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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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