Didier Stevens

Friday 29 July 2011

My Home Surveillance System

Filed under: Arduino,Hacking,Hardware — Didier Stevens @ 10:21

Aside from having installed my own Home Automation and CCTV system, I also designed and installed a surveillance system at home. This post will discuss some of the design decisions I took. Some of them are different from more conventional alarm systems.

The surveillance system has many sensors in and around the house (passive infrared (PIR) sensors, reed switches, temperature sensors, …) and can take several actions, like starting sirens, turning on lights, sending text messages, making phone calls, taking pictures, … Which  actions are taken depend on the alert level that was set.

First design decision : this system is designed to deter common burglars, not burglars with inside knowledge of the system.

Second design decision is that the system will log all events coming from sensors, regardless of triggering an alarm.

Third design decision is that there is no alarm delay: if a sensor triggers that would cause the alarm to sound, then the alert sounds immediately. There is no delay or pre-alarm phase. I believe an immediate alarm has a greater deterrent effect. With this design, it’s best to avoid false-positives as much as possible.

Fourth design decision : use analogue PIR detectors, not binary PIR detectors. A classic (binary) PIR detector will just tell you that movement occurred. With an analogue PIR detector, you get the amplitude and duration of the movement, which is useful information to weed out false alarms, or ignore movement from small pets.

Now on to some interesting or unusual use cases.

I have a sensor on the doorbell too. When someone rings the doorbell, the event is logged and the system takes pictures of the front door. I’ve seen some interesting events since this doorbell sensor was installed. For example, I expected a package to be delivered after 18:00. The sender had instructed our national courier company to deliver the package after 18:00. You can probably guess they didn’t follow the instructions. I have evidence they attempted to deliver well before 18:00, and what’s even worse, they left a note saying they had passed around 18:15…

Like modern, commercial alarm systems, I have several alarm zones. For example, I can set the alarm level for when we go to bed. In this mode, the alarm will go off if there is movement inside the house, except in the bedroom and nearby rooms/hall. But come morning, you have to remember to switch off the alarm before you leave the bedroom.
Not with my system. If my system detects movement in the protected zone, and if there has been movement in the bedroom zone just before, it will disable the alarm in stead of sounding the alarm. So no false-alarms triggered in my house by sleepy-heads.

Outside lights that switch on when movement is detected are supposed to deter burglars, but they are so common that I believe the deterrent effect is negligible. My system turns on some lights inside the house when it detects movement outside while it is dark and there is no movement inside. I believe this has a much greater deterrent effect, because it’s so uncommon. And it will also take pictures. I now have a large picture collection of neighborhood cats in my back garden 😉

I’ve recently installed wireless interconnected smoke alarms. I will connect one smoke alarm to my home surveillance system, so that my system is aware when smoke alarms trigger and can act appropriately.

Testing all these functions is fun. I’m ” testing in production “, you can imagine that I don’t have a second home that I can use as a test system.
So sometime you can see me run around the house like a madman, but I’m just testing a new feature I programmed… 😉

Blog at WordPress.com.