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Andy "Not Dice" Dice
  
location:
We can live in dumpsters if we have to, MA
Gender: Male
UE'ing is for pussies, bro.
 |  |  |  | Magnetic door sensors.
< on 5/4/2005 5:51 AM >
|  | | | I'm certain that this has been a topic before, but I'm ALSO certain that the issue to which I am curious about was not addressed in it, or at least not addressed at any great length.
Magnetic door sensors.
Most of them just sense when the circuit is cut, right? But does there exist a sensor that detects ANY significant change in resistance or anything like that, thus making it a bad idea to carefully unscrew the magnet from the door and jury-rig it into place right next to the static part of the device?
It's an active structure that I'm curious about, so I know that there's power to the sensors. There's nobody on security there whom I would be willing to talk to about it without fear of incriminating myself, so if this type of sensor exists, I'm going to have to assume that these doors have them, and that could kinda be a bummer as it would hinder my EFFORTS.
Thanks a bunch!
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sleek
 
Gender: Male
| |  | | Re: Magnetic door sensors.
<Reply # 1 on 5/4/2005 6:00 AM >
| | | | I work with the very things. if you can remove the bar that is attached to the door the circuit is still complete, therefore no alarm. however they are very hard to remove generally. There is usually a box nearby with a small philips screw that when opened releases the maglock, Beware this may or may not be alarmed.
hope this helps
Whats going on I wanna see... let me through I'm a nosy bastard |
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Andy "Not Dice" Dice
location:
We can live in dumpsters if we have to, MA
Gender: Male
UE'ing is for pussies, bro.
| | | | Re: Magnetic door sensors.
<Reply # 2 on 5/4/2005 6:10 AM >
| | | | I checked out some doors in unsuspicious areas of the building that have the sensors and it looks like the magnet on the door is just screwed on, so I plan on unscrewing it and taping it to the box on the doorframe; this is what you just suggested doing, right?
So I don't have to worry about tripping an alarm by putting the door-piece CLOSER to the box on the doorframe?
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ofberenonehand
location:
Minn-e-snow-ta
"Where now is Boromir the Fair? He tarries and I grieve."
| | Re: Magnetic door sensors.
<Reply # 3 on 5/4/2005 9:30 PM >
| | | | How is it possible to unscrew it without opening the door and thus setting off the alarm?
"That's What Government Is For; To Get In A Man's Way" -Mal |
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Chainsaw
This member has been banned. See the banlist for more information.
location:
Underground, Colorado
Gender: Male
| | | Re: Magnetic door sensors.
<Reply # 4 on 5/4/2005 10:32 PM >
| | | | These sensors are fairly common, basically the magnet is pulling down a reed switch, when the magnet is moved away the switch opens.
There should be no problem just sticking the magnet right on the sensor, they aren't sensitive that way.
Instead of unscrewing the magnet from the door a small magnet with a piece of tape over the sensor would have the same effect, just make sure you get the right pole facing the right way. You can tell which way the pole on the door magnet is by taking your dummy magnet and seeing which way attracts and which repels...if you can't figure it out from there might I suggest you leave the alarm systems alone?
Sleek- he's not talking about a magnetic lock - just a door open sensor.
Probably looks like this:
http://homesecurit....asp?productID=527
As far as making the swap - just do it when the alarm system is disarmed.
[last edit 5/4/2005 10:32 PM by Chainsaw - edited 1 times]
Quid latine dictum sit, altum sonatur.
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totensiebush
location:
Berkeley, CA
Gender: Male
neophyte
| | Re: Magnetic door sensors.
<Reply # 5 on 5/5/2005 8:40 PM >
| | | | Posted by ofberenonehand
How is it possible to unscrew it without opening the door and thus setting off the alarm?
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He is probably on the same side of the door as the sensor - maybe for getting out or something?
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ofberenonehand
location:
Minn-e-snow-ta
"Where now is Boromir the Fair? He tarries and I grieve."
| | Re: Magnetic door sensors.
<Reply # 6 on 5/5/2005 8:42 PM >
| | | | Posted by totensiebush
He is probably on the same side of the door as the sensor - maybe for getting out or something?
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But aren't they usually mounted inside the door frame?
"That's What Government Is For; To Get In A Man's Way" -Mal |
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grit1
location:
University Campus - Minneapolis, MN
Gender: Male
Got Shear Line?
| | | Re: Magnetic door sensors.
<Reply # 7 on 5/5/2005 8:59 PM >
| | | | A lot of them are surface mounted on the top of the door and on the frame.
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ofberenonehand
location:
Minn-e-snow-ta
"Where now is Boromir the Fair? He tarries and I grieve."
| | Re: Magnetic door sensors.
<Reply # 8 on 5/5/2005 9:08 PM >
| | | | Posted by grit1
A lot of them are surface mounted on the top of the door and on the frame.
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Ahhh, I see. I bought a bunch of those at Ax-Man for a few bucks a piece when I was young. Alarmed my door and windows. I always thought that was too cheap for any decent security system.
"That's What Government Is For; To Get In A Man's Way" -Mal |
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grit1
location:
University Campus - Minneapolis, MN
Gender: Male
Got Shear Line?
| | | Re: Magnetic door sensors.
<Reply # 9 on 5/5/2005 9:51 PM >
| | | | Posted by ofberenonehand
Ahhh, I see. I bought a bunch of those at Ax-Man for a few bucks a piece when I was young. Alarmed my door and windows. I always thought that was too cheap for any decent security system.
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Yay for Ax-man's ... best Twin Cities Store...well...yeah...best store ever! ~Grit.
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Avatar-X
 
Alpha Husky
location:
West Coast
Gender: Male
yay!
| | | Re: Magnetic door sensors.
<Reply # 10 on 5/5/2005 10:18 PM >
| | | | The easiest way to defeat these types of sensors is to simply bridge the connection between the two screws on it.
If you do this, you can open the door without tripping the sensor.
Note: This only applies if the sensor is a NC type, where opening the door cuts the circuit. This is the most common in alarm systems.
-av
huskies - such fluff. |
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grit1
location:
University Campus - Minneapolis, MN
Gender: Male
Got Shear Line?
| | | Re: Magnetic door sensors.
<Reply # 11 on 5/5/2005 10:27 PM >
| | | | Posted by Avatar-X
The easiest way to defeat these types of sensors is to simply bridge the connection between the two screws on it.
If you do this, you can open the door without tripping the sensor.
Note: This only applies if the sensor is a NC type, where opening the door cuts the circuit. This is the most common in alarm systems.
-av
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You could run a continuity test with your handy-dandy multimeter to see if its NC or NO ... if the door is closed and the meter beeps ... normally open ... no beep and it's normally closed.
I didn't know they had power across the screws though. ~Grit.
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Chainsaw
This member has been banned. See the banlist for more information.
location:
Underground, Colorado
Gender: Male
| | | Re: Magnetic door sensors.
<Reply # 12 on 5/5/2005 10:48 PM >
| | | | Yeah, I doubt that very much, but I'll take av's word on it - seems really stupid to build it that way.
Quid latine dictum sit, altum sonatur.
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Andy "Not Dice" Dice
location:
We can live in dumpsters if we have to, MA
Gender: Male
UE'ing is for pussies, bro.
| | | | Re: Magnetic door sensors.
<Reply # 13 on 5/5/2005 11:35 PM >
| | | | I AM on the same side as the sensor, inside the building. The door is on the roof. What I need to do is be able to go up there while the system is disarmed, during the day, jury-rig it if possible, and put a piece of duct tape over the bolt on the lock (it's one of those doors that's always unlocked from one side, but locked on the other, y'know?) so that I can go up onto the roof and access the building through this door during the night. I can't do this to the ground-level doors because the building is active and people use them during the day.
I'm not stealing or vandalizing anything, don't worry about it. It's a long story.
I shoulda explained this earlier to avoid confusion, sorry 'bout that.
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Avatar-X
Alpha Husky
location:
West Coast
Gender: Male
yay!
| | | Re: Magnetic door sensors.
<Reply # 14 on 5/6/2005 12:54 AM >
| | | | I didn't mean the screws holding it to the frame -- I mean the screw terminals the wires are attached to.
If there are no screw terminals, gently scrape away the insulation on each of the two wires coming out of the device and then tie them together or bridge them somehow.
-av
huskies - such fluff. |
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SinisterMessiah
location:
South Portland, ME
Gender: Male
| | | Re: Magnetic door sensors.
<Reply # 15 on 5/6/2005 4:39 AM >
| | | | There's an electronic device we have in the military for dealing with those types of things. We got to use them during urban assault drills when trying to get into a seized building that would have the alarm still intact to let the hostiles know where us good guys are coming in.
Basically you put it on the outside of the door and frame where you are trying to get into, and it shorts out the magnetic response in these security devices. They also have them so that it creates an illusion to the system that the circuit is still complete, even when you open the door. Tricky little suckers, they be.
Generally speaking slipping a magnet between them, doesn't really do anything. I've never heard of these types of security devices running any sort of current or circuit through the screws. These are designed to be straight-cut entry detection systems.
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Avatar-X
Alpha Husky
location:
West Coast
Gender: Male
yay!
| | | Re: Magnetic door sensors.
<Reply # 16 on 5/6/2005 5:28 AM >
| | | | Here is a quick diagram of how these things work:
Essentially, the current flows down one wire, into a hinged metal bar attached to a spring, into a contact point, and then back out through the second wire.
The proximity of the magnet causes the bar to overcome the spring and touch the contact.
As soon as you take the magnet away, the spring will pull the bar away from the contact point and the circuit will be broken.
This is why cutting the wire won't work -- it will also break the circuit.
You also can't just place magnets arbitrarily -- chances are your magnet will pull the bar away from the contact point.
-av
[last edit 5/6/2005 5:30 AM by Avatar-X - edited 1 times]
huskies - such fluff. |
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Andy "Not Dice" Dice
location:
We can live in dumpsters if we have to, MA
Gender: Male
UE'ing is for pussies, bro.
| | | | Re: Magnetic door sensors.
<Reply # 17 on 5/6/2005 7:13 AM >
| | | | Jimini Jelliquers, Av! The thought never even crossed my mind. Short circuit it, of course! I'll definately check that out as an option, thank you.
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Octothorpe
Noble Donor
location:
Citizen of the World
Gender: Male
| | Re: Magnetic door sensors.
<Reply # 18 on 5/6/2005 1:46 PM >
| | | | I've done (legitimate, authorized) work on alarm systems before, and it's in your best interests that I tell you the following: there are magnetic sensors whose reed switches are held in the MIDDLE of their range of movement internally by the magnet, so that removing the magnet will cause the reed to hit the top contact due to the spring that eternally tries to pull it up, and that adding a magnet will pull the reed down further than usual and cause it to hit the bottom contact.
As far as shorting out the screw terminals goes, any alarm system worth anything will use what are called end-of-loop resistors (EOLRs) that are intended to prevent silly people (or any people, for that matter) from shorting things out to get around things. However, instead of the resistor being soldered in line with the loop right at the panel as is usually done, in the magnetic switches I describe above the resistor is IN the switch, which is solid molded plastic.
So, when the reed is held between the two contacts by the correct magnet, there's maybe 1200 ohms across the two terminals. Allowing the reed to touch either contact (by removing or adding magnetic field strength) will short the contacts (since electricity prefers the path of least resistance, it won't bother with going through the resistor anymore and will go through the reed instead, which is a dead short). Cutting the wires will open the loop. Either opening or shorting the loop will set off the alarm.
Either way, you're basically fucked.
So, be careful before attempting this. It may not be as simple as it looks.
[last edit 5/6/2005 2:34 PM by Octothorpe - edited 1 times]
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Avatar-X
Alpha Husky
location:
West Coast
Gender: Male
yay!
| | | Re: Magnetic door sensors.
<Reply # 19 on 5/6/2005 2:38 PM >
| | | | Yeah, maybe in super-fancy alarm systems. But every magnetic reed switch I've seen in an alarm system is just a simple switch like in my diagram. The reason it works is because usually the sensor is on the INSIDE of the door, and the people you want to keep out are on the outside.
A floating bar like you describe with short points at the top and at the bottom would work, but would require an incredible amount of time to balance when attaching to the door, since moving the magnet 1CM closer or further would affect the position of the bar.
-av
huskies - such fluff. |
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