Home Improvement → [Security] Alarm contacts for french doors - questions

We are replacing a sliding glass door that uses a single magnetic switch with french doors where both doors are hinged to open. I need to decide what to do relative to the alarm.

While I am able to call my alarm company to come and do the rework, it seems a waste of money given the likely simplicity. Given that it is currently a single suface-mounted magnetic switch, I am not seeing where I can get by with that same single switch since both french doors can swing.

If I were to install plunger switches for both doors on the hinge side of each, I presume the net-additional switch will require its own home run back to the panel. Is this correct? Or can both switches be spliced to share the same wire ack to the panel? (I think not, but that's why I am asking.)

If it requires a new homerun for the 2nd switch back to the alarm panel, then I will probably end up calling the alarm company. But if I can DIY it without touching the panel then that would help save some bucks.

Don't you have to open one door before the other will open?

Don't know for sure yet, but possibly. Specs we received from our renovation contractor via email are:

Smooth fiberglass
Fulllite - no grills
Low E
In swing - both doors operable
Double bored
PVC jamb and PVC brick molding on exterior
4 9/16 jamb ( for 2x4 wall construction )
Latch for fixing one door
Aluminum threshold
6/0 size
Satin nickel hinges

You can have multiple contacts on the same run. You want to run one wire from one contact to the next and then back to the panel from that contact. Run them in sequence so if one breaks it breaks the chain.

If you splice 2 wires to the one going back to the panel it won't trigger unless both contacts are triggered as one would provide a complete path.

This was 5 years ago installing analog security systems that we did this.

I can't imagine arming the alarm without latching the opposing door.

Neither can I. But not being an owner of french doors (yet), I don't know for sure if the latched door could be broken open independent of the other door.

As others said, you can wire 2 switches in series. Just moved into a new house and they did that for double Windows. If either window is opened the circuit is broken and the zone trips. It will only show as one zone in the system.

Wiring them in series is the correct way to do French doors even if one must be opened before the other one. You can either series them in the panel or at the door but whichever way you do it the 2 contacts should only end up providing one input to the panel. Normally it's done at the door for simplicity.

be careful here, if the lines are "supervised" he need to make sure the resistor ratings don't need to be changed, that said unless there is a middle post in the door, generally 1 door needs to be opened first

Thanks everyone. The replies make perfect sense. I am pretty sure I can accomplish this myself. Yet, paying for a service call has that added benefit of making sure the alarm company is on the hook for correct operation. Oh well. I fret about these things for a while before finally committing and buying the parts and having the courage to drill into the new expensive door.

One more question. As I said, the alarm contact on the door currently is a surface mount magnetic switch. Do we suppose that is a "normally closed" switch?

Open when the door is shut, likely with a resistor across it to detect a wire break.

Short = door opened, no current = wire cut, resistor-limited current = door's closed and wire's OK.

I have double sliding glass patio doors. Both of my doors have a magnetic switch & both are on the same zone. I believe they are tied together at the panel.

Most alarms use normally open switches (they're "closed" when the door is closed). See »www.diyalarmforum.com/di ··· m-faq32/ for illustrations on how they can be wired.

Actually that method of wiring is mainly used on fire alarms and not security alarms and will require all the switches to be wired in parallel. Security alarms usually just loop through normally open switches with a EOL resistor somewhere in the circuit (a cut wire will just show up as an open door).

I just opened up the panel box to take a look. My original alarm panel lasted about 25 years, and I had a new panel put it in about two or three years ago. They did not have to run any new wires to new sensors. Just hooked up the old sensors/switches to the new panel.

I recall the alarm guy saying, and confirmed with my eyes, that he did install resistors at the panel. Even left a pack of additional resistors inside the panel box.

The more I dig at this, the more I think paying the alarm company to come out and do the install of switches for the new doors is the best plan. But scheduling them at the same time the contractor is going to put in the door in order to maximize ease of access for drilling will the trick.

If the circuit is normally open and closes when the door opens you need to wire the two switches in parallel.

If the circuit is normally closed and opens when the door opens you need to wire the two switches in series.

The latter makes more sense for alarms to use to me because it would also get triggered by cut wires.

resistors at the panel completely defeat the purpose of supervised lines, the lines can be tampered and you would never know

Not entirely true. The only temper/damage detection loss would be if something happens to short the wires together; and in that case only sensors after the short would be bypassed. The system will still detect an "open" just fine.

/M

if a burgler actually wanted to defeat this, he simply needs to get near the lines, short them together, thus bypassing every single sensor in the system, without tripping an alarm, to come back later and walk through the door.

considering its a different of 5 minutes per sensor to put them at the end i find it to be a big issue

How would that bypass them all? I have never seen a one zone system, most I've seen have 4 to 7 zones. Also if the system was put in when the house was built none of the wires would be exposed.

If this has happened you have already lost.

He can also tap-splice in his own resistor and cut the line to get by your method. If there are multiple sensors it is trivial to bypass all but the one with the resistor attached.

Lets be honest though, a thief is most likely just gonna smash something to get in and ignore the alarm going off while grabbing what he wants and leaving. By the time the police get there 10+ minutes later (assuming they even respond to alarms) he's gonna be long gone.

Most doors I've seen have one primary door that opens and closes easily, and another that requires the primary door to be opened first. So, you should only need one sensor. As others have noted, you can always just connect them in series should you need or want two.

I wired mine up myself. It was almost a necessity; had I not jumped in while they were installing the door, they would have run the wires to the wrong place, and the trim would have had to have been pulled and reinstalled to hide the wire correctly .

The wires needed to be extended from the old slider location, to above the door near the center.That's not how my Ademco system works. When the door is shut, the wires are connected (switch is closed), so there is current. Wire is cut or door is open = zone is faulted. In my case the wires are inside the wall so it's not easily cut anyway.

how can he tap splice, changing the resistor value at all will fire the alarm.

i agree with all your other points though

It really is moving into "impractical" territory, but if you could simultaneously tap in the resistor while cutting the original one out the alarm won't notice. It depends on the quality of the alarm but I can see a residential grade alarm not sounding on a resistance change (not a complete open or short) that lasts only a few ms.

At any rate you would need to know if the resistors are on the last sensor or at the panel before doing either one (tap or simple short).

Most thieves aren't going to care about an alarm. They'll bust the door or window then grab as much as they can in 5-8 minutes then flee before anyone responds to the alarm. Let's not pretend they are going to waste the time to tamper with the alarm system. If they need more time they just add more people to the job but the 5-8 minute timer remains the same.

LittleBill is right. They're called "end of line resistors" for a reason.