Alarm systems are often found in modern buildings, both occupied and vacant. The basic purpose of an alarm system is to draw attention to an intruder. Alarm systems accomplish this either through the use of sirens and flashing lights, and/or by remotely notifying a security office or police.
Systems range everywhere from cheap unmonitored sensor/siren units that operate on battery power to large central systems with concealed sensors and real-time monitoring.
The typical modern alarm system found on a house or small commercial building consists of sensors with wiring leading back to a central location, a central control panel, a control keypad, and a circuit of sirens and/or strobe lights. It is usually activated when the building is being left unattended.
Most alarm systems have multiple zones, allowing for easier configuration and troubleshooting. For instance, a home alarm system can be set up with all the door and window sensors on zone 1, and the motion sensors on zone 2. At night, the residents can arm the system with zone 2 disabled, allowing them to walk around in front of the detectors, but causing an alarm if an intruder were to open a door. The system's controls also allow a malfunctioning zone (for example, a zone with a defective sensor that indicates a window open when it's not) to be easily identified.
Addition by Zorb: A zone fault (trip) at an entry zone will also generally allow a grace period of between 30 and 90 seconds, before activating the actual alarm, in order to facilitate entry into the alarmed structure and alarm disarmament. The zone fault would usually be indicated at your alarm panels only by a loud repeated beeping or continuous (usually very shrill) tone until the alarm itself starts, at which point the tone will either continue, be replaced by silence, or be replaced by a siren tone from the panel and other locations.
A zone fault in an armed, non-entry zone will almost always immediately trigger the alarm, which likely will not be resettable from the console, and will require telephone contact with the alarm service. (Read: Bad!) If you're afraid of the bogeyman coming out to play when you go into the windowless bathroom in the center of your house for a few minutes, don't set your alarm!
The sensors provide input to the system when a person enters a secure area. These may be any of a number of different devices. The types of sensors include pressure sensitive mats, pin switches, magnetic reed switches, infrared beam transmitter/receiver units, passive infrared (PIR) motion detectors, microwave sensors, sound sensors, glass break sensors, and wire/tape loops.
Pressure sensitive mats are usually pretty obvious; they're surfaced in black rubber. Many older automatic door systems (especially those that use a swinging door) have a pressure mat on the entrance side of the door to trigger the mechanism, and a pressure mat on the exit side to allow it to detect if someone is in the way of the moving door.
Pin switches and magnetic reed switches are used to detect if a door has been opened. The most common switch you will find on modern systems is a magnetic reed type, and consists of a small plastic rectangle stuck to the door, containing a magnet, and another rectangle mounted to the door frame, containing a reed switch. When the magnet is moved away from the switch, the switch triggers. Another type of magnetic reed switch can be drilled into and concealed within the frame of the door itself, and resembles a small plug inserted into the frame, matching another small plug in the door, containing the magnet. Pin switches have a mechanical actuator that sticks out, and is normally held down by the closed door or window. Releasing it triggers the switch. I've also seen pin switches mounted inside the alarm system's central control box and siren cabinets, set up as tamper switches.
Glass break detectors and foil tape loops are used to protect windows against unwanted entry. These take a few different forms. Often, on older installations, you will find conductive tape applied to the glass. Breakage of the glass shears the tape, breaking a circuit. Another form of window sensor is hidden within a window screen; a small insulated wire runs in a serpentine fashion through the screen, such that the circuit will be broken when the screen is torn. You will usually see a small terminal block mounted to the screen's frame, and a grid (about 8 cm * 8 cm) of thicker filaments in the screen. On windows where conductive tape or screens cannot be used, a plastic puck will be attached to the glass, containing a shock-sensitive switch. This will trigger the alarm if the glass is shattered, or, unfortunately, in the event of a nearby thunder clap. The final and most easily concealed method of glass breakage detection is a small audio sensor, which is basically tuned to the acoustic properties of glass breaking. These sensors usually have three lights, green (to indicate it's alive), yellow (probably used when configuring the detection threshold), and red (TILT!). These can be adjusted to ignore the ambient noise level in an indoor location and left active continuously.
Motion sensors are usually what get us in trouble. They come in three basic flavors, passive infrared (PIR) (most common), microwave, and infrared beam. All of these sensors can be reasonably concealed.
Microwave sensors are the easiest to conceal; fortunately, they are also uncommon. It is possible to detect them using an inexpensive multi-band radar detector (the kind you would otherwise use to find speed traps) will alert you to their presence. The detector may have to be tuned for lower sensitivity, as I have heard reports of supermarkets using microwave detectors for their automatic door openers triggering radar detectors at a range of 70 feet.
Infrared beam sensors are used to detect an object crossing a certain path, doorway, or hall. These consist of either a seperate transmitter and receiver on opposite sides of the opening, or a combined transmitter/receiver and a reflector on the other side. These are ofter installed in such a manner as to make them easy to get over, under, or around. IR beams are now most commonly used to detect something in the path of a closing motorized door (garage, elevator, etc.)
Passive infrared (PIR) sensors are now very inexpensive and commonplace. These are the sensors you usually find which have a curved white semi-transparent plastic lens and indicator light. These are found nearly everywhere, from alarms in utility tunnels to automatic energy-saving light switches. These basically detect moving warm objects, such as human bodies. They are very sensitive and fairly immune to false triggering. As they are passive, it is not possible to detect them.
Sensors can also be used to detect non-security issues, such as a basement or tunnel section flooding, a piece of equipment malfunctioning, smoke, fire, or excessive heat.
Some alarm systems merely make noise. Others do much more. Some may even be configured as silent alarms. Here, I'll explain how the monitoring of systems works.
A typical small monitored system that is armed when a facility is unoccupied contains a phone dialer. This is similar to a modem, and calls a remote monitoring service, notifying them that an alarm condition has been reported, and what zone is active. Depending on the type of alarm and the response prearranged with the monitoring service, they will either notify the police or fire department, or call a predefined emergency contact first. The main vulnerability on these systems is that the telephone lines can often be disconnected or cut from outside the building, disabling the monitoring. However, there are add-ons available to protect against this, such as wireless (cellular/PCS) dialer units, and a telephone company service that automatically notifies police if the telephone line has been cut. BellSouth calls it Watch Alert service, and I believe it works by checking the line impedance.
On a larger system, the alarms can be monitored by a security officer within the facility. A typical system of this type will notify the security officer of where the intrusion has taken place, and will automatically activate CCTV monitoring and recording in the area if present. This is usually a silent alarm, and you may not know you triggered it until security arrives! If no security officer is present, the system may be set up to forward alerts through a phone dialer.
Usually a large gray box mounted on the wall somewhere. Contains a logic controller that operates the system and watches sensor inputs, provides power to sensors that require it, and provides power to audible/visual alert devices when an audible alarm is triggered. Often contains a sealed lead acid gel cell to provide a few hours of backup power. After a day or more without AC power, this can be assumed to be dead as a doornail if no lights are visible on the system keypad.
Unless you have the code to deactivate the alarm system, this is only really useful to figure out if it's got power or is armed. Lights marked "ARM", "ARMED", or "STAY" indicate the system is armed. A light marked "AC" indicates that outside AC power is present to the system. A "READY" light will usually be on if all zones are clear (all doors closed, etc) and the system is not on. If no lights are on, it is quite possible that the system is dead.
The keypad also usually contains an audible alert, which will start beeping up a storm if you've triggered the system and it's waiting 45 seconds for you to enter the code... (oops again)
Detecting alarm systems
In most cases, it's pretty obvious if a building has alarms. You will see the installed sensors (before or after triggering them, *oops*). In Miami-Dade County, FL, businesses with alarm systems are required to obtain and display an alarm permit, usually in a front window. On truly ancient systems, you may come across a wall plate with an Ace-style lock, a green LED, and a red LED. Of course, those stupid stop-sign shaped "PROTECTED BY FOO SECURITY" signs are a dead giveaway..