To make my door alarm, I've got a new MakeCode 
project. Let's get rid of the 'on start' block

but keep the 'forever' block because we're going 
to keep checking the magnetic field measured by

the micro:bit's magnetometer, its compass. Go to 
'Logic' and find the 'if' block and drag that in,

and also in 'Logic' we'll find the comparison 
block, so get that hexagonal block and drop

it on top of 'true'. Then, if we go to 'Input' 
and click on 'more', you'll find a block called 'magnetic force'.

Drop it in the first hole in 
the comparison block, and we're going to change it

so that it measures the overall strength of the 
magnetic force. Your micro::bit's magnetometer can

measure magnetic force in different dimensions, 
but we're just interested in just how strong it is overall.

And we're going to pop a number 
in here that we might need to change later but

I'm just going to pick 200 for the moment. It 
measures the strength of the magnetic field

in a unit called 'microTeslas' and I'm going to 
try 200 to start with. If the magnetic field

falls below 200 microTeslas, I'm going to show an 
angry face because what that will mean is that the

magnet has gone away from my micro:bit, somebody's 
opened the door, and we want to know if someone's

been in the room so it's going to show an angry 
face on the display. How do I know what number

to pick for the magnetic force? How do I know 
how strong my magnet is? Well, we're going to

do something to sort that out. So, if I go 
to 'Input', I drag in 'on button A pressed',

and we'll add a 'show number' block. Let's copy 
the magnetic force block if I right-click on it,

duplicate, and drag it in here and drop it there. 
What this means now is that when I press button A,

I can measure the amount of magnetic force 
measured by the micro:bit. I can see it on the display.

So, we're ready to try this 
out now. It doesn't work in the simulator,

so we're going to put this on a real micro:bit. 
So, I'm going to call it 'door alarm',

and I'm going to download the hex file, and 
transfer it to my micro:bit. I've fixed the

micro:bit and the battery pack to the door frame 
using something like Blu Tak and put the magnet

on the door itself, very close to it. Have a look 
on the back of the micro:bit board and see where the compass is.

You need to make sure the magnet 
is close to the compass chip on the back of your micro:bit.

Now we can calibrate it. With the door 
closed, press button A and get a reading of

the magnetic field strength. In my case, it's around 
453 microTeslas. I'm then going to open the door,

press button A again and take a reading with the 
door open. It's 129 microTeslas now.

It can still measure some magnetism, don't forget the Earth 
is magnetic, and there are other things probably

in my house that are magnetic as well, so the 
micro:bit is still picking up some magnetic field.

So my number of 200 should work pretty well. If 
it's more than 200, it means the door is shut,

it sounds likely. Because with the door shut 
that's 453 microTeslas. Below 200, that probably

means somebody has opened the door, in this 
case I know when my door is open I have 129 microTeslas.

So that should work OK now, I should 
be able to tell when someone has opened my door,

an angry face will appear on my micro:bit's 
display, and I'll know that somebody has been in my room.

There are lots of things you could do 
to this project to augment it. You could perhaps

have a delay to give yourself some time to get 
out the room. Remember if you need to reset it,

you could press button A to show the magnetic 
force again and that will clear the angry face

off the display of your micro:bit. Maybe you 
could add an audible alarm too or

you could add a music block so that when somebody opens 
your door, it makes an audible sound to warn you

that somebody has been inside your room. There are 
all sorts of ways you could modify this project.