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Arduino Project Log: Getting Started with a Digital Compass

Andrew ThumbAs I mentioned during the last Blue Pints episode, this year I’m going to be attempting to build a low-cost open source CTD for basic oceanographic measurements. This is in addition to my ongoing work with the OpenROV. I have a pretty solid electronics background, but in order to accomplish this goal, I also need to learn how to program microcontrollers, something that I’ve never done before. For the next several months I’m going to tackle various small Arduino project to get comfortable with the fundamentals. I’ll be working out of Environmental Monitoring with Arduino: Building Simple Devices to Collect Data About the World Around Us and Arduino Projects to Save the World, both of which feature beginner to advanced projects based around environmental monitoring and data collection, as well as Programming Arduino Getting Started with Sketches for as a basic programming primer.

For my first project, I wanted to start with something that could eventually be implemented with the OpenROV–a magnetometer to provide compass headings while navigating the robot. As a first step, I wanted to learn how to program the Arduino board to calculate compass headings from a magnetometer and develop some sort of rough visual readout of that heading.


Step 1: Connecting the compass module to the Arduino analog outputs.

I mounted the compass module to a small breadboard so that I could move it around independent of the rest of the setup. The pin labeled VIN connects to the 5V output on the Arduino, SCL connects to analog ouput A5 while SDA connects to A4. Ground connects to ground.

The compass module connected to an Arduino Uno.

The compass module connected to an Arduino Uno.

Step 2: Connecting the LED array to the Arduino digital outputs. 

I arranged the 4 LED’s in a diamond shape, with the red LED indicating North. Connect the ground from the Arduino board to the negative bar of the breadboard. Each 220-ohm resistor connects the negative bar to the negative lead of the LED. If you’re following my program, pin 13 connects to the North LED, 12 connects to West, 11 connects to East, and 10 connects to South.

The LED array connected to an Arduino Uno.

The LED array connected to an Arduino Uno.

That’s all the set up you need to do.

Step 3: Programming.

I used the open source example program provided with the compass module as a template to add the LED commands (and removed some extraneous data being sent to the serial monitor. For the LED lights, I just added a series of 4 “If ” loops to turn the lights on or off depending on which way the compass is pointing (note the the X-axis on the 3-axis compass module corresponds to your bearing). I also had to adjust for my declination angle (the difference between compass north and true north). This varies depending on where you are in the world and should be inserted into the code in radians.

Final Thoughts

This was my first project beyond the basic “make an LED blink” tutorials. It was simple enough that I could tackle it with some basic programming knowledge but complex enough that I had to spend some time figuring out how to get the program right. I also produced a “cool thing” that will eventually lead to something practical for my larger projects.

Full Arduino Code reproduced below:

#include <Wire.h>
#include <HMC5883L.h>

HMC5883L compass;
int error = 0;

void setup()

Serial.println(“Starting the I2C interface.”);

Serial.println(“Constructing new HMC5883L”);
compass = HMC5883L();

Serial.println(“Setting scale to +/- 1.3 Ga”);
error = compass.SetScale(1.3); // Set the scale of the compass.
if(error != 0) // If there is an error, print it out.

Serial.println(“Setting measurement mode to continous.”);
error = compass.SetMeasurementMode(Measurement_Continuous); // Set the measurement mode to Continuous
if(error != 0) // If there is an error, print it out.

pinMode(13, OUTPUT);
pinMode(12, OUTPUT);
pinMode(11, OUTPUT);
pinMode(10, OUTPUT);

void loop()
// Retrive the raw values from the compass (not scaled).
MagnetometerRaw raw = compass.ReadRawAxis();
// Retrived the scaled values from the compass (scaled to the configured scale).
MagnetometerScaled scaled = compass.ReadScaledAxis();

// Values are accessed like so:
int MilliGauss_OnThe_XAxis = scaled.XAxis;// (or YAxis, or ZAxis)

// Calculate heading when the magnetometer is level, then correct for signs of axis.
float heading = atan2(scaled.YAxis, scaled.XAxis);

// Once you have your heading, you must then add your ‘Declination Angle’, which is the ‘Error’ of the magnetic field in your location.
// Find yours here: http://www.magnetic-declination.com/
float declinationAngle = -0.1742;
heading += declinationAngle;

// Correct for when signs are reversed.
if(heading < 0)
heading += 2*PI;

// Check for wrap due to addition of declination.
if(heading > 2*PI)
heading -= 2*PI;

// Convert radians to degrees for readability.
float headingDegrees = heading * 180/M_PI;

//Light North LED
if(headingDegrees > 325 or headingDegrees < 45)
digitalWrite(13, HIGH);
digitalWrite(12, LOW);
digitalWrite(11, LOW);
digitalWrite(10, LOW);

//Light South LED
if(headingDegrees > 135 and headingDegrees < 225)
digitalWrite(13, LOW);
digitalWrite(12, LOW);
digitalWrite(11, LOW);
digitalWrite(10, HIGH);

//Light West LED
if(headingDegrees > 225 and headingDegrees < 325)
digitalWrite(13, LOW);
digitalWrite(12, HIGH);
digitalWrite(11, LOW);
digitalWrite(10, LOW);

//Light East LED
if (headingDegrees > 45 and headingDegrees < 135)
digitalWrite(13, LOW);
digitalWrite(12, LOW);
digitalWrite(11, HIGH);
digitalWrite(10, LOW);

Output(raw, scaled, heading, headingDegrees);

void Output(MagnetometerRaw raw, MagnetometerScaled scaled, float heading, float headingDegrees)
Serial.println(” Degrees \t”);

Deep-sea biologist, population/conservation geneticist, backyard farm advocate. The deep sea is Earth's last great wilderness.

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