Monday, April 7, 2014

Home security system false alarms: One mystery solved

Have you ever had your home security system fail and produce false alarms and you just can't figure out why it's doing that?  If you try to search the problem on the internet, you're going to be hit with an avalanche of sales sites and other useless information.  I'm going to describe one means by which a common type of residential alarm can fail periodically and erratically in a way that was surprisingly difficult for us to determine.  This is one thing you can check for as you are troubleshooting your own system.
Few things in everyday suburban life are as annoying as an alarm system that refuses to function.

Screengrabbed from
Most simple home security systems use magnetic sensors on windows and doors.
They look something like this, usually screwed to the window sill.  If anything interferes with the magnetic signal between the magnet attached to the window and the sensor attached to the frame, the system will fault and produce a false alarm if it happens to be armed at the time when the fault occurs.    
In our case, the fault problem was ultimately caused by an aluminum window blind.  I had initially discounted that blind as a possible cause because of three assumptions that I subsequently learned were not correct:
NUMBER ONE - I had assumed that an aluminum blind was, in fact, made of aluminum (duh) and, as a non-ferrous metal, it would not interfere with the magnet.  I had paid a lot of money for these blinds because they are mounted on our two garage windows.  I wanted all-aluminum to ensure that they would not rust over time (we live in the humid subtropics and so corrosion is a concern).

As it turns out, the blinds themselves are aluminum, but a few of the trimmings are made of steel.  And this particular little trimming just happens to fall exactly where the magnet is located.  
NUMBER TWO - I had underestimated the *exquisite* degree to which the magnet was sensitive to the placement of the bottom rail of the window blind.  Literally a half an inch separated "no alarm problems" from "nothing but alarm problems".  For as long as the blind was 100% lowered, there would be no problem.  But if someone left it just a bit elevated, the alarm system would behave erratically, producing faults.
There's also a third reason why it was very difficult to troubleshoot this issue.  When we first installed those blinds several years ago, they were neat and pretty and new, and everyone in the house basically left them in place, lowered all the way down.  We went for three years with no alarm faults.  Then all of a sudden, things started going haywire.  I'd be sitting quietly at my desk working, nobody else would be in the house, and I'd hear the alarm chime, indicating that a fault had just occurred.  Why would anyone logically suspect window blinds as the cause of a thing like that, given that they'd already been on the windows for years without incident?

But of course what happened over time is that family members had been raising and lowering them, sometimes leaving them in sloppy configurations.  And at one window, some garage equipment had gotten shoved up against the blind, further pushing the bottom rail up against the magnet.  And those changes were enough to tip the magnetic balance and make the sensor connection fade in and out unpredictably.  

This was the only way to reliably fix the weak link in this failure chain (remember, if the alarm faults and you can't get to your phone, the police come to your house and all heck breaks loose):
This will prevent a future family member from "forgetting" and leaving the blind in the wrong position, triggering a false alarm.
Go through your house and check all your window blind bottom rails.  Even if they're not aluminum, they might contain some metal parts that could be contributing to your alarm issues.
We also have faux wood and real wood blinds in our house.  I checked both for metal pieces.  The faux wood had a hollow bottom rail, but no metal was visible in there.  
The real wood blinds had no metal anywhere on or in the rail.  

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