QMC5883L on OLED: Digital Compass

What it is: The QMC5883L is a 3-axis magnetometer that senses the Earth's magnetic field, so you can compute a compass heading (0–360°). It's the chip on most "GY-271" breakouts sold today. How it works: I2C device at address 0x0D. It reads magnetic field strength on X/Y/Z; the library turns X and Y into an azimuth (heading) in degrees. Shares SDA/SCL with the OLED. Wiring (shared I2C bus with the OLED): QMC5883L SDA -> GPIO4 QMC5883L SCL -> GPIO5 QMC5883L VCC -> 3V3 QMC5883L GND -> GND OLED VCC=3V3, GND=GND, SDA=GPIO4, SCL=GPIO5 Libraries: "QMC5883LCompass" by MPrograms, plus "Adafruit SSD1306" and "Adafruit GFX Library". On screen / Serial: heading in degrees (0 = North). Rotate the board flat and watch it track like a compass. Gotchas: Older GY-271 boards use the HMC5883L chip (address 0x1E, different library) — check which you have. Keep it away from motors, speakers and metal, which distort the field. For accuracy, run the library's calibration by rotating the board through all orientations.

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <QMC5883LCompass.h>

Adafruit_SSD1306 display(128, 64, &Wire, -1);
QMC5883LCompass compass;

void setup() {
  Serial.begin(115200);
  Wire.begin(4, 5);
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  compass.init();
}

void loop() {
  compass.read();
  int heading = compass.getAzimuth();          // degrees, 0 = North

  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println("QMC5883L Compass");
  display.setTextSize(3);
  display.setCursor(0, 22);
  display.printf("%d", heading);
  display.setTextSize(1);
  display.setCursor(0, 54); display.print("degrees");
  display.display();

  Serial.printf("Heading: %d deg\n", heading);
  delay(200);
}