mobile-3-axis-magnetometer

This is a setup of a general-purpose magnetometer I assembled for my postdoctoral project at UCLA. The magnetometer measures all three components of the magnetic field vector in the range of $\pm4$ gauss ( $\pm400$ uTesla) or $\pm16$ gauss ( $\pm1600$ uT).

Parts

Here is the list of all parts from the adafruit website (you need an account to access).

In this setup, the magnetometer is driven by a microcontroller board from adafruit, but an Arduino or a Raspberry Pi can be used instead. I used an OLED display soldered directly onto the board to display the magnetic field values, but might not be needed in the case when data is sent directly to a computer via a USB cable. Also, there might be display options that do not require soldering.

As of October 2022, the total price of the setup is slightly above 70 USD, excluding a USB-C cable and power adapter necessary to power the device unless it’s connected to a computer. The magnetometer can become truly portable with a rechargeable battery, which is charged when the board is connected to a power source via USB.

Setup

1. I prefer to start with soldering the OLED display onto the board first, but it can be done after installing the board software. For some soldering tips see https://learn.adafruit.com/adafruit-128x64-oled-featherwing/assembly . It is easier to start by soldering the pins to the display.

1. Install the board firmware as described in https://learn.adafruit.com/adafruit-stm32f405-feather-express/dfu-bootloader-details . I would do it in the following order:

   1. Download and install STM32CubeProg on your computer (if Windows OS)

   2. Download the board firmware

   3. Connect the magnetometer via stemma cable to the board, and connect the board to the PC via USB-C cable

   4. Enable DFU bootloader mode by shorting the pins. I used slim alligator clips for that.

   5. Install the firmware by following STMCube instructions from the link above. A new drive should appear on your PC.

      

2. Download a software bundle corresponding to your firmware version from https://circuitpython.org/libraries .

3. Move the following files to the lib folder

Note that the above files might be specific to particular display and magnetometer models. For more information see https://learn.adafruit.com/adafruit-128x64-oled-featherwing/circuitpython and https://learn.adafruit.com/lis3mdl-triple-axis-magnetometer/python-circuitpython. This repo contains lib folder and code.py specific to my device.

1. In the main folder, open code.py with any text editor and update with the following code:

import time
import board
import displayio
import terminalio
import adafruit_lis3mdl
import math

# can try import bitmap_label below for alternative
from adafruit_display_text import label
import adafruit_displayio_sh1107

displayio.release_displays()
# oled_reset = board.D9

# Use for I2C
i2c = board.I2C()
display_bus = displayio.I2CDisplay(i2c, device_address=0x3C) # connect to the display
sensor = adafruit_lis3mdl.LIS3MDL(i2c) # connect to the magnetometer

########################
#Set up the display

# SH1107 is vertically oriented 64x128
WIDTH = 128
HEIGHT = 64
BORDER = 2
REFRESH_TIME = 0.5

display = adafruit_displayio_sh1107.SH1107(
    display_bus, width=WIDTH, height=HEIGHT, rotation=0
)

# Make the display context
splash = displayio.Group()
display.show(splash)

# Draw a smaller inner rectangle in black
inner_bitmap = displayio.Bitmap(WIDTH - BORDER * 2, HEIGHT - BORDER * 2, 1)
inner_palette = displayio.Palette(1)
inner_palette[0] = 0x000000  # Black
inner_sprite = displayio.TileGrid(
    inner_bitmap, pixel_shader=inner_palette, x=BORDER, y=BORDER
)
splash.append(inner_sprite)

###########
# continuously measure and display the magnetic field

SAMPLE_NUM = 10

while True:
    #measure the field values and average over SAMPLE_NUM    
    mag_x, mag_y, mag_z = sensor.magnetic
    for i in range(SAMPLE_NUM-1):
        mag_x2, mag_y2, mag_z2 = sensor.magnetic
        mag_x += mag_x2
        mag_y += mag_y2
        mag_z += mag_z2
    mag_x, mag_y, mag_z = mag_x/SAMPLE_NUM, mag_y/SAMPLE_NUM, mag_z/SAMPLE_NUM
        
    #display Bx
    yb = 8
    text1 = "Bx = " + str(mag_x) + " uT"
    text_area1 = label.Label(terminalio.FONT, text=text1, scale=1, x=8, y=yb)    
    splash.append(text_area1)
    #display By
    text2 = "By = " + str(mag_y) + " uT"
    text_area2 = label.Label(terminalio.FONT, text=text2, scale=1, x=8, y=yb+15)    
    splash.append(text_area2)
    #display Bz
    text3 = "Bz = " + str(mag_z) + " uT"
    text_area3 = label.Label(terminalio.FONT, text=text3, scale=1, x=8, y=yb+30)    
    splash.append(text_area3)
    #display the magnitude of B
    text4 = "|B| = " + str(math.sqrt(mag_x**2+mag_y**2+mag_z**2))+ " uT"
    text_area4 = label.Label(terminalio.FONT, text=text4, scale=1, x=8, y=yb+45)    
    splash.append(text_area4)
 
    #refresh the display
    display.show(splash)
    time.sleep(REFRESH_TIME)
    splash = displayio.Group()

After you save the updated code, the display should start showing the magnetic field values.

For information on calibration, see https://learn.adafruit.com/adafruit-sensorlab-magnetometer-calibration . We did not calibrate it, but a comparison with an expensive commercial hall probe showed excellent agreement.