Can an open/close sensor be "defeated"

(Monte Montemayor) #1

In theory, could a person put a small magnet in front of an open/close sensor to simulate it still being closed? Not ready to use ST for Security but just thinking out loud here.

(Ray) #2

It’s not a theory. You can definitely defeat the sensor this way but depending what you are trying to protect. Mounting it in an easy accessible location for an intruder is not ideal for a contact sensor.


Open/close sensors should be placed with the magnet and sensor pieces on the interior of the building, not the exterior. At that point, the person has to already have access to the restricted space before they can place a second magnet to maintain the closed reading. So for most residential deployments that’s sufficient from a security standpoint.

In commercial buildings where someone might be allowed access to the room at some times, but not at others, then you do have to address this possibility.

(Edward Niedziejko) #4

Door contracts in conjunction with motion sensors are usually used in commercial buildings. Even if the door contact is somehow comprised the motion sensors make up for it.


Yep. It’s also a good idea if you have some things secured inside the home against other household residents, such as the liquor cabinet.

(Larry) #6

i imagine even on the inside if you had a powerfull enough magnet on the outside you could still defeat it through the wall. if i could find my old tape eraser i would try it.

(Edward Niedziejko) #7

Interesting, if you set the field backwards to the existing magnet, the contact could momentarily disengage and set the alarm off. You’d only have a 50% chance of getting the magnetic field the same direction as the existing one.


People try this every year or so and write blog articles about it for various security magazines and almost always come to the conclusion that there’s less than a 50% chance of A stranger being able to do it. It’s just really hard to do. And that’s without worrying about being noticed while you’re doing it. And even if they did succeed, they couldn’t be certain that they had, which makes it less likely to be tried.

First, if there’s any kind of metal plating on the outside, even a screen door, no matter how powerful the magnet is that you were trying from the outside, the field will diffuse, which means it won’t move the reed on the other side of that wooden door. So it’s really easy to design something to defeat that specific issue.

Second, as @Edward_Niedziejko just mentioned, you have to get the second magnet in place without dropping the connection from the first magnet even for a second. That’s harder than it sounds because you have to know both the position and the polarity of the first magnet.

Again, if the person who’s trying to defeat the system has access to the original sensor from the inside, yes, they can probably do it easily and quickly if it’s at a time of day when no one is going to worry if That door/window is open.

But if you’re talking about a burglar from the outside who doesn’t already know everything there is to know about your security system, the practical risk is extremely low.

You can try the extra powerful magnet from the outside if you want, but give it to someone who’s never seen the inside of the house and doesn’t know where your sensor is and then see what happens. Also remember that they won’t know if they succeeded so they have to make the decision about when to try it without any feedback information.

And meanwhile, of course, any other security devices you have such as motion sensors or cameras may have detected them in the process of trying to use the magnet.

A teenager who wants to defeat a contact sensor on their parents’ liquor cabinet can probably do so, especially if they have access to the Feedback information. Which is why you put a motion sensor inside the cabinet as well if you’re concerned about thieves in your own household.

But there’s a reason the contact sensors are so popular in low cost security systems – – they work reasonably well against typical break-ins.

(Joel W) #9

I have actually seen on a jib way back the open close sensor was defeated using a huge fishing magnet. Many homes and buildings have metal doors on the exterior, or garage entrance to home. The thief takes a large what we at one time called a salvage magnet (the ones treasure hunters drop off boats) 4" x 4" large place it on the outside of the door in the corner the magnet is located. The magnet sticks to the door, creating a very strong magnetic field, allowing the thief to open the door just enough to insert a magnet on a stick in from to the contact. He now can tape a temporary magnet in place of the original. He got caught by a security guard, but in a home, he would have gotten away with it. After that day we changed the exterior contacts to plunger type in the frame.

(Michael Deffendall) #10

This is why it is important to have “layers” of defense. Locks + Door Sensor + Motion Sensor + Alarm. More layers=More security

(Monte Montemayor) #11

Thanks everyone - part of my motivation to ask such a question has to do with wanting to secure things like liquor cabinets - other things too. We have an antique cabinet that doesn’t have a lock but there is a place for something like a NYCE NCZ-301 or Sensative Strip.

(Michael Deffendall) #12

Put a contact sensor on your favorite bottle :grin:

(Monte Montemayor) #13

LOL, those of us with teenagers know where I’m going with all of this… because once WE were teenagers. Today’s kids are just a little more cleaver than I was.

(Realy Living Dream) #14

" In theory" a lot of things are “possible”. The issue to be concerned with is how likely is it to happen? In theory ( and this has been done successfully ) somebody could be sitting in the mall parking lot with a signal sniffer and get the RFID codes your key fob uses to unlock and start your car. However unless you are driving a mid 6 to 7 figure supercar, chances of it happening are infinitesimal. If I lived in a multi-million dollar estate, I would not be trusting a $10 sensor to guard my treasures.
If the crack hoes and heroine junkies that might attempt to break into the majority of houses had the tools ( other than a rock or screwdriver ) to bypass residential security, it would have already pawned.
For guarding the liquor cabinet from unauthorized access, given enough motivation and time, I’m sure some forward thinking person could easily find a way to bypass. Of course the simplest way would be to simply disconnect the hub for a couple minutes, get what they wanted, then reconnect it before it timed out and reported the disconnect.

(Monte Montemayor) #15

Do tell! Where can I get one of those ;^)

(Monte Montemayor) #16

I think I am going to use WebCore to send me a text if that happens. Good thought! (See, I didn’t even think of that, that’s why I’m asking).

(Realy Living Dream) #17

I got mine from Lowe’s when they were on sale. A few cases of the ST multi-sensor for $15 when they were on sale too.
If you are serious about home automation, you need to watch the sales or have a bottomless bank account.
I was inventorying the other day, realized I have probably $2000 retail of “extra” devices. Just waiting for a project I need them for.


Just wrap the sensor in some tin foil for a few seconds while replacing the good stuff with colored water. Difficult for the hub to differentiate between sleeping sensors and extremely temporarily unreachable sensors. Signal strength from a battery based sensor isn’t that great to begin with so probably doesn’t have to be a full coverage wrap even. But if you’re at that level, you may want to investigate padlocks instead. :slight_smile:


(Edward Niedziejko) #19

Put a motion sensor in the back of the cabinet instead. You really can’t defeat a motion sensor that’s mounted facing a door inside a closed cabinet, unless you defeat the whole system, but that is something you can monitor. Also, you can just buy a second smartthings hub, put it in an obvious spot, and hide the “real” one elsewhere, but set the second one with tamper warnings. :wink:


The reason I got into SmartThings was my son was sneaking out the side door of the house at night, sooooo I started looking at door sensors and now a year later I have a ton of sensors, Echo Dots, lights, automation, cameras, etc… Sigh… Its definitely more fun and less stressful catching your kid sneaking out than burglars sneaking in.