4 MAGICAL MAGNETISM

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4
MAGICAL MAGNETISM

As you saw in Chapter 1, the micro:bit has a built-in magnetometer that serves a variety of purposes. In this chapter, we’ll use it to turn your micro:bit into a compass that tells you which direction you’re facing. We’ll also measure the magnetic fields of neodymium magnets. Then, we’ll make a magnetic alarm that rings whenever someone opens the door.

PROJECT: COMPASS

Difficulty: Easy

In this project, shown in Figure 4-1, you’ll use the micro:bit’s built-in magnetometer as a compass. It will display an arrow on the screen that points toward magnetic north.

Figure 4-1: Finding north with a micro:bit compass

Unlike a conventional compass, this compass doesn’t always point north. Instead, it shows you which way to turn in order to be facing north.

Rotate until the arrow is facing straight ahead, and you’re looking north!

What You’ll Need

For this experiment, you will need:

Micro:bit

Battery pack

You only need a micro:bit to make this compass, but if you plan to navigate outdoors, then you’ll need a battery pack as well.

Construction

Go to https://github.com/simonmonk/mbms/ to access the book’s code repository and click the link for Compass. Once the program has opened, click Download and then copy the hex file onto your micro:bit. If you get stuck on this section, head back to Chapter 1, where the process of getting programs onto your micro:bit is explained in full.
If you prefer to use Python, then download the code from the same website, along with instructions for downloading and using the book’s examples. The Python file for this experiment is ch_04_Compass.py.
Once you’ve loaded the program onto your micro:bit, you’ll see a message prompting you to move your micro:bit in a certain way. Follow the instructions. This will happen every time you flash a program onto the micro:bit that uses the magnetometer.
The purpose of moving your micro:bit this way is to calibrate its magnetometer. The magnetometer chip on the micro:bit is sensitive to local magnetic field variations. By moving the magnetometer in different directions, you help its internal filtering software to compensate for the local distortions to Earth’s magnetic field. This is why it’s a good idea to calibrate your compass again if you take it outside, away from the metal objects commonly found indoors. Also, a battery pack can affect the magnetometer, so it’s best to calibrate with the same hardware you’ll use on the final project.

Calibrating your micro:bit’s magnetometer is a bit like doing a puzzle. As you move your micro:bit, more LEDs will light up (Figure 4-2).

Figure 4-2: Calibrating your micro:bit’s magnetometer
To use the compass, attach a battery pack to your micro:bit and take it outside, well away from items such as computers and household appliances. Hold the micro:bit level (so it’s flat). If the arrow points to the right or left, slowly turn in that direction until the arrow points straight ahead. When it does, you’re facing magnetic north.
If the compass isn’t pointing north, try calibrating it again by pressing button A.

Code

The Blocks and MicroPython versions of the code follow the same pattern. The heading, or bearing, (the direction the micro:bit is pointing) is measured and then the program decides which of the three arrows (north, west, or east) it should display in order to direct the user in the right direction.

Blocks Code

Here is the Blocks code for this project.

In this project, we’ll set up the code so that you can calibrate the magnetometer any time you want by pressing button A.

The calibrate compass block is inside the on button A pressed block. The calibrate compass block will display the dot we used to calibrate the magnetometer earlier. You can then roll it around the display, as we did at the start of this project.

In the forever block, assign the value of compass heading (degrees from 0 to 359) to a variable called heading, which will contain this heading.

Then create a large if block that consists of an if, an else if, and an else. This big block will test which direction the micro:bit is facing and display an arrow on the LED display showing you which way to turn to be facing north.

If you’re writing this code yourself, you will notice that the Blocks code library seems to offer only two kinds of if blocks: if then and else. To add the else if then section, click the + circled in Figure 4-3.

Figure 4-3: Adding another section to if blocks

The first line of the large if block tests whether heading is between 10 and 180 degrees, where 0 degrees is north. Thus, a heading value between 10 and 180 would indicate that you were facing east. In this case, the micro:bit would display the west arrow, which points left. This arrow tells you to turn to the left in order to face north.

If heading is not between 10 and 180, then the else if part of the block tests whether heading is greater than or equal to 180 and less than 350 degrees. If it is, then the micro:bit displays the east arrow, which tells you to go right.

If heading is between 350 and 10 degrees, the else part of the if block will display the north arrow, which points forward—you’re on the right track! Figure 4-4 shows how this works (the numbers are possible values of heading).

Figure 4-4: The arrow will point in one of three directions, guiding you north.

MicroPython Code

Here is the MicroPython code for the project:

from microbit import *

while True:
    heading = compass.heading()
    if heading > 10 and heading < 180:
        display.show(Image.ARROW_W)
    elif heading >= 180 and heading < 350:
        display.show(Image.ARROW_E)
    else:
        display.show(Image.ARROW_N)

    if button_a.was_pressed():
        compass.calibrate()

This code uses the same logic as its Blocks code equivalent: it checks the reading of the magnetometer and tells the micro:bit to display an arrow. Again, if the heading is between 10 and 180 degrees from north, the micro:bit shows a west arrow; if it’s between 180 and 350 degrees, the micro:bit displays an east arrow; and if the heading is within 20 degrees of north, an arrow points straight up, telling you to keep going in the same direction.

Things to Try

See whether you can use a magnet to confuse the compass about which direction is north.

How It Works: The Earth’s Magnetic Field

Earth’s magnetic north and south poles have strong charges, creating a magnetic field all around the globe, as shown in Figure 4-5.

Figure 4-5: Earth’s magnetic field

The micro:bit’s magnetometer measures the strength of Earth’s magnetic field to deduce the direction the micro:bit is facing.

Interestingly, the magnetic north pole is not exactly at the top of the spinning ball that is Earth. The magnetic field can be centered up to 20 degrees away from the geographic poles, depending on where you are in the world.

Another interesting fact about the magnetic poles is that they’re shifting at a rate of about 6 miles (10 km) a year. One day, the poles will flip completely, meaning that magnetic north will be at the southern end of the globe. This process happens on Earth about every 450,000 years.

EXPERIMENT 4: MEASURING MAGNETIC FIELDS

Difficulty: Medium

As we just saw, the micro:bit’s built-in magnetometer is sensitive enough that you can use it as a compass to find north. It’s also a great tool for measuring the strength of a nearby magnet. In this experiment (shown in Figure 4-6), you’ll measure the strength of a magnetic field at various distances.