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Magnetic door alarm

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Hi... I'm asking you guys because i have a serious problem. I'm about to make a project in school and i don't quite know how to get started on it...
I would just say before you start accusing me of cheating that i came up with the idea for this project by myself.

But here we go... I have chosen to make a door alarm that works by placing a magnet on the door and and a magnet of the opposite charge (+/-), on the door frame. When the door is opened without shutting down the magnets the alarm sounds...
Here comes the tricky part
I want it to work so the two different poles creates a field of magnetism that is strong enough to work over the opening in a door!

Hopes it makes sense... If not, ask me!!!
No its not clear. Can you show a graphic of what you want to do?
Virtually all door alarms only have one magnet, and a magnetic switch. The magnet is embedded in the door, and the switch (+associated wiring) is embedded in the door frame.

If you want, you could replace the permanent magnet, with an electromagnet (but I can't see the point in doing that).
It kinda sounds like you're looking for two different types of circuits here. Are you wanting a basic alarm that sounds when the door is opened or do you want to know if someone has walked through the door if it were already opened?
I want the two magnets to create a magnetic field that sounds an alarm when broken.
So Basically you want to to mount a magnet on each side of the doorframe, use them as sensors and the alarm goes of when someone walks between the magnets? Does your experiment have to involve the use of magnets? I ask because this would require some powerful/sensitive magnets and complimentary circuitry. And the same could be accomplished without the magnets.
the way i seee it you need to hava an alarm sounded when you open the door? you can simply use a reed swith on the door frame and a magnet on the door? how about this idea?
bogdan, that's what I was thinking. Since he mentioned this was a "project" for school, I thought maybe he had to do something involving a magnetic field, not just any old way to make a door alarm. He could also use a photo diode, proximity switch, a coil around the door frame. There are a lot of possibilities.
depends of you want to sound the alarm when the door is opened or when someone passe trough it...
if you need to detect when someone passes trough it there are lots of you said, coli-inductive swicth, photodiode and source, laser beam, even make a small wire across wich you will brake when you enter.
but i dont understand the part with putting 2 magnets to generte elctromagnetic field ...
maybe you need to make a device that opens the door so that when you get near it it will open and sound an alarm?
No one is going to accuse you of cheating - that's between you and your teacher. If you want something to happen when a door opens, this group can work with you - but you are going to have to help with clear information.

How far must the door open before the alarm sounds?

Normally, what you are describing is done with one magnet and one switch. When the magnet (attached to the door) is within 2" - 3" of the switch (reed switch attached to the door frame), the switch is at rest - when the magnet is moved away (by opening the door) the switch activates. The switch is used to trigger an alarm circuit. This may not be what you had in mind.

If you want to experiment with two magnets, that a possibility too. But if the idea is to work with a magnetic field across a 36" door opening, that may be a bit much.

BTW, it would help us respond better if we knew your age. Let's get started!
Hi Tulkas,

This sounds very much like an arrangement i have tried
some time ago.

I used two magnets, arranged in opposition, and a
reed switch between the two.

I found that the stationaty magnet near the reed switch
only needed to be small, in fact i used a broken piece
from a magnet that had got broken.
Presumably cos its closer.

The positioning is not as critical as it looks.

I have used this arrangement and i can confirm that
it works ok.

With the usual arrangement the circuitry has to be
made to activate something when the reed switch opens,
this means that there will be a small current at rest.

With this arrangement there is no current drain until
it operates.

Reed switches are available with a normally closed
contact, but they seem a rarity, ive only seen one,
it had the normal, with an extra brass contact inside.

This may or may not be the sort of thing you mean,
but it fits your description very closely.

Regards, John


  • Doorswitch.jpg
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First of all, forget the magnets. Suppose someone wore a suit with a bunch of metal (magnetic) rings on it and they were walking through that door. what happens? the suit will stick to the magnet. So, forget the magnets.

There are two other ways of doing this. The results will be better. however, the parts will cost pennies more.

One way is to make a radio transmitter and a radio receiver. pick a frequency that the two units will share. Just make sure that they dont affect radio stations. The distance between the two units will determine the volume of the tone (of your choice) the transmitter produces.

Or a better option is to use infrared technology. You need a transparent transistor with the base pin taken out and a transparent diode. These two should come in one package. I think the circuitry will be simpler here.
why do you think that the magntes are so strong to attract metal parts from your can sut small magnets that will not attaract a needle from 2 cm....
the radio is too complicated for such a thing....
but a better solution than radio is light, use a laser or IR beam....
My alarm

My house alarm, and every other house alarm I have seen uses a magnet and a reed switch, so there cant be much wrong with that.

Anyway, no alarm is safe, you just need a big hammer.... it doesnt matter if that hammer hits the alarm or the alarm hits the hammer.... :D

you just need a big hammer

Prof - In our industry, we call it "percussive maintenance" :lol:
The instrument to perform percussive maintenance is a "peripheral weight-o-knock-o-meter" :lol:
Here we go... i've made a sketch of it and here we go.

I only want the alarm to go off when the door is opened. Because i don't think it is possible for even the skinniest person alive to crawl in between a door and a doorframe.
Well i can't seem to get the attachments to work but anyway... The pic ythat John1 has posted is exactly what im looking for.

And for those of you who asked: The alarm has to go off when the door is opened
Hey i was about to buy one of those type of alarms, but i thought it was kinda shabby. Cant you just put a magnet on both of em and open the door without triggering the alarm? If not, i might wanna go ahead and buy it :lol:
Hi iwant2drive,

You might think that you could do that,
but these are quite small items, the reed switch
is generally around an inch or less, and they are
sunk into the frame from the inner part of the frame
and there should be no indication of their location
externally. With one magnet the reed switch would be
closed when the door is closed, and the alarm would
operate if the reed switch opened.

In order to maintain a magnetic field to prevent the
reed switch from opening, you would have to provide
one externally, whilst the door was opened.
Such an externally applied field would have to be in
the correct direction, because if it were applied in
opposition, the reed switch would open and trigger
the alarm. You would also need to know pretty closely
where to apply such an external magnetic field,
because even a few centimetres away would mean that
the field would have to be exceptionally strong.

In the arrangement shown earlier in this thread, the
arrangement is reversed. The reed switch is open
with the door closed, because the two magnets are
arranged in opposition, the alarm is triggered when
the reed switch closes in this case.
I would imagine that it would be very difficult to
arrange for a changing magnetic field to ensure the
reed switch remains open whilst the door is opened.

If you applied an external magnetic field to the
second arrangement it would operate the reed switch.

Defeating these kinds of alarm can of course be done,
but it is not as easy as all that, and for the most
part they are easy to set up and to understand.

Best of luck with it, John :)
These devices are mounted on barriers and are used primarily to detect an attack on the structure itself. The technology relies on an unstable mechanical configuration that forms part of the electrical circuit. When movement or vibration occurs, the unstable portion of the circuit moves and breaks the current flow, which produces an alarm. The technology of the devices varies and can be sensitive to different levels of vibration.**broken link removed**
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