The micro:bit's built-in magnetometer chip is intended for use as compass to detect magnetic north. Like the compass app on your phone, this requires calibration and in my experience doesn't work very reliably. However, it is great for detecting the presence of a magnet!
Magnet Detector
The following project will make the micro:bit's display show a cross when a magnet comes near. This works best with a strong 'neodymium' magnet like the one in the side panel.

When you are looking at the top of the micro:bit, the compass chip is more or less behind button B.
Note that I have had problems with this project using battery power from the battery connector. However, its fine powered from a USB lead.
JavaScript Block Code
When the program starts the micro:bit will automatically take you through a calibration process for the compass chip. This involves tilting the micro:bit around in a circular motion in order to light up a ring of LEDs on the display.
The JavaScript Blocks Code editor is embedded directly on this page below. From the editor, you can click on Download button (bottom right) and then copy the downloaded file onto your micro:bit. Alternatively, you can Click here to open the editor in a separate browser tab.
In this program the block magnetic force is used to measure the intensity of the magnetic field near the compass chip on your micro:bit. This is used in the on start
block to establish a baseline
reading of magnetic strength with no magnet near the micro:bit. This happens when the micro:bit is first powered up or the reset button is pressed.
The forever
loop repeatedly takes readings and checks to see if the absolute value of the difference between the new reading and the baseline reading is above 100 and if it is it displays the cross symbol on the display. The absolute
value is just the value with the sign removed. This is necessary because, depending on which pole of the magnet is pointing towards the compass chip, the magnetic force
reading may decrease rather than increase.
You may need to increase the threshold of 100 if the cross appears even without a magnet being near.
MicroPython
To run the MicroPython version of the code, open up the online Python editor here and paste the following code into the editor window.
from microbit import * baseline = compass.get_field_strength() # Take a baseline reading of magnetic strength while True: if abs(compass.get_field_strength() - baseline) > 10000: # Magnetic field strength increased by 10000 display.show(Image.NO) # Show a cross symbol else: display.clear()
The program first import
s the microbit
library that contains the compass
and display
objects needed in the program.
The while
loop works just like its JavaScript Blocks sibling. It repeatedly takes readings and checks to see if the absolute value of the difference between the new reading and the baseline
reading is above 10000 and if it is it displays the cross symbol on the display. The abs
value is just the value with the sign removed. This is necessary because, depending on which pole of the magnet is pointing towards the compass chip, the magnetic force reading may decrease rather than increase.
You may need to increase the threshold of 10000 if the cross appears even without a magnet being near.
Arduino
Make sure that you have your Arduino environment set up for micro:bit by following this guide.
Now start a new Sketch by clicking on the File menu and New. Then paste the following code into the editor window.
#include "Wire.h" #include <SparkFun_MAG3110.h> #include <Adafruit_Microbit.h> Adafruit_Microbit_Matrix microbit; MAG3110 compass = MAG3110(); // The compass chip int baseline = 0; void setup() { microbit.begin(); compass.initialize(); // Initializes the compass chip compass.start(); // Puts the sensor in active mode baseline = readStrength(); // Take a baseline reading of magnetic strength delay(500); } void loop() { if (abs(readStrength() - baseline) > 15000) { // Magnetic field strength increased by 15000 microbit.show(microbit.NO); } else { microbit.clear(); } } int readStrength() { int x, y, z; while(!compass.dataReady()) {}; // Wait for data to become available to read compass.readMag(&x, &y, &z); // Read data into variables // calculate the RMS power combining x, y and z readings int power = sqrt((sq(float(x)) + sq(float(y)) + sq(float(z))) / 3); return power; }
The Arduino version of the compass library does not include an equivalent to the magnetic force block of the Blocks editor or the get_field_strength
of MicroPython. This means that we have to write our own, called readStrength
. This takes the x, y and z readings from the compass chip and combines them using a mathematical technique called RMS (root mean square). That takes a kind of average of the readings.
You may need to increase the threshold of 15000 if the cross appears even without a magnet being near.
Page last edited March 08, 2024
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