Most alarm systems operate on the principle of either an open or a closed circuit. With an open circuit, the connection between the power source and the alarm system is not yet made. When the alarm is activated (say, by opening a door), it completes the connection and activates the alarm. With a closed circuit system, the principle is reversed. A constant flow of electricity prevents the alarm from going off. When that connection is broken (again, by opening a door or detecting movement within a given field), the alarm detects the loss of electricity and activates.
Most pressure-sensitive alarms follow an open circuit format. A pad or a rug is placed above a pressure plate, which consists of two pieces of metal slightly apart. When someone steps on the pad, the metal pieces come into contact, completing the circuit and setting off the alarm. Pressure-sensitive alarms can be fine-tuned so that only a certain weight will activate them, thus preventing pets or small children from triggering false alarms. (The exact same principle is sometimes used in to create different effects; for instance, some stores will place pressure pads in front of their doors so that when a customer steps on them, the doors open automatically.)
Pressure-sensitive alarms have slowly fallen out of favor since more sophisticated electronics can provide more reliable security more easily. An intruder wishing to avoid detection can simply step around the pad, which means a pressure-sensitive alarm needs to cover much more space in order to work. A motion-sensing alarm is much smaller (using just a fraction of the materials in its construction) and yet can scan an area of equal or greater size just as effectively.
For households with wandering members, however, a pressure-sensitive alarm makes for a very useful tool. Children who are prone to sleepwalking may injure themselves if adults are not alerted, while relatives suffering from Alzheimer's disease may similarly become confused and wander off in the middle of the night. Motion detectors may not be as effective in alerting the other members of the household, because they will pick up incidental shifts made while sleeping or detect innocuous behavior such as going to the bathroom. But a pressure-sensitive alarm can be placed around the bed or in danger areas such as at the landing to a stairway. The family member can then move around in bed or in "safe" parts of the house, and only trigger the alarm if he is in danger.
Pressure-sensitive alarms can also be used to help train pets. If you don't want a dog or a cat going near a particular door or section of the house, you can put down a pressure mat to set off the alarm when the pet comes too close. (You can deliver a scolding to reinforce the notion, if needed.) As with cases of infirm relatives, the pressure-sensitive alarm can discriminate better than a motion detector, which will go off regardless of whether the pet enters the forbidden area.