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===== Oak: Using an Ultrasonic Sensor=====

======= WORK IN PROGRESS =======


{{http://digistump.com/wiki/_media/oak/tutorials/ultrasonic_hc-sr04_shifter_bb.png?300|
{{http://digistump.com/wiki/_media/oak/tutorials/ultrasonic_hc-sr04_shifter_bb.png?500}}


In this lesson, we will connect an ultrasonic sensor to the Oak. An ultrasonic sensor, using the properties of sound, not only detects an object, but also detects how far away the object is. The sensor generates a high frequency sound, then listens for an echo received back to the sensor. The time interval from send to receive allows you to calculate the distance.

===== Components =====

^ Part       ^ Quantity          ^Identification^
| Oak with soldered headers |1| |
| Breadboard|1| |
|Ultrasonic Sensor|1|HC-SR04|
|Level Shifter|1|TXB0108|
|Jumper wires|11| |


**Note:** The HC-SR04 sensor is a 5v sensor, while the Oak is 3.3v. We will be using a bi-directional level shifter for compatibility. Given the plethora of micro electronic devices, and the race to use less and less power, it's not unusual to have a need for level shifting.

Particle.publish will be used to send the sensor data to the cloud. To view the data, you will need your free Particle account. You most likely created one when you first set up your Oak. If you don't have one already get yours here: [[https://build.particle.io/signup]].

===== Concepts =====

==== level shifting ====



==== pulseIn ====

pulseIn() can be thought of as a specific timer. pulseIn() takes at least 2 parameters, and up to 3, which are: pin, HIGH or LOW, and timeout. pulseIn() reads the pulse width value for the designated pin during a signal HIGH or LOW condition (designated by the state parameter). Pulse width value is the length of time the pin’s voltage remains HIGH or LOW.

For example, when the state parameter is set to HIGH, the pulseIn() function waits for the pin’s voltage change to HIGH and measures the length of time the pin’s voltage remains HIGH until it changes to LOW. The length of time is measured in millisecond. The maximum pulse width is 71 minutes. If nothing is detected within the specified time-out period, the function abandons the read attempt and returns 0.

==== Particle.publish ====

The Particle.io cloud offers many features that Oak users can take advantage of, some of which are quite in depth, and allow various devices to "communicate" with each other by passing information (like Particle.variable). If you just need something simple, such as a notification when something has happened, Particle.publish() may be what your looking for. 

Particle.publish() can either be public (available to anyone who has the event name and uses Particle.subscribe) or private (only visible to you, your devices, etc). 

Particle.publish() takes the name of the event, the data (char or String), TTL (length of time the data is good for), and if its private.


===== Circuit =====

The HC-SR04 has 4 pins: VCC, Trig, Echo, and Gnd. The particular bi-directional level shifter being used in this example has 20 pins, we will be using 8 of them. The connections get a little tricky, be sure to refer to the schematic above. 

  *      Connect the VCC pin to the Oak VCC
  *      Connect both GND pins to the Oak GND (via GND rail)
  *      Connect the Trig pin to Oak pin #2
  *      Connect the Echo pin to Oak pin #4


{{http://digistump.com/wiki/_media/oak/tutorials/hc-sr04_level_shifter.png?300|
{{http://digistump.com/wiki/_media/oak/tutorials/hc-sr04_level_shifter.png?600}}
     

Here's a picture of the actual setup.


===== Code =====

The first part of our sketch is the variable declaration:

<code>
/*
 HC-SR04 with TCB0108 level shifter
 Oak VCC to level shifter VCCA + OE
 Oak GND to level shifter GND
 Oak Pin 2 to level shifter A1
 Oak Pin 4 to level shifter A2
 Oak VIN to level shifter VCCB
 Level Shifter VCCB to sensor VCC
 Level shifter B1 to sensor Echo
 Level shifter B2 to sensor Trig
 Level shifter GND to sensor GND
 */


int trigPin = 2; // Trigger Pin
int echoPin = 4; // Echo Pin
int LEDPin = 1; // Onboard LED

long duration; 
long inches; 


char post_Distance[8]; //particle post variable

</code>

Next we tell the Oak what the pins will be doing in the setup section:

<code>

void setup() {
 
 pinMode(trigPin, OUTPUT);
 pinMode(echoPin, INPUT);
 pinMode(LEDPin, OUTPUT); // Use onboard LED for visual feedback
 
}

</code>

Now the loop portion. First we tell the sensor to send a sound:

<code>
void loop() {

 //Make sure the trigger pin is off
 digitalWrite(trigPin, LOW); 
 delay(100); 

//tell the sensor to send its pulse 
 digitalWrite(trigPin, HIGH);
 delay(10); 
 
 //end trigger
 digitalWrite(trigPin, LOW);

</code>

Then listen for the echo:

<code>

// pulseIn HIGH starts measuring the time until it hears the echo
Time_Echo_us = pulseIn(echoPin, HIGH);  
 
// Calculate distance to object. Divide by 2 because echo time is roundtrip
 inches = (duration/2) / 74;

</code>

Now that we have our data, let's use it:

<code>
//Send distance to cloud for viewing by Particle dashboard, and OakTerm
 sprintf(post_Distance, "%03d", inches); //convert int to string for Particle.publish
  Particle.publish("distance:", post_Distance);
 
/*
Use onboard LED for visual feedback, adjust values based on what you see via Particle.publish
Could attach an RGB LED to change colors based on distance           
*/
 if((inches < 36) && (inches > 1)) {   // limit the distance from the sensor's overall range
  
digitalWrite(LEDPin, HIGH); 
 }
 else { 
 digitalWrite(LEDPin, LOW); 
 }                            
   
 delay(1000);                                
 
}
</code>


===== Conclusion =====

You did it. You properly connected, and used an ultrasonic sensor to your Oak, and sent the data to the cloud. Make sure that you log in, and visit your Oak's logs at [[https://dashboard.particle.io/user/devices]]. If you have registered multiple Oaks (why wouldn't you have more than one?), be sure to view the logs for the correct Oak.

Look into other Particle functions at your disposal, such as Particle.variable, that allow you to post your variable, and even access it with another (read 2nd or 3rd oak) device.

What can **you** use an ultrasonic sensor for?
  * "eyes" to keep a robot from running into something
  * Connect an RGB, put in your garage for a parking distance guide
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