is happening... Interestingly, by removing the digitalWrite lines from the code, the sensor returns the correct values (i.e. no "0" value when my hand is in front of the sensor).  </p>
  
  <p>Any ideas on how I can fix this? </p>
  
  <p>Thanks in advance!  </p>