Simple Magnetic Alarm


Introduction: Simple Magnetic Alarm

Wary of intruders in the night? Don't feel secure knowing that there are expert lockpickers prowling the neighborhood? Perhaps you live in an apartment and you don't trust the landlord because he seems to watch you? Well here's a simple solution for your problems (at least in your room).

I apologize for the crappy photos and for the fact that I kind of broke my smoke detector's alarm so until I find the other one that is wandering around the house somewhere, I have created this temporary set-up to give you all the plans on how to create this alarm.

1. Disc magnet
2. Small flashlight
3. Alarm (old smoke detector, alarm clock, etc.)
4. Metal right angle (or plate)
5. Wire
6. Conductive foil
7. 9v battery
8. Electrician's tape
9. 9v clip (optional, but easier)
10. Capacitor (optional unless you need it)

Step 1: Only the Beginning...

Ok, confession time. I broke my smoke detector's alarm, so I am unable to show it to you correctly (though I did salvage parts for other projects). Hopefully you understood the intro photos in how this works. So I'm showing it using an old speaker (which I'm converting later, so I didn't mind cutting the wires).

1. Cut the wires to your alarm (or solder some on, if neccessary) and strip the ends.

2. Take the cover off of your flashlight and keep only the plastic lens holder (don't keep the lens)

3. Put your disc magnet into the plastic housing. Should have enough room to move freely, but not enough to slide or flip around.

4. Now take your 9v battery and put a piece of electrican's tape on it.

Step 2: This Is the End of the Instructable.

1. Take a piece of wire and tape it (or use the 9v clip and twist the wires together) to the negative terminal of the battery. Attach the other end to the middle of the battery on the electrician's tape (I taped the tip down, you can glue it).

2. Now take the negative wire coming from the alarm and place it in the middle of the battery on the electrician's tape, but not touching the other wire.

3. Take the postive wire from the alarm and attach it to positive battery terminal (or clip)

4. Place the metal plate (or right angle) on top of the magnet inside the housing. Put the housing over the wires on the battery and tape the housing to the battery. Now you have a primed alarm.

Some other ideas that I didn't include in this instructable:
1. Drill a small hole through the center of the magnet and thread some string in and tie a knot on the other side. Now you have an easy reset string.

2. Install a hidden on/off button somewhere so you can set up the alarm easy.

If you have any improvement ideas, I'd be glad to hear them...

Step 3: Smoke Alarm's Design (I Found the Other One)

More or less the pictures below will be your guide. The smoke detector you use may be set up differently, but the way to figure out which points to solder is the same. You'll need a multimeter to test between the points or some electric wire.

1. Hook the circuit board up to the 9v battery.
2. Using the multimeter/wire, test which points react to others.
3. After several tests, you should have figured out that two of the points underneath the detector (located right next to each other) will allow you to by-pass the detector.
4. Unhook the battery and just use some solder to connect these two points together.

Now you have an alarm to hook into your system. Good Job...



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    14 Discussions

    i had made something similar about a month ago..
    i used a timed buzzer from another instructable and added a magnet switch that i made my self the only problem is i have to be in the room with the door closed to set it... which makes it kinda useless..

    When You close the door the alarm turns off. You can fix this by wiring a relay to click and hold down. This way the only way to shut it off is to bipass the coil which you can use a key cylinder for. You can also find buzzers at radioshack for five dollars

    3 replies

    in that case, the keylock switch(i am assuming thats what u were talking about) would have to function as a momentary off. Apem has those (free sample...) however, it can also be done with multiple relays and a normal push switch, wiring it through the keylock as u desired (in this case it is a normal one) i am using the same concept as the arm/disarm feature in my diy estes rocket launcher -- one button is used to arm and another to disarm. the idea is that the one relay will click and stay up when the 1st button is pressed. it is wired through the normally closed pin on the 2nd relay, which will interrupt the circuit when the other button is pressed a 3rd relay might be needed if u want to keep the circuits separate

    But when you do it that way you always have elctricity flowing. I don't quite understand your design. I don't understand why there would be a need for a third relay. I designed one where I could set it from the outside but the best way is to buy a remotr control circuit switch from orchard. My design that I have now failed in my eyes because electricity is always flowing. The remote control will allow you to complete the circuit from the outside but the magnetic switch will then take over not allowing it to flow.

    the 3rd relay, like i said is there to separate the circuits we are talking about standard 5 pin relays (2 power, 1 in, 2 out, -- so to speak..) the design i use involves 4 of the pins to complete a circuit and keep the relay "alive" once it has been manually set to the up position through a push switch. then it can power that 3rd relay mentioned (using only 2 pins - power) leaving the other 3 free to route the circuit in this instructable though... now -- instead of using a push off switch to break the circuit and "kill" the relay, i am using another relay, previously the 2nd one... the other circuit is wired through its in/out in the off position, and turning the relay on breaks that. now -- honestly, that explanation confuses me too and a schematic probably wont help so im gonna put up some pics tmrw from this cool interactive electronics program i found lying around -- its pretty good at showing what will happen in the circuit and how

    Someone could use say a knife (or any other ferrous metal) to disarm this (bad if your using this for security), i would suggest putting the magnet on the other side of this system and use a small spring to push a metal plate towards the contacts.

    Can I use it say if I were to go to a hotel. Like put it on the inside of the door? Because I am goning to a retreat and my freinds which will be in another room like to prank people.

    1 reply

    Hmmm, never noticed that. In step C, my conductive foil disappeared from the picture. To clarify: The foil will still be there when the connection is completed so just imagine it being there in step C. Sorry about that, I thought it was there when I posted it. Oh well....

    Hey, that's a good idea. I've been trying to figure out how to set an alarm like that, and this has def helped. Ty!

    1 reply

    JPEGS are bad :)

    It may, but I didn't feel like messing around to create one. I'll try to post your idea along with the actual alarm when I find my other old detector. Thanks for the input.