This library provides support for the QMC5883L chip which is found in some inexpensive compass boards marked as GY-271 and sold through a variety of online vendors.
The GY-271 is supposed to have an HMC5883L, but some knockoff boards have a QMC5883L which has the same basic capability, but is not compatible with the H.
If you have a GY-271 board and observe an I2C address of 0x0C and read back nothing but zeros from the board, then you probably have a QMC5883L chip! Try this library and see if it works for you.
To install this library into your Arduino environment, download the latest release file to your desktop: https://github.com/dthain/QMC5883L/releases
In the Arduino app, select the menu Sketch->Include Library->Add Zip Library and then select the downloaded file.
Then within your sketch, declare a compass instance at global scope:
#include <QMC5883L.h>
#include <Wire.h>
QMC5883L compass;
Initialize the compass, which will reset the hardware and begin continuous measurement mode:
Wire.begin();
compass.init();
Read raw magnetometer and temperature values from the compass.
int r;
int16_t x,y,z,t;
r = compass.readRaw(&x,&y,&z,&t)
Read calibrated heading values from the compass.
This function will initially return 0,
indicating that the compass is uncalibrated.
Continue to call readHeading() while rotating the compass on all axes.
Once enough data is collected, readHeading() will begin to return
integer heading values between 1 and 360 degrees.
int heading = compass.readHeading();
Note that readings are only periodically available, depending
on the sampling rate of the compass. If a reading is not immediately
available, both readRaw and readHeading will block until it is available.
If you want to perform a non-blocking check instead, use the
ready() method:
if(compass.ready()) {
int heading = compass.readHeading();
}
You can adjust the performance of the chip with the following methods:
compass.setSamplingRate(rate);
compass.setRange(range);
compass.setOverampling(ovl);
Allowable values for rate are 10, 50, 100, or 200 Hertz.
range may be 2 or 8 (Gauss).
ovl may be 512, 256, 128, or 64.
Datasheet on which the code is based: http://wiki.epalsite.com/images/7/72/QMC5883L-Datasheet-1.0.pdf