Hi Jeckson,

Do a search for "alarm sensors." The burglar alarm industry is a mature industry with many well developed (and often inexpensive) types of sensors available off-the-shelf.

Obviously the magnetic reed sensors are a staple to the industry. They are inexpensive and they come in a variety of packages for varied applications ranging from mounting to the floor for an overhead door to mounting concealed in the frame of a pedestrian door. I even configured one to secure a pet door once by using an RC time constant to prevent triggering the switch long enough for the dog to pass through, but not long enough for a burglar to shimmy through. It had the added benefit of preventing false alarms triggered by wind blowing the flap open. They can also be mounted behind paintings (triggered if someone lifts the frame away from the wall).

If you want to protect individual objects (like a vase on a pedestal or a gun in the rack), simple momentary pushbutton contacts are available. Lift the object off the button and the contacts trigger the alarm. One of my favorite applications of a reed switch involved one that had the magnet and contact in a single package, so all that was needed was a metalic plate to pull the switch closed. It was designed for use with a metal door, to be mounted concealed in the frame. I mounted some in the barrel rests of a gun rack. The barrel of the gun pulled the contacts closed, and removing the gun allowed the contacts to open. I mounted the switches behind the felt strip in the rack so you couldn't tell they were there.

There are many other types of sensors, all becoming progressively more expensive. There are pressure mats that can be placed under rugs, but I found them to be prone to wearing out in high traffic areas. There are photoelectric beams which trip if the beam is broken, but I found these a bit expensive. There are shock detectors that attach to glass panes, but I found them prone to false alarms in thunder storms (unless you set them so insensitive that you risked not detecting breaking glass).

There are several sonic-based sensors, like noise discriminators that monitor rooms for high frequency sound spectrums associated with breaking glass. And there are ultrasonic detectors that sense high frequency ripples in the air associated with a person walking through the room. Again, I found these sonic-based sensors to be more prone to false alarms than I liked.

One sonic-based sensor that I liked, though I never got a chance to test it, was called an infrasound sensor. Instead of monitoring for high frequency disturbances in the air, it monitored for extremely low frequency waves. In particular, it would detect someone opening a door. Pushing a door through the air generates a very low frequency pressure wave, and this thing would do a pretty amazing job of detecting them. It had promise for securing areas where mounting sensors to the doors wasn't an option (like in a rented apartment). Unfortunately I never tested one so I can't say how reliable they are.

The reason I never tested one is because the passive infrared (PIR) detector made them unnecessary. For general purpose protecting of internal spaces you can't beat the PIR. The optics can easily establish boundaries of protected space in 3 dimensions (like leaving space for a pet to walk around at floor level, or leaving a passageway for a security guard to walk through on his rounds), and they are inexpensive.

Of course, all sensors can be defeated, with sufficient knowlege and expertise, so do your own research. First learn what sensors are available, their strengths and weaknesses (in both reliability and cost), and only then try to make choices about which will most suit your application.

Good luck,

Joe