Sensors
Ignoring direct audio or video monitoring, security system sensors act to convert conditions to on/off representations, typically as switch closures or openings. You can build sensors for nearly anything; here (in no particular order) are the types we found readily available on the Internet:
Door and window — Perhaps the easiest sensor type to build, door and window opening sensors work using either mechanical or magnetic switches. Pressure from the door or window frame holds a mechanical switch against a spring; opening the door or window releases the pressure and actuates the contacts. A permanent magnet mounted on the moving part of the door or window holds a small reed switch actuated; opening the door or window releases the reed switch.
Floor — You can detect weight on floors with pressure-sensitive switches. The ones you can buy are relatively small — just a few square feet — so if you need to cover a larger floor space, you’ll need to be inventive. Suspending a large floor on pressure switches probably isn’t practical, particularly in existing buildings, but Hollywood-style infrared lasers or other optical sensors may be. If you have a large area to cover, however, you’ll probably want to use motion detectors.
Motion — Detecting motion is a good solution for seeing intruders in relatively large spaces, particularly ones where it’s impractical to monitor the entire perimeter. The most common motion detectors use passive infrared (PIR), a simplified form of imaging sensor that maps out the heat levels across the scene. PIR sensors have a specific field of view, and you may need multiple sensors to adequately cover an area. PIR sensors are subject to false alarms from medium to large size pets and other animals, something you’ll have to consider in your overall system design.
Smoke or carbon monoxide — There are many concerns you might monitor for besides break-ins, including fire and heater malfunction. The same sort of smoke and carbon monoxide detector technology you use in your home can trigger switches that feed your remote security system. If you use battery-driven units, be sure to replace the batteries regularly, because there’s no practical way for the security system to test them. (What you could do, though, is to program your software to send you reminder e-mails periodically.)
Glass breaking — One of the ways to circumvent a window alarm switch is to break the glass, entering without opening the window frame. Breaking glass creates high-frequency sound with well-known characteristics distinct enough that you can get sensors to listen for those sounds. The sensors will close a switch when they hear the right noise, triggering your alarms.
Wind, water, and temperature — High wind, flood water or water in the basement, and freezing temperatures can all do significant damage. Measured data doesn’t itself fit the on/off switch model directly, though, so you’ll want to pick sensors that compare the measured value against a threshold and trigger switches based on those decisions.
Vehicle — Your security system need not stop at the building walls. Vehicles that don’t belong are one good indication of a potential problem, and can be detected with a low false alarm rate. Motion detectors are another good way to monitor outdoor areas, but are much more likely to generate false triggers.
Power failure — It’s not hard to generate an alert when an electrical circuit loses power. You can get monitors that generate a switch action, or can receive the warning in software from an uninterruptible power supply (UPS).
Monitoring for trouble and sending alerts is harder if all the power goes out. You’ll need at least a UPS to make sure your power failure message gets out (we use APC units such as those at , but if you’re really intent on having all the power you need no matter what, consider something like the Generac Guardian Air–Cooled 12KW Standby Generator, about $3,200. Connect it to a natural gas supply and you could probably run all winter long no matter how bad the local power.
Either way, make sure both your communications equipment and your computers still have power after the UPS kicks in, and test the system with the power cut to make sure you haven’t overlooked anything.Test that the system powers up and resumes normal operation without any user interaction after power is restored, too.
Perhaps the least obvious sensor technology — one you’re near constantly but never think abou t— is the vehicle detector. People tried many different technologies, including treadles to sense the weight on the tires, before settling on a coil buried in the pavement. A large enough mass of metal near the coil alters its magnetic properties enough to change the frequency at which an oscillator connected to the coil runs, a change you can detect with a frequency counter.