Alarm Question

[Marzka]

Will an alarm for a house or car go off if I attempt to open a door or window? Or will it only go off if broken?

[Æternaeon]

Depends.

Car alarms tend to break down into microwave or ultrasonic varieties. Microwave sensors put out microwaves and detect reflections typically within 3 feet. These are the ones that warn you when you get near. If you're around for longer than the set time, it sounds the alarm. Very annoying. The ultrasonic ones - your typical modern car alarm - will go off if any change in cabin pressure or movement is detected inside of the interior. So basically no matter how you enter the car it will sound the alarm if it is not disabled prior.

On a building it's much more complex an depends entirely on the systems used and where the sensors are located. There are glass break sensors that will detect of the glass is broken, they appear as almost foil strips on the glass wired into the main system.

You also have magnetic switches. When you open the window, the two pieces of the system separate which opens the circuit and sounds the alarm. You can see these as two "boxes" right next to each other on the window, one is on the frame and the other - typically the magnet - is on the moving pane.

Another issue for buildings is that if there is a motion sensor of any variety - PIR, ultrasonic, microwave, combo, etc. - facing the window and within range, any method of entry at all will trigger it.

Same applies to doors needless to say, just that there's no "door break sensor".

[Marzka]

Is there any way to disable a house alarm by cutting some wires? I'm guessing the alarm companies would think of a way around just having wires cut, though...

[TheBurnedMan]

depends, some of them are connected to the phone lines as well so they will still be operational if the power is cut, some have a battery and will use the cellular service to call the security company, not any good ways to disable those without being inside the building that I know of

[Zerg]

I wouldn't mess with disabling the hardware. There are hundreds of possibilities as to what goodies you'll find once you cut a wire or unplug a cord. It is almost always simpler and more effective to just watch over someone's shoulder as they type in the code. Or intercept messages that include the code.

[Æternaeon]

As for the alarm in and of itself, it's actually a REALLY simple thing that you can understand with only fundamental electronics knowledge.

The control panel has several simple linear circuits that branch out to various areas. Each of these circuits are almost always "closed" in modern systems. That is to say that there is a constant flow of power continuing through the line. Cutting the line "opens" the circuit.

The panel throws a switch if one of the circuits opens up, which creates the alarm condition. On a decent system, the panel has a switch for each circuit so that the alarm company knows exactly which area has been compromised. It also allows parts of the system to be shunted from the panel if there's a problem such as false alarms while the rest of the system remains active.

One way to open up the circuit is to trip a sensor. Separating a magnetic switch, setting off a glass break sensor, or having a motion sensor detect you will open the circuit, sounding an alarm. An obvious way to open it, of course, is to cut a line. So not a good idea.

Panels are typically backed up with batteries and usually use a wireless signal of some kind - cellular most commonly - anyways so there's not really a magic line you can cut to disable the system from the outside.

There's a lot to be said about alarms, but for now the important thing is to look at them as simple high school level circuits. That's really all they are, they're just extremely effective if installed right. Even knowing the work-arounds is no guarantee of success in defeating them, in fact it often doesn't help at all.

[Marzka]

Thanks, Æternaeon. That's a good explanation.
If only there was some way of opening a circuit while making the control panel believe the circuit is still closed...
I'm showing my ignorance here but if you had time at day to turn the alarm off and switch the wires leading to the windows to a small circuit you've created yourself than the alarm wouldn't realise you opening windows.
Than again, I may be just misunderstanding and taking the 'circuit' thing too literally.

[Zerg]

Well, theoretically, you could redirect the wires. But most modern systems also have voltage meters, so that would be extremely difficult.
Edit: You said you would this when the alarm was disabled so I guess this wouldn't really apply.

[Æternaeon]

Switching the wires to your own counterfeit circuit is a technique called "jumpering" in the tiny niche that is alarm circumvention. It can work but there are two major considerations and for the practical operative who starts out on the outside of a secured building it's all but impractical.

The first is the obvious fact that you need access to the wiring. You are usually outside or sometimes the circuit is hidden and protected by structuring or conduit. If you can access it during the day inconspicuously, this helps your cause but the process is time consuming and you will need to have lots of alone time with the system.

The second is the measure that Zerg alluded to, the 'end of line resistor'. EOLRs tend to be installed at or just after the final security element in the system so that if you were to install a line across any of the sensors, switches, etc. the panel would be able to pick up easier on a difference in resistance. What the panel does from here again depends on some conditions.

Alarm panels can often be set up to negate this feature. This, along with simply not using the EOLR or placing it at the panel instead are somewhat common practices because doing so eliminates a lot of false alarms that are due to surges through the system because of storms, faulty wiring, dirty power, or whatever other reason. It's especially seen as unnecessary on systems where the wiring is hidden because the potential for false alarms does not outweigh the risk of tampering because it's assumed any would-be troublemakers can't get to the wiring anyhow.

But you can never assume that unless you know for sure. So otherwise, the panel will consider the deviation in power to be either a trouble condition to be diagnosed and/or an alarm condition. Often a trouble condition sounds an alarm all the same so the practical effect for you, the operative, is still the same.

To be honest I'm not sure if panels record a known resistance so I can't say for certain if jumpering with the system deactivated will prevent an issue with the EOLR.

However, what I do know is that you can make any circuit match the resistance of the sensor by using a variable resistor to "program" the circuit to match resistance. I've never done this before but in theory, the process would work like this; you approach the deactivated system with a multimeter and make contact with the wiring at both sides of a sensor/switch you want to jumper. Record the resistance value at those two points of contact and leave.

Then what you do is create your counterfeit circuit. Basically a wire with a variable resistor on it, you can use something simple from a busted radio or anything with a knob to change volume or channels usually which will be a variable resistor. Then you'd attach your multimeter to both ends of your counterfeit circuit and run a resistance value test. Tune the resistor until your circuit exactly matches the alarm circuit in value. Lock the variable resistor at the setting however you can, then double check to make sure it's still the same afterward. Now you have a drop-in counterfeit circuit that is guaranteed to be able to jumper the system.

You can wire the counterfeit circuit in with the system shut off if you can access it in the daytime unnoticed again, just make sure you are unnoticed and then cut off the sensor or switch from the circuit. Hide your handiwork afterward.

It's a bit impractical since if you can reach the wiring with it activated you can probably just avoid the security element at hand, but if need be you could probably install this with the system on, too. To do that you would need to include an instant relay switch in your circuit and then have another one on hand. It would take a custom switch set up to do. Basically you'd wire in your counterfeit circuit with the switch off and then add your other switch into the line with the switch on so that now you can switch that relay off and open that part of the circuit. That done, you'd (and by "you" I mean some electronic set up that turns them at precisely the same time) simply turn the switches at the same time so that the counterfeit circuit takes over and hopefully it doesn't cause any spikes in power and the panel never detects a change. I've never done this either and don't have the time to test anytime soon so I couldn't say. I would recommend a safe test by having the two switches on a dummy circuit with a multimeter testing it and see if there are any spikes in resistance either way.

Sorry for the long winded nature of this point, but there's so much to go over. Again it's all simple electronics, but the practical application of it can get quite complicated.