Arduino

We'll use the Arduino IDE for the compass code - download & install it if you haven't already. 

If you're new to Arduino, check out the Getting Started with Arduino guide.

Install Libraries

Along with Arduino IDE, we'll need to install the following three libraries.

Open up the Arduino IDE and from the top menu, go to Sketch --> Include Library --> Manage Library, search for Unified Sensor and install the latest version of the Adafruit Unified Sensor library.

Next follow the same steps as above to install the Unified LSM303 library (or the library to match the accel/mag sensor you are using) and the Adafruit NeoPixel libraries.

Install Board Support Package

Additionally, go to Tools --> Board: --> Board Manager and update the Adafruit SAMD boards library if you haven't already.  Additional info for installing boards in the Arduino IDE is available here.

Upload Code

Go to Tools --> Board, and choose Adafruit Circuit Playground Express. Then go to Tools --> Port and choose the corresponding port for your board.

Create a new sketchcopy the code you see below, and paste it into that new sketch.

/* Circuit Playground Express compass. */

/* Adafruit invests time and resources providing this open source code. */
/* Please support Adafruit and open source hardware by purchasing */
/* products from Adafruit! */

/* Written by Dave Astels for Adafruit Industries */
/* Copyright (c) 2018 Adafruit Industries */
/* Licensed under the MIT license. */

/* All text above must be included in any redistribution. */

#include <Wire.h>
#include <Adafruit_NeoPixel.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_LSM303_U.h>
#include <math.h>

/* Assign a unique ID to this sensor at the same time */
Adafruit_LSM303_Mag_Unified mag = Adafruit_LSM303_Mag_Unified(12345);

// Replace these two lines with the results of calibration
//---------------------------------------------------------------------------

float raw_mins[2] = {1000.0, 1000.0};
float raw_maxes[2] = {-1000.0, -1000.0};
//---------------------------------------------------------------------------

float mins[2];
float maxes[2];
float corrections[2] = {0.0, 0.0};


// Support both classic and express
#ifdef __AVR__
#define NEOPIXEL_PIN 17
#else
#define NEOPIXEL_PIN 8
#endif

// When we setup the NeoPixel library, we tell it how many pixels, and which pin to use to send signals.
// Note that for older NeoPixel strips you might need to change the third parameter--see the strandtest
// example for more information on possible values.
Adafruit_NeoPixel strip = Adafruit_NeoPixel(10, NEOPIXEL_PIN, NEO_GRB + NEO_KHZ800);

// Map direction pie slices (of 30 deg each) to a neopixel, or two for the missing ones at USB & power.
int led_patterns[12][2] = {{4, 5}, {5, -1}, {6, -1}, {7, -1}, {8, -1}, {9, -1}, {9, 0}, {0 -1}, {1, -1}, {2, -1}, {3, -1}, {4, -1}};

#define BUTTON_A 4

void fill(int red, int green, int blue) {
  for (int i = 0; i < 10; i++) {
    strip.setPixelColor(i, red, green, blue);
  }
  strip.show();
}


// Do some initial reading to let the magnetometer settle in.
// This was found by experience to be required.
// Indicated to the user by blue LEDs.
void warm_up(void)
{
  sensors_event_t event;
  fill(0, 0, 64);
  for (int ignore = 0; ignore < 100; ignore++) {
    mag.getEvent(&event);
    delay(10);
  }
}


// Find the range of X and Y values.
// User needs to rotate the CPX a bunch during this.
// Can be refined by doing more of the saem by pressing the A button.
// Indicated to the user by green LEDs.
void calibrate(bool do_the_readings)
{
  sensors_event_t event;
  float values[2];

  fill(0, 64, 0);

  if (do_the_readings) {
    unsigned long start_time = millis();
    while (millis() - start_time < 5000) {

      mag.getEvent(&event);
      values[0] = event.magnetic.x;
      values[1] = event.magnetic.y * -1;
      if (values[0] != 0.0 && values[1] != 0.0) { /* ignore the random zero readings... it's bogus */
        for (int i = 0; i < 2; i++) {
          raw_mins[i] = values[i] < raw_mins[i] ? values[i] : raw_mins[i];
          raw_maxes[i] = values[i] > raw_maxes[i] ? values[i] : raw_maxes[i];
        }
      }
      delay(5);
    }
  }

  for (int i = 0; i < 2; i++) {
    corrections[i] = (raw_maxes[i] + raw_mins[i]) / 2;
    mins[i] = raw_mins[i] - corrections[i];
    maxes[i] = raw_maxes[i] - corrections[i];
  }
  fill(0, 0, 0);
}


void setup(void)
{
  strip.begin();
  strip.show();

  pinMode(BUTTON_A, INPUT_PULLDOWN);

  /* Enable auto-gain */
  mag.enableAutoRange(true);

  /* Initialise the sensor */
  if(!mag.begin())
  {
    /* There was a problem detecting the LSM303 ... check your connections */
    fill(255, 0, 0);
    while(1);
  }

  warm_up();

  // If reset with button A pressed or calibration hasn't been done, run calibration and report the results
  if (digitalRead(BUTTON_A) || (raw_mins[0] == 1000.0 && raw_mins[1] == 1000.0)) {
    while (!Serial);
    Serial.begin(9600);
    Serial.println("Compass calibration\n");

    raw_mins[0] = 1000.0;
    raw_mins[1] = 1000.0;
    raw_maxes[0] = -1000.0;
    raw_maxes[1] = -1000.0;
    calibrate(true);

    Serial.println("Calibration results\n");
    Serial.println("Update the corresponding lines near the top of the code\n");
    Serial.print("float raw_mins[2] = {"); Serial.print(raw_mins[0]); Serial.print(", "); Serial.print(raw_mins[1]); Serial.println("};");
    Serial.print("float raw_maxes[2] = {"); Serial.print(raw_maxes[0]); Serial.print(", "); Serial.print(raw_maxes[1]); Serial.println("};\n");

    while(1);
  } else {
    calibrate(false);
  }
}


// Map a value from the input range to the output range
// Used to map MAG values from the calibrated (min/max) range to (-100, 100)
float normalize(float value, float in_min, float in_max) {
  float mapped = (value - in_min) * 200 / (in_max - in_min) + -100;
  float max_clipped = mapped <  100 ? mapped : 100;
  float min_clipped = max_clipped > -100 ? max_clipped : -100;
  return min_clipped;
}


void loop(void)
{
  // Pressing button A does another round of calibration.
  if (digitalRead(BUTTON_A)) {
    calibrate(true);
  }

  sensors_event_t event;
  mag.getEvent(&event);

  float x = event.magnetic.x;
  float y = event.magnetic.y * -1;

  if (x == 0.0 && y == 0.0) {
    return;
  }

  float normalized_x = normalize(x - corrections[0], mins[0], maxes[0]);
  float normalized_y = normalize(y - corrections[1], mins[1], maxes[1]);

  int compass_heading = (int)(atan2(normalized_y, normalized_x) * 180.0 / 3.14159);
  // compass_heading is between -180 and +180 since atan2 returns -pi to +pi
  // this translates it to be between 0 and 360
  compass_heading += 180;

  // We add 15 to account to the zero position being 0 +/- 15 degrees.
  // mod by 360 to keep it within a circle
  // divide by 30 to find which pixel corresponding pie slice it's in
  int direction_index = ((compass_heading + 15) % 360) / 30;

  // light the pixel(s) for the direction the compass is pointing
  // the empty spots where the USB and power connects are use the two leds to either side.
  int *leds;
  leds = led_patterns[direction_index];
  for (int pixel = 0; pixel < 10; pixel++) {
    if (pixel == leds[0] || pixel == leds[1]) {
      strip.setPixelColor(pixel, 4, 0, 0);
    } else {
      strip.setPixelColor(pixel, 0, 0, 0);
    }
  }
  strip.show();
  delay(50);
}

Click the Upload button and wait for the process to complete. Once you see Done Uploading at the bottom of the window, the Circuit Playground Express should warm up and enter calibration (be sure to open the serial monitor first).

Compass Calibration

The compass needs to be calibrated before use. This adapts it to the variation in the chip as well as the local magnetic environment.  Simply spin the compass when the NeoPixels are green. If the compass acts erratically, pressing the left button (A) will do an additional calibration phase, which will hopefully improve the calibration. Be sure to spin it a couple full rotations for the widest range of values.

To calibrate you need the serial console open in order to get the calibrated values if you wish to set them in the code.

There are a few ways to run a calibration. If the compass has never been calibrated (the raw_mins array contains all 1000.0) it will be calibrated. Additionally it will be done if the A/left button is pressed when the power is turned on or reset is pressed. In both these cases it is assumed that you have a USB serial connection. When calibration completes, two lines are output, similar to:

Download: file
float raw_mins[2] = {-181.30, 216.09};
float raw_maxes[2] = {-136.96, 262.61};

Copy these into the code, replacing the two that look the same (other than the values), rebuild, and load onto the Circuit Playground Express. Subsequently powering on or resetting will then skip calibration.

A Guided Tour

The warm_up function is used when starting up to make multiple reads from the magnetometer. Experience showed that the initial reads were unreliable; the magnetometer needs to warm up it would seem. The NeoPixels are set to blue during this phase.

Download: file
void warm_up(void)
{
  sensors_event_t event;
  fill(0, 0, 64);
  for (int ignore = 0; ignore < 100; ignore++) {
    mag.getEvent(&event);
    delay(10);
  }
}

Calibration works by making about 1000 readings, and keeping track of the lowest and highest values along the X and Y axes. At the end, the origin offset is calculated by finding the center point of the readings (averaging the minimum and maximum values). Now we have the offset for readings as well as the expected value ranges. If the SAMD21 had EEPROM this could be stored and subsequent automatic calibration could be skipped, without having to rebuild the code.

Download: file
    void calibrate(bool do_the_readings)
{
  sensors_event_t event;
  float values[2];

  fill(0, 64, 0);

  if (do_the_readings) {
    unsigned long start_time = millis();
    while (millis() - start_time < 5000) {

      mag.getEvent(&event);
      values[0] = event.magnetic.x;
      values[1] = event.magnetic.y * -1;
      if (values[0] != 0.0 && values[1] != 0.0) { /* ignore the random zero readings... it's bogus */
        for (int i = 0; i < 2; i++) {
          raw_mins[i] = values[i] < raw_mins[i] ? values[i] : raw_mins[i];
          raw_maxes[i] = values[i] > raw_maxes[i] ? values[i] : raw_maxes[i];
        }
      }
      delay(5);
    }
  }

  for (int i = 0; i < 2; i++) {
    corrections[i] = (raw_maxes[i] + raw_mins[i]) / 2;
    mins[i] = raw_mins[i] - corrections[i];
    maxes[i] = raw_maxes[i] - corrections[i];
  }
  fill(0, 0, 0);
}
  

The main loop runs about 20 times per second. It starts by checking for the left button being pushed. If it is, more calibration is done. Next the magnetometer values are read for X and Y. 

Download: file
    if (digitalRead(BUTTON_A)) {
    calibrate(true);
  }

  sensors_event_t event;
  mag.getEvent(&event);

  float x = event.magnetic.x;
  float y = event.magnetic.y * -1;
  

Notice that the Y value is negated since the breakout is mounted upside down, flipping the Y axis.

Occasionally there will be a problem and the read values will both be zero. If so the reading is ignored.

Download: file
if (x == 0.0 && y == 0.0) {
  return;
}

Assuming the values are good, they are normalized. This is a simple mapping of the value from the range that resulted from calibration, to the range from -100 to +100.

Download: file
float normalize(float value, float in_min, float in_max) {
  float mapped = (value - in_min) * 200 / (in_max - in_min) + -100;
  float max_clipped = mapped <  100 ? mapped : 100;
  float min_clipped = max_clipped > -100 ? max_clipped : -100;
  return min_clipped;
}

Once we have normalized values the atan2 function is used to compute the angle of the vector made by those values. The angle will be in radians (ranging from -pi to +pi). That gets converted to degrees by multiplying by 180/pi. Now we have a value in degrees that ranges from -180 to +180. That gets 180 added to it to give a value from 0 to 360. This is then divided by 30 to give the number of a wedge which corresponds to 1 of 10 NeoPixel positions. Because the wedge at the top is defined by the angles at +/-15 degrees, 15 is added (and the result clipped to 360 by using the modulus operator %) before the division.

Download: file
  float normalized_x = normalize(x - corrections[0], mins[0], maxes[0]);
  float normalized_y = normalize(y - corrections[1], mins[1], maxes[1]);

  int compass_heading = (int)(atan2(normalized_y, normalized_x) * 180.0 / 3.14159);
  // compass_heading is between -180 and +180 since atan2 returns -pi to +pi
  // this translates it to be between 0 and 360
  compass_heading += 180;

  // We add 15 to account to the zero position being 0 +/- 15 degrees.
  // mod by 360 to keep it within a circle
  // divide by 30 to find which pixel corresponding pie slice it's in
  int direction_index = ((compass_heading + 15) % 360) / 30;

The final step is to update the NeoPixels. The led_patterns array that is defined at the top of the code is used to specify which pixels correspond to each wedge. Notice that two of the wedges specify two pixels, while the other 10 use only one. This is because the Circuit Playgrounds only have 10 NeoPixels; the top and bottom positions are used for the USB and battery connectors. To work around this, the two adjacent NeoPixels are used.

Download: file
  int *leds;
  leds = led_patterns[direction_index];
  for (int pixel = 0; pixel < 10; pixel++) {
    if (pixel == leds[0] || pixel == leds[1]) {
      strip.setPixelColor(pixel, 4, 0, 0);
    } else {
      strip.setPixelColor(pixel, 0, 0, 0);
    }
  }
  strip.show();

The complete code is below:

/* Circuit Playground Express compass. */

/* Adafruit invests time and resources providing this open source code. */
/* Please support Adafruit and open source hardware by purchasing */
/* products from Adafruit! */

/* Written by Dave Astels for Adafruit Industries */
/* Copyright (c) 2018 Adafruit Industries */
/* Licensed under the MIT license. */

/* All text above must be included in any redistribution. */

#include <Wire.h>
#include <Adafruit_NeoPixel.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_LSM303_U.h>
#include <math.h>

/* Assign a unique ID to this sensor at the same time */
Adafruit_LSM303_Mag_Unified mag = Adafruit_LSM303_Mag_Unified(12345);

// Replace these two lines with the results of calibration
//---------------------------------------------------------------------------

float raw_mins[2] = {1000.0, 1000.0};
float raw_maxes[2] = {-1000.0, -1000.0};
//---------------------------------------------------------------------------

float mins[2];
float maxes[2];
float corrections[2] = {0.0, 0.0};


// Support both classic and express
#ifdef __AVR__
#define NEOPIXEL_PIN 17
#else
#define NEOPIXEL_PIN 8
#endif

// When we setup the NeoPixel library, we tell it how many pixels, and which pin to use to send signals.
// Note that for older NeoPixel strips you might need to change the third parameter--see the strandtest
// example for more information on possible values.
Adafruit_NeoPixel strip = Adafruit_NeoPixel(10, NEOPIXEL_PIN, NEO_GRB + NEO_KHZ800);

// Map direction pie slices (of 30 deg each) to a neopixel, or two for the missing ones at USB & power.
int led_patterns[12][2] = {{4, 5}, {5, -1}, {6, -1}, {7, -1}, {8, -1}, {9, -1}, {9, 0}, {0 -1}, {1, -1}, {2, -1}, {3, -1}, {4, -1}};

#define BUTTON_A 4

void fill(int red, int green, int blue) {
  for (int i = 0; i < 10; i++) {
    strip.setPixelColor(i, red, green, blue);
  }
  strip.show();
}


// Do some initial reading to let the magnetometer settle in.
// This was found by experience to be required.
// Indicated to the user by blue LEDs.
void warm_up(void)
{
  sensors_event_t event;
  fill(0, 0, 64);
  for (int ignore = 0; ignore < 100; ignore++) {
    mag.getEvent(&event);
    delay(10);
  }
}


// Find the range of X and Y values.
// User needs to rotate the CPX a bunch during this.
// Can be refined by doing more of the saem by pressing the A button.
// Indicated to the user by green LEDs.
void calibrate(bool do_the_readings)
{
  sensors_event_t event;
  float values[2];

  fill(0, 64, 0);

  if (do_the_readings) {
    unsigned long start_time = millis();
    while (millis() - start_time < 5000) {

      mag.getEvent(&event);
      values[0] = event.magnetic.x;
      values[1] = event.magnetic.y * -1;
      if (values[0] != 0.0 && values[1] != 0.0) { /* ignore the random zero readings... it's bogus */
        for (int i = 0; i < 2; i++) {
          raw_mins[i] = values[i] < raw_mins[i] ? values[i] : raw_mins[i];
          raw_maxes[i] = values[i] > raw_maxes[i] ? values[i] : raw_maxes[i];
        }
      }
      delay(5);
    }
  }

  for (int i = 0; i < 2; i++) {
    corrections[i] = (raw_maxes[i] + raw_mins[i]) / 2;
    mins[i] = raw_mins[i] - corrections[i];
    maxes[i] = raw_maxes[i] - corrections[i];
  }
  fill(0, 0, 0);
}


void setup(void)
{
  strip.begin();
  strip.show();

  pinMode(BUTTON_A, INPUT_PULLDOWN);

  /* Enable auto-gain */
  mag.enableAutoRange(true);

  /* Initialise the sensor */
  if(!mag.begin())
  {
    /* There was a problem detecting the LSM303 ... check your connections */
    fill(255, 0, 0);
    while(1);
  }

  warm_up();

  // If reset with button A pressed or calibration hasn't been done, run calibration and report the results
  if (digitalRead(BUTTON_A) || (raw_mins[0] == 1000.0 && raw_mins[1] == 1000.0)) {
    while (!Serial);
    Serial.begin(9600);
    Serial.println("Compass calibration\n");

    raw_mins[0] = 1000.0;
    raw_mins[1] = 1000.0;
    raw_maxes[0] = -1000.0;
    raw_maxes[1] = -1000.0;
    calibrate(true);

    Serial.println("Calibration results\n");
    Serial.println("Update the corresponding lines near the top of the code\n");
    Serial.print("float raw_mins[2] = {"); Serial.print(raw_mins[0]); Serial.print(", "); Serial.print(raw_mins[1]); Serial.println("};");
    Serial.print("float raw_maxes[2] = {"); Serial.print(raw_maxes[0]); Serial.print(", "); Serial.print(raw_maxes[1]); Serial.println("};\n");

    while(1);
  } else {
    calibrate(false);
  }
}


// Map a value from the input range to the output range
// Used to map MAG values from the calibrated (min/max) range to (-100, 100)
float normalize(float value, float in_min, float in_max) {
  float mapped = (value - in_min) * 200 / (in_max - in_min) + -100;
  float max_clipped = mapped <  100 ? mapped : 100;
  float min_clipped = max_clipped > -100 ? max_clipped : -100;
  return min_clipped;
}


void loop(void)
{
  // Pressing button A does another round of calibration.
  if (digitalRead(BUTTON_A)) {
    calibrate(true);
  }

  sensors_event_t event;
  mag.getEvent(&event);

  float x = event.magnetic.x;
  float y = event.magnetic.y * -1;

  if (x == 0.0 && y == 0.0) {
    return;
  }

  float normalized_x = normalize(x - corrections[0], mins[0], maxes[0]);
  float normalized_y = normalize(y - corrections[1], mins[1], maxes[1]);

  int compass_heading = (int)(atan2(normalized_y, normalized_x) * 180.0 / 3.14159);
  // compass_heading is between -180 and +180 since atan2 returns -pi to +pi
  // this translates it to be between 0 and 360
  compass_heading += 180;

  // We add 15 to account to the zero position being 0 +/- 15 degrees.
  // mod by 360 to keep it within a circle
  // divide by 30 to find which pixel corresponding pie slice it's in
  int direction_index = ((compass_heading + 15) % 360) / 30;

  // light the pixel(s) for the direction the compass is pointing
  // the empty spots where the USB and power connects are use the two leds to either side.
  int *leds;
  leds = led_patterns[direction_index];
  for (int pixel = 0; pixel < 10; pixel++) {
    if (pixel == leds[0] || pixel == leds[1]) {
      strip.setPixelColor(pixel, 4, 0, 0);
    } else {
      strip.setPixelColor(pixel, 0, 0, 0);
    }
  }
  strip.show();
  delay(50);
}
This guide was first published on Oct 25, 2018. It was last updated on Oct 25, 2018. This page (Arduino) was last updated on Apr 23, 2019.