Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

A simple & small magnetic door alarm

Not open for further replies.
Buy a megnetic-reed switch set and screw one part of it to the door and other to the fixed frame of the door. The switch should be N/O type. As soon as the door is opened the switch will be closed and any device connected to it will turn on.
the reed switch is the simplest way. but instead of buying the set, you can buy only the switch, because this way you can mask the contact better. and put it on the doorframe, and on the door put a small magnet,. . i think that you need a normally closed switch. when the door is closed the magnet near it will keep it opened. when you open the door, the magnet mooves away and the switch closes and activates the circuit.
but you can find small alarms like that already built.
I agree with kinjalgp,use a normally open type reed,because this method protect also against sabotage of wires.
question ....

Just as an aside here,
Do ya'aal think it might be possible to stroke
a reedswitch repeatedly with a magnet, enough to
cause the reedswitch to close without
the magnet being present?

john1, nope, won't happen. I've been installing and servicing security systems for over 7 years now and you wont be able to "magnetize" the reed switch.
I have seen a magnet installed in a steel door that has distributed the magnetic field onto the steel door so much that it (the magnet) has lost it's magnet-ticity... if that's a real word?

By the way, in the residential security business, we use normally closed type switches but normally open type would work fine depending on what you want to activate.

And to protect against sabotoge, the system looks for a resistance value to "supervise" the loop. We install a resistor in line at the switch end of the wire run. I don't feel this is a very necessary feature unless you're going for high security.
first...i think it is language problems. a switch is opened when current passes or not?
and about the magnetization of the reed switch...there is no posibility to remain conected no matter how long you keep the magnet neat it.
it is made of a special matterial and desides, if it would magnetize then the contacts would reject even more.
so guys, dont forget to tell me what does an opened switch mean...
Hope this picture solves your problem, bogdanfirst :)


  • Switch.gif
    462 bytes · Views: 1,466
Hi Scratch,

Thanks for dropping in!
I may not have made myself clear ....
I do realise that repeated operation of a reed switch,
even after many years would not cause it to retain
much magnetism, that wasn't quite what i meant.

One can magnetise a nail by stroking it with a magnet
so long as you repeatedly use the same end of the
magnet, it doesnt get very much magnetism transferred
to it, but usually after a while, it could pick up
another nail.

I doubt if one could magnetise a nail by moving a
magnet close to it, then away again on the same route,
which is what a reed switch gets.

When one is magnetising a nail in this way, the
stroking action is intended to 'line up' the molecules
in a way that does not happen by moving the magnet
towards, and away again, like the action of a door.

Now, i have never tried this with a reed switch,
but i am very curious as to whether or not it would
retain any magnetism, and thereby remain closed.

It may be that the reeds are made of some ferrous
material that does not retain magnetism very well
but still responds as required.

I may see if i can find one somewhere to give it a try.

Thanks for mentioning the 'resistor with the switch'
i had thought that something like that would be done.

In instrumentation, it is something that used to be
done quite a lot. The zero reading from a sensor is
often about 12mA (i think), this low current can be
'zeroed' on the meter at the readout by the pointer
If the meter is reading below zero then the repairman
knows that its not getting its 12mA, and that helps to
locate the fault.

If i cannot get a reed switch to do this, maybe i
could stick a little magnet behind it to keep it shut.
I would prefer it to close without me doing that.

Regards, John
thnaks for the open switch. i was wrong you were wright, but it wasnt a technical problem.
here is why the reed sw doesnt magnetize, actually it does. but think of something two poles from the magnet like south and soth or north and north reject. in the same way when you put a magnet near the switch it only magnetizes the contacts with the same pole, fo after the magnet is removed they will reject.
Hi bogdanfirst,

Thank you for your reply.
Although to be honest, i wasn't sure what you meant.

I know they dont become noticeably magnetised in normal use.

I was thinking of deliberately trying to magnetise one,
so that it would remain closed, on its own.

I got one yesterday, and stroked it with a magnet
for about half an hour.
It made absolutely no difference at all.
It closed at the same distance with the magnet
in either direction.

I find that very puzzling.
I would have expected it to close a little nearer one way,
and to close a little further the other way.

But it was just the same.

I dont understand why no magnetism was induced in it.

I may try 'flashing' which is using a lead-acid battery to
provide a heavy current in windings, often used with
dynamoes for the field winding.

That may cause it to be more sensitive in one direction.

you can try, i tried some time ago, but i coudnt magnetize it. good luck in trying, but i am telling you they are made specially not to magnetize.
Yes bogdanfirst,

I think you're right.
I will try 'flashing' it,
but its not looking likely.

Maybe i could get a small magnet and glue it on the back,
that should hold it shut.

Why were you trying to magnetise a reed switch?

i was curious like you i wnated to know if you could magnetize it.
why do you want to make it stay closed?
Hi bogdanfirst,

I have some of the normally open type.
They are the usual ones.
I want to magnetise it to stay closed when left alone.
The intention is that when the magnet comes over to it,
it would open.
I would have to make sure that the magnet was the right
way round, to do that.

So far i have had no success.

The intention is that i could then use the ones i have
as normally closed types, which would suit me better.

Maybe i will end up using a small magnet glued to it,
but i would be happy if the 'flashing' works.

Maybe i will find that its too fiddly to be a practical

Your English is much better than my Romanian.
Regards, John
try this, place a small magnet at a distance near it and another one, maybe a larger one, so that when the second magnet comes near it it will reject the first and open the contacts.
i would try, but i dont have a reed swtch.
Hi bogdanfirst,

Yes, thats the sort of thing,
it sounds easy enough.

but first i want to try 'flashing' it
because that seems an easier way as
there is nothing else involved.

I will try the two magnet thing now,
and let you know if it works.
I see no reason why it shouldn't.

Regards, John
Hi bogdanfirst,

yes, it works perfectly well.
just a bit of positioning and its ok.
the reed switch will stay closed
with just small bits of broken magnet on it.

And when the moving magnet comes up to it,
it will cheerfully open.

It was so little trouble that i think
i will forget about the flashing.

Regards, John
Not open for further replies.

Latest threads