Security systems are designed to protect a premises from unwanted intruders and control access to sensitive areas. This writeup is intended to introduce you to security systems and how they work.

You begin with two things, a panel and a keypad. A panel monitors everything that goes on in a system and communicates with a central monitoring station to tell it such important things as "somebody just forced a door open so call the cops’. A keypad allows us measly humans to communicate with the panel.

Here’s how they work. Mr. Burglar has decided to break into the local Fat Cat National Bank. He does so by forcing open the back door. A set of door contacts tell the panel that one of its zones is in fault, which the panel interprets as "The door is open’'

Now if Mr. Burglar has chosen the wrong door, the panel will sound the alarms and its internal dialer is already dialing the central monitoring station to send a digital ‘call the cops now!’ message. The reason for that is that the programmer (me!) has decided to make some of the doors and motion detectors go instant alarm. Mr. Burglar now has until the cops arrive to get into the vault and grab the dough, while the neighbors are all waking up and staring at the Fat Cat Bank thanks to an overly loud siren.

Of course all doors and motion detectors can’t alarm instantly. Fat Cat Banker needs a way to open for business without calling the police. So one or more of the doors and motion detectors are set for delayed alarm. So when Mr. Burglar forces the back door the panel still knows the door is open. But since this is the same door used by Fat Cat Banker to open shop it allows for a delay—usually between one minute and thirty seconds---- for the Fat Cat to get to the keypad and type in his authorized four digit code that disarms enough of the system for the bankers to get in and go to work.

Presumably Mr. Burglar doesn’t have an authorized code. So he has an additional thirty seconds to clean out the vault. Only the vault and the approaches to it are on different zones, zones the installer has programmed for ‘instant alarm' So he gains only a few seconds by choosing the right door.

But assume Mr. Burglar is knows this. He’s seen the Pink Panther movies and wants to emulate his role model, the Phantom. So he has his spray can to tell him where the light beams are. Only in the real world we don’t use light beams anymore. Beam detectors are available, but they’re expensive to install and maintain, and are far easier to defeat. We use area detectors that detect motion and he can't move slowly enough not to set one off.

Unless it’s an inside job, and Mr. Burglar can disarm the system. Then he has all the time he wants to take the money and run. Of course when the robbery is discovered, Inspector Clouseau will call the installer (me!) out to the site, and in about two minutes flat I’ll use my code, scroll back in time and tell the good inspector whose code disarmed the system. Which means that guy had better be on a beach in Argentina by now or he’ll have a lot of questions to answer.

This fictional crime story was designed to introduce you to how a security system works, and some of the terminology used. At the center of it all is a plain tan box without we call the panel. It's the brains of the system. Panels generally operate on 16 volts AC and thus can readily be powered from a standard ice cube transformer. All panels will have one or more batteries to back them up in the event of power failure or sabotage. At the minimum a panel will have one or more programmable outputs, which supply voltage when commanded and can control all sorts of interesting devices. It will have a com loop, which is how it interfaces with one or more keypads and the other functional modules in the system. It will have a bell circuit to let the world know it’s in alarm. It may have a polling loop to which addressable modules can be added to monitor other points. It will have terminals to supply 12 volts of DC power to power things like motion detectors and a variable number of zones, which will be covered in more depth later. All the needed circuitry is located on a single circuit board, though the system may be expanded by the addition of additional modules. A module is a board dedicated to a single purpose, and it can be anything from zone expansion, fire system monitoring or communicating with the central monitoring station.

A panel will have a number of zones. A zone guards a particular area of a building. For our bank example, we had a door contact for zone 1, and a motion detector near the keypad for zone two. But it is possible to have more than one device on a zone, but more than two is rare. Zones are wired as normally closed contacts, which means that when the door is closed a magnet on the door will pull closed a contact mounted on the door. Two wires go back to the panel. The wires are supervised through the use of one or more resistors. Normally one is located in series with one of said wires at the panel. When the panel sees the resistor all is well. When the circuit opens. Because the door is open, or if Mr. Burglar decides to cut the wire the panel then decides that zone is faulted and does whatever its programming tells it to do. Like call the cops now.

The normally closed contact in security systems is why cutting a zone wire doesn't work as a way of defeating the system. Most zone type fire alarm systems operate with the smoke detector as normally open When smoke is detected or a pull station is pulled the positive and negative wires are shorted and you get an alarm. Here you have continuity all the time. Of course a clever burglar might decide to strip the wires and connect them to each other. In applications where that sort of activity is seen as a real threat we use a dual resistor system, putting one resistor at the panel and one at the device. If the panel doesn't see one of the resistors it faults the zone and calls the cops.

Zones generally monitor three types of devices. Most door contacts operate as described above— they use a magnet on the door to hold the contact closed. Sometimes the contact is mechanical. A spring will push a set of contacts open unless the door is in place to push back and close the contacts. Window contacts work the same way.

Door contacts are nice because they don’t draw much power or cost very much. All they are is a set of contacts and are thus quite reliable provided they are properly installed. They don’t false alarm as they age, which motion detectors often do. But they cover only one door or window. Few buildings besides banks and jewelry stores cover every possible entrance, so if Mr. Burglar finds an unprotected entrance he's in.

So must buildings use a second line of defense, and that’s a detector. We use two types. A break glass detector listens to the environment and is programmed to alarm when it sees a frequency pattern characteristic of glass breaking. They’re expensive, and you don’t want to use them where cats might meet glassware. But they do work.

More common are motion detectors. Like a break glass sensor, these are area detectors. Inexpensive wall mount detectors see in the infrared and paint a pixelated heat picture the detector memorizes. When the pixels change something has moved and the alarm is sounded.

Of course you pet lovers out there might be gnashing your teeth. Detectors are available that ignore small animals, which includes most dogs smaller than rottweilers. Cats are pretty safe but the detector cannot be mounted on a wall above a shelf that kitty likes to climb on. The detector requires some distance in order to make the determination between a small and large animal. However, you own a bull mastiff motion detectors probably aren't for you. On the other hand, if you have a bull mastiff wandering your halls you probably don't need a burglar arm.

Motion detectors come in all types and sizes. Some use elaborate mirror systems to see a long way down a hallway. Ceiling mount detectors often use a conical mirror to provide 360 degree protection. They cost more, of course. All require four wires to run. Two wires power the device and two more cover the zone.

You may have noticed the word ‘wires’. Hardwired systems are nicer because they are more reliable and in the long run cheaper. If installed during construction the additional costs are limited. But running wires isn’t so easy in that nice Queen Anne you recently purchased. Wireless door contacts and motion detectors are available. They have some disadvantages. First of all, they run on batteries and batteries have to be changed. The models I install use lithium batteries. They're supposed to last from three to five years and sometimes they do. When the battery starts to go down, they will false alarm, usually in the middle of the night. Which leads the police to knock at your door. The cops will do that, for free, a few times every year. Any time they catch or chase off an actual burglar comes free. But if they start coming back all the time they start charging and they don't come cheap. So change your batteries on a regular schedule. If you start getting false alarms from one detector change the batteries in all of them, as it will soon have company. For that reason I don’t recommend using wireless motions very often, as motion detectors really suck the juice.

Also, wireless devices have a limited range. That can be overcome partly by adding receiver modules at remote locations, but again they aren’t cheap. Run wires when you can.

Most basic house systems stop there. A basic size zone system with one motion, a siren, panel and one keypad can be purchased for under $200 (plus installation). Bigger panels do more, but the biggest difference is you get more zones.

We measly humans interface with this system in three ways. At a minimum you will have one or more keypads with an LCD display. Fat Cat Banker and Suzy Homemaker use them to arm and disarm the system. Sometimes you have more than one option. A perimeter arm option is available. That turns off the zones with motion detectors but arms the door and window contacts, and is used when people are settled in for the night. Just don’t forget to disarm the system before you take out the garbage! I’d recommend some delay at the door for that reason to give you time to disarm it before all hell breaks loose.

Second, all panels have a dialer. The panel dials out to tell the monitoring station (usually run by the installing company) if it has a trouble-- say loss of power or a cut horn circuit--- or an alarm. The central monitoring agency or the installer can also dial in to the panel. We use that capability to install new software, to diagnose problems, or to simply see what has happened on the system.

Finally, a card reader can be used to arm and disarm the system, or simply to open a door. Security systems (at least the models I install) can be expanded for many functions.

To operate a system there are several levels of codes required. The highest level is the Installer’s Code. The owner will not be given this code. I don’t want the user mucking around in the system, screwing it up and then blaming me for the problem. You can do anything with the installer's code.

The second is a master or maintenance code. I use that to access information and operate the system, but when I don’t want to change something in programming. The building owner doesn’t need or get this code. You can’t change the programming but there is stuff the owner can screw up. And I don't want Mr. Burglar’s accomplice using my code to disarm the system.

The supervisory code belongs to the owner. That allows the end use to go in and put in codes and program keycards for people authorized to use the system. You can also delete the codes of disgruntled employees (though you can tell the system to remember if they attempt to enter). That code allows the owner to see the event history-- who armed and disarmed the system, problems, etc. All panels remember every operation for at least a week back, and most remember a whole month. You can also set the date and time in the event of a power loss.

Finally there are user codes assigned to a particular user that allow him or her access to arm and disarm certain parts of the system, and access to controlled areas. Which leads us to partitions.

In a large security system, say a high school, a football coach may have a legitimate need to open up the gym and locker rooms but no need to visit the computer lab. So the installer will divide the building into partitions. Normally one or more keypads is assigned to each partition, though a skilled user (and one with the right codes) can use any keypad to disarm anyplace. So we put the gym in partition one and the computers in partition two. Coach’s code opens the gym but will not open the lab. In this way large buildings can limit access within the building.

Security systems can be used to control all nature of devices. If the proper modules for door control or relay control are installed, they can be used to control door strikes or door magnets that limit access. They can be used as fire alarms with certain provisos. First of all all, a commercial fire alarm system requires two separate phone lines, most security panels have provision for just one. A dedicated fire alarm module provides the needed lines. As mentioned earlier most fire alarm initiating devices are normally open, while the circuits are normally closed for security. That requires some different programming, which is fairly easy. But the bigger barrier is voltage. Security systems operate at 12VDC. Most fire alarm devices operate at 24VDC. That voltage difference doesn't matter for pull stations or the tamper and flow switches that monitor sprinkler systems. Smoke detectors are available for 12 volt operation. But duct detectors that protect a large area mostly require 24 volts. Almost all horn/strobes designed to empty the building require 24 volts. By code, all fire alarm circuits must be supervised. meaning the panel must know if anything goes wrong in the circuit. Auxiliary power circuits must be supervised (a relay can accomplish this). That means auxiliary power supplies must be used that are designed for the purpose, or you must purchase higher cost 12VDC devices.

Rule of thumb: If all you need are a couple smokes, some sprinkler supervision and some pull stations, add the expansion card to your security system. If it’s a good sized building, and you want duct detectors and the like, get a dedicated fire alarm.

If you are contemplating the installation of a security system the first question to ask yourself is what do you intend to accomplish? Commercial products are almost infinitely expandable, but if you have a clear idea of what you want the installer can do a better job of giving you the system you want at the lowest possible cost. If you think you may want something down the road the installer needs to know that as well, so the system will be designed in a way that makes expansion easy. My company makes a point of designing in expandability, it makes the initial system slightly more costly but cuts costs down the road. Second, get the installer involved before a major remodel or new construction. If wires and boxes can be roughed in before the walls are finished costs go down and the system is much more aesthetically pleasing.

The cost of maintenance is one big variable in security system installation. Many companies offer homeowners free or very low cost installation in return for a contract to use their maintenance services. They lose money on the installation but make it up through more expensive monitoring and maintenance costs. For example, the batteries that power the panel during power outages wear out after four or five years. So do the batteries in the wireless devices they install to cut installation costs. One customer of mine told me he was charged $100 for a single battery. My system is vastly more expensive to install, but I charge $20 for the same battery and my monitoring costs are about half most of the 'free install’ people. My hardwired system requires less maintenance and is far more expandable. And if you don't want me working on the system any more I'll come in and return all the codes the manufacturers defaults, give you the manuals and wish you luck. Try that with some companies where everything is proprietary. Over the long haul free or low cost installation deals lose their luster.

A properly designed, installed and maintained security system can make a real contribution to building security. Screwing up any of the above creates very real and potentially expensive headaches. There are many products and installers out there. Start by taking the time to decide what you have to have, what you’d like to have, and where you think you want to go. If you know that, your decisions will grow much easier, and the overall costs much lower.

rootbeer277 rightly pointed out that the spray cans don't work on infrared light, which is the kind of light beam detectors use these days. Back in the sixties, they did use conventional light beams, But i'm not really sure the 'spray can' technique would prove very useful, even if the burglar is a twister addict.

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