Most commercially available burglar alarm systems use zones to break the system down to manageable units. The simplest systems will have instant zones and delayed zones. The instant zones respond without immediately to intrusions like broken windows. More on delayed zones further along.
The simplest loop detection circuits are for normally open (N.O.) or normally closed (N.C.) contacts. Normally closed contacts are generally preferred here because you will know about a faulty wire as soon as it happens since a broken wire will send an identical signal to a tripped device.
A weakness inherent in the simple N.O. loop is the fact that there is no way of knowing if it is functioning at all unless you set it off from time to time. The primary weakness of the N.C. loop is that someone who knows about it can simply apply a bypass jumper across the contact. This might be done by a contractor working in your home or a guest of a guest at a party. The system will appear to function normally for you with no way of knowing there is a point of entry that can now be breached. Below is a depiction of two ways burglar alarm connection panels are frequently labeled. The example on the left uses schematic symbols for normally open and normally closed alarm contacts. Within the alarm industry, normally open or closed refers to the condition of the device when no alarm is present. This may be confusing to some if you integrate other electrical and electronic devices into your system. In the non-alarm electronics world, normally open or normally closed refers to the condition of contacts absent any outside influences.
| For instance a magnetic reed switch that closes when a magnet is nearby, will be retailed in the alarm industry as normally closed, while the rest of the technology industry refers to it as normally open because when there is no magnet nearby (outside influence) the contacts are open. |
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There is a more advanced system loop that uses an end of line resistor. This is installed at the furthest end of the loop from the panel and provides a constant verification that the wire is complete all the way out to the end and that nobody has bypassed the contact or sensor. This type of loop requires a bit more technology due to the fact that you are having to detect something other than a yes or no (or in the digital world 1 or 0). For this type of loop, there is no difference in reliability between N.O. and N.C. contacts, but the detection circuits are different. These are not going to be discussed in this chapter because you are unlikely to find low-end home alarm systems that support fully monitored loops.
Premise Cabling:
Once you have decided what combination of loop sensors is right for your system, you will need to connect them to your alarm panel. Most simple home alarm systems will have three zones (or loops) for contacts. In addition to the two above, there is almost always a “delay” zone. Delay zones are used for the main entry door to give authorized users time to leave the protected area after arming the panel and to disarm the system via keypad after entering. This offers the advantage of keeping the keypad in a protected space indoors and lessens the possibility of an ambush while working the alarm code. Generally these delayed zones offer an optional warning tone to remind the operators to disarm the system. A common mistake made is placing the keypad in an easily visible place and putting the warning tone device inside the keypad. This is simply too much information for the intruder since most homeowners don’t need to be reminded where the controls are even if they forget to use them. I advocate locating the keypad in a closet near the usual entry. In a closet, it is less likely to be identified by brand and model by tradesmen having regular access. The less information strangers have about your security system, the better.
Because home alarm systems generally only have three zones, they are wired with multiple devices in each zone. There is a right and a wrong way to do this. For instance, if you are running a loop to all the windows in the house, you could run a single strand of wire from the common connection (COM) on the panel to the first window. Connect to the one of the window sensor terminals, then run another single strand of wire from the second terminal at this window to the first terminal in the next window. This would be followed until you get to the last window in your circuit around the house and from there, you would run a single strand of wire back to the alarm panel and connect it to the appropriate loop connection. I dislike this from a troubleshooting standpoint. I have spent too much time in attics chasing down a bad connection in systems that go from window to window at the narrowest parts of the attic (the edges).
| I think wire is the cheapest part of any installation like this and so would always recommend running every device or contact all the way back to an easily accessible point if not to the alarm panel itself. At the very least, you can mount a small terminal block to the face of a rafter in the attic right next to the attic access hatch. By doing this, in combination with accurate labels, a fault can quickly be identified and localized. In a pinch, the faulty device can be bypassed until the problem can be fully addressed. |
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In the diagram above, 12Vdc has been pulled from the alarm panel and is provided here to supply active devices (those requiring external power) like motion detectors. In addition, notice the green jumper bypassing the bottom three “unused” connections. It is always wise to create your system with future expansion in mind even if you think you have thought of everything.
Time to introduce the multimeter:
As with most alarm systems, the one in our example runs on 12Vdc. If the alarm panel is showing a bad loop because of wiring or contact failure, it will not arm. There will be an indication on your alarm panel and/or remote keypad alerting you that one of your sensors is tripped. This is perhaps a good place to discover how to use a multi-meter for testing real-world problems. In this case, we will find the bad contact by using DC voltage readings. Since we are using an N.C. loop, there should be close to a dead short across the entire loop, but the alarm panel is telling us that is not the case. Refer to the owner’s manual that came with your meter and configure it to read the lowest DC voltage that is at least 12 volts. Place the meter leads across each pair of terminals until you show a voltage. If a voltage drop is present between the terminals marked as “Greenhouse”, you can go check the greenhouse and see if the contact device is faulty or broken, then check all the wire between the wiring block and the greenhouse. In many cases, you will have these problems when immediate troubleshooting is not practical. For now, you can just install a short piece of wire across these two terminals and make a note to yourself that the greenhouse is not being protected until you get around to further troubleshooting. If your meter doesn’t automatically sense polarity of DC voltages, then you need to start out at the highest available DC voltage range and watch what happens to see if the common is the more negative side of the circuit or the more positive. Once you know the polarity you can step the meter down to the lowest scale that is 12Vdc or higher.
In a residential attic, the hazards to your alarm wiring include rodents and squirrels chewing through the wires as well as various workers in the attic stepping on the relatively delicate low-voltage wiring and breaking it or tripping over it and pulling it out of the connectors. For this reason, I build pathways for the wire using fence staples driven loosely into the bottom edges of rafters overhead. The wire can be pulled through after the staples are in place if you just tap them in far enough to be firmly attached. By doing so, most of your wire faults will be at the very ends of the cable, where they are within rodent reach as they pass down through the ceiling or wall framing.
Passive Sensors
Windows:There is a seemingly endless variety of sensors out there for alarm systems. Almost nobody uses window foil anymore because it is costly to install, looks terrible and frequently breaks in normal useage. Likewise, putting magnetic contacts on window sashes is not generally done anymore. The amount of labor involved in putting contacts on all the windows in a home makes this strategy cost-prohibitive. More on window sensors under active Sensors below.
Doors:
There is a drill bit you can purchase from electrical supply houses. It is called a bell puller’s bit. Usually around three feed in lenth, they come in ¼ and 3/8 inch sizes to match the flush mounted reed switches discussed here. There is a small hole drilled sideways through the tip of a bell puller’s bit so that you can drill straight up through a doorway into the attic and when the bit stops spinning, your partner can slip the alarm wire through the hole so the bit pulls the wire down as you retrieve it. The corresponding magnet drops into a matching hole in the top of the door so that when the door is closed, the magnet is directly beneath the reed switch.
One thing about door contacts like this is they are both free and relatively easy to install. An alternative to motion detectors inside the living space might be to put contacts on some of the interior doors. Candidates for this would be doors that are normally closed when you are away from home, but are going to be opened in the first moments of a burglary during an initial sweep of your premises to look for especially valuable items.
Other Passive components:
Magnetic reed switches can also be used to protect smaller items of value indoors and out. If you can glue a rare earth magnet to the underside of an object, the reed switch can be under the countertop or even under a bit of loose sand if the object is outside which would also serve to conceal the wiring.
There are a variety of vibration sensors on the market for alarm system work. These can be used to detect unauthorized access to your valuables indoors or out. I once had a motorcycle that I had to park in an alleyway behind my office. My desk was only about ten feed from the motorcycle, but through a windowless concrete wall. The only place to hide anything on the bike was a container for a tool pouch that was about the size of a pack of cigarettes. I took one of those life-alert neckace fobs apart and wired it to a small adjustable vibration sensor. The receiving unit was AC powered and sat on my desk. That system never gave me a false alarm, but you could set it off with as little as bumping the back tire with your foot.
Active Sensors
Passive Infrared:Passive infrared motion detectors operate by sensing rapid changes in black body radiation along specific pathways defined by the Fresnel lens covering the sensor. These lenses focus infrared radiation from specific directions (as many as six or eight rays fanning outward from the front of the unit. If there are pets in the home, care should be exercised to ensure the rays of coverage are out of reach of the animals. There have also been reports of false alarms when previously blocked sunshine suddenly warms a window pane within the coverage area. These units should never be positioned so they can see windows and probably shouldn’t be used in rooms exposed to direct sunlight. <
Microwave and Ultrasonic:
Microwave and ultrasonic sensors both work in similar ways but with obviously different types of energy. They flood a room with low levels of energy waves which bounce around and find their way back to the sensor. Motion is detected by a rapidly changing phase difference between emitted and received energy. They have the advantage of providing full coverage to a protected room with few if any dead spots. As with any technology, there are limitations. I had installed a microwave motion detector in the offices of a large legal firm on the upper floors of a tall building. The microwave motion detector caused a false alarm every week night at 2:15am. I was there one sleepless night feeling very frustrated that we had been completely unable to force the unit to false alarm. As long as we were unable to duplicate the error, it was going to be unsolvable. I was standing across the room from the misbehaving unit a few minutes before the nightly false alarm. My back and head were pressed firmly against the wall behind me to help me remain motionless. At almost exactly 2:15am, there was a brief vibration in the wall behind me accompanied by a whooshing sound. Two floors up, I met with the custodian of another law firm washing his hands after having stepped out of the executive washroom. Tucked under one arm was the newspaper he had been reading during his regularly scheduled break. Drywall and plastic sewage pipes were transparent to the microwave energy. Solid human waste accompanied by several gallons of water are not. The solution here was to install a dual-technology sensor. Both the microwave and ultrasonic sensors have to trip at the same time for there to be an alarm. Either can false by itself with no effect on the system status.
Audio Discriminators:
There is a class of sensor used in homes for the last 30 years or so called an audio discriminator. Rather than go around the perimeter of the house with wire and gadgets for all the windows. They are typically installed high on a wall opposite the windows, they discriminate between normal sounds, even very loud ones, and the sharp splintering sound of breaking glass or splintering wood. My father discovered decades ago the alarm system should not be armed while loading the dishwasher because glasses clinking together can fool it. A beautiful thing about these units is they are capable of having the microphone wired back to the panel so that during alarm status, someone monitoring the system can hear what is going on and make an intelligent decision as to whether a burglary is taking place.
Bank vaults are protected from the repeating heist movie theme of digging your way in after hours. Not only are they very thick and very well reinforced, but there is a simple microphone inside and almost any noise from within will cause an alarm. Things like this make me want to put an old wind-up alarm clock in a safety deposit box. It would run down before they got the door open, so troubleshooting would really be a bear.
Electric Eyes:
Electric eyes are not used much anymore in home security systems, and they do have some limitations on what they cover, but they do have their applications. Across a roadway or driveway, they work well and can be located far enough to the side as to not be noticed. I would use them outdoors only as a dual-technology solution or perhaps with two in a vertical stack so that small animals would not cause false alarms. Flying insects are apparently attracted to the infrared light emitted and while they probably would not be a problem, I have found that the birds who feed on those insects often cause false alarms.
Other Active Sensors:
Even something as simple as a table lamp can be an effective alarm trip when combined with a concealed light detector pointed at it. If the lamp is the only available light in the room, it might be switched on at some point. I subscribe to the “defense in depth” philosophy. Rather than rely on any one solution, I want to employ them all so far as is feasible.
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