Security Watch: Magic Detectors

A look into the inner workings of today’s alarm systems.
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Security Watch: Magic Detectors

Oct 1, 2005 12:00 PM, By Steve Filippini

A look into the inner workings of today’s alarm systems.

It isn't enough just knowing that alarm devices carefully placed and wired throughout a home or business are working correctly. I'm one of those hard-to-please folks who needs to know the how's and why's of their operation. I know it stems back to my youth when I needed to dismantle (destroy) everything in sight in order to see how it functioned, but I can't help it. Unfortunately, time is not abundant and supporting the desire with the opportunity to learn something new has sometimes evaded me over the years. It wasn't until recently, during a conversation with an intern alarm designer, that I attempted to right a few wrongs in his understanding of how some detection devices actually worked. My efforts quickly turned into a full-out debate with my colleagues as we argued over our beliefs on how some things operated. Eventually, I pulled out a few dusty manuals and surfed the Internet long enough to support what I was preaching. What follows are a few things you may or may not know regarding the inner mechanics of some security detection devices.


Ademco offered a model 1401 proximity alarm long before I became a service technician. This meant that I experienced my first run-in with this black-magic device during a service call with no prior training or explanation from my peers.

As I entered the manager's office of a respectable dining chain, I quickly assessed the situation. The safe in question, situated atop four plastic hockey puck-like disks roughly 1in. to 1.5in. tall, sat in the corner. Each puck supported one corner of the safe and provided a separation from the safe to the floor. The 1401 panel mounted above the safe produced a green ground wire connected to the back of the thick-steeled container. On the front of the 1401 panel there was an analog meter with a red and green background, red meaning alarm and green meaning normal. There was also a silver disk cap near the meter that, when removed, allowed access to a sensitivity-setting variable capacitor.

Having never worked on a proximity safe alarm panel before, it took me a while to figure out that the object of the game was to set the sensitivity to a point where the analog meter showed “Green” when no one was near the safe. Touching the safe, on the other hand, magically moved the meter reading to “Red,” thus creating an alarm condition. It sounds simple enough, but as it turned out, I couldn't find the right sensitivity balance to do it. Eventually I had to leave the account with the 1401 panel bypassed in the hopes a more experienced technician would swing by to correct the problem.

A few months later, my supervisor taught me the correct way to calibrate the 1401 panel. He didn't tell me how it worked, but he did show me how to repair it. Later in life, I figured out the rest on my own. First of all, the 1401 panel used a capacitance-activated circuit. The human body, when it comes into contact with a ground-sensing device, can measure up to several thousand Pico-farads of capacitance. The variable capacitor on the 1401 panel allowed you to set the threshold at which a specific capacitance reading triggered an alarm condition.

So there I was, during that initial service call in my uneducated youth, with a metal-tipped screwdriver in my hand, my body leaning over the safe to reach the sensitivity control. Every time I touched the 1401 or leaned close to the safe, I was changing the capacitance reading. As if that wasn't tough enough, customers, even more ignorant to the physics of the alarm system than I was, would often lean dustpans and other metal objects against the safe out of my line of sight. No wonder it was a service nightmare.

You don't see many proximity safe alarms these days. Now there are durable safe door contacts, vibration sensors, and robust motion sensors at our disposal to protect these armor-clad storage boxes.


Protecting an open area from unauthorized personnel is always a challenge. Either the customer is expecting total coverage with the least amount of purchased hardware, or the protected environment is hostile toward the detection equipment selected. Again, during my carefree days as a service technician, the options were limited. Back then covering open spaces or internal paths of pedestrian traffic was handled by ultrasonic motion detection.

Ultrasonic motion detection (sometimes called microwave, which is similar in operation but different in frequency) has two parts (or transducers) to it, the emitter and the receiver. The emitter sends out ultrasonic waves that bounce off objects in a room and the receiver, well, receives them. The reflected waves arrive at the receiver in constant phase if nothing is moving in the area. If something is moving, then the transmitted signal shifts in phase before reception. The receiver's built-in phase comparator detects the shifted phase and initiates an alarm condition.

Ultrasonic detectors were once popular in banks, open office areas, and sales floors. They were effective under the right conditions, but had their share of false alarms. Promotional balloons, advertising banners, and environmental vibrations were often the cause of nuisance activations. Ultrasonic waves also had a tendency of going through walls, which resulted in unstable reflected signals.

Speaking of the environment, dust and dirt would often settle on the emitter's transducer and vibrate, causing a very high-pitch frequency to saturate the room. If the frequency was too high to hear, you might have felt pressure against the eardrums instead. During a service call at a local bank, my mentor, an elderly gentleman known by many as “Old John,” told me to watch the employees as he turned off the system's power. When he did, everyone looked up from what they were doing and looked around the room. They all noticed the pressure in their ears had gone away. The bank manager told us some of the employees were experiencing frequent headaches and wondered if the system could have been contributing to them. Old John said he wasn't aware of any correlation, but he knew it did. Unfortunately, it was all we had to offer at the time.


The field started using Passive Infrared (PIR) motion detectors not too long after that. Since a person or animal emits infrared radiation (body heat), this type of detector was designed to look for it. Of course, without proper instruction or training, we field technicians assumed a few things on how it operated, and initially we were wrong. All of the provided documentation showed drawings depicting “fingers” of detection spread out over an open area, so it was a safe assumption the detectors emitted beams across a room looking for infrared radiation. Had we actually read the instruction sheets we would have understood that the word “passive” meant the sensor wasn't emitting anything, just collecting heat signatures.

The operation of this device is simple. In the evening, after everyone vacates the protected area, the room “cools down” and stabilizes. The PIRs are then looking at inanimate objects (walls, chairs, desks, counters) at room temperature. If someone enters the room and walks across the path of the sensors, the detection of a sudden introduction of a concentrated 98.6-degree heat generator sets off an alarm. This type of detection product is looking for a rapid plus or minus 10-degree temperature difference, which means air conditioning and heating vents can generate false alarms if improperly located.

Thankfully, gradual temperature changes due to weather shifts are compensated and ignored by the detectors, although halogen headlamps from cars and direct glares of sunlight into the PIR's lens can also cause sensor havoc. These problems were initially overlooked by the installer because they assumed PIRs were capable of going through walls and glass doors, just like the ultrasonic detectors, and focused their efforts elsewhere. Since the PIR's were collecting and not emitting, this proved to be an incorrect assumption.

PIR detectors have multiple (or divided) infrared-sensitive phototransistors inside of them and are spatially positioned to break up an area of coverage through the use of a multi-faceted reflective lens. This design reduces false alarms by requiring the heat source to move from one “finger” of coverage to another before the detector initiates an alarm.

In case you were wondering what an “Active PIR” detector is, look no further than your Photo-Electric (PEC) beam used primarily to cover long distances from a perimeter or skylight intrusion. This type of beam is commonly used in liquor stores and small shops. The next time you walk into one and a remote buzzer or doorbell announces your arrival, look at the door jamb you just entered from and you might see a pair of them spanning the opening.

On the lighter side of these types of motion detectors, technicians, in their ever-festering need to understand and beat the systems they installed, devised various methods to defeat the devices. Some proved that wearing a cardboard box insulated the body's radiation from the detector while others wrapped themselves up in aluminum foil and plastic wrap. One customer even went as far as making his way across the room by taking tiny steps to do it. I argued that if a burglar wanted to take a half-hour to cross in front of a PIR undetected, he earned whatever he could carry away.

Of course the one thing that gave us the most headaches were pets, specifically dogs and cats, because they generated heat, too. I can't tell you how many times I was reduced to crawling around on all fours across rooms and up stairwells just to see if the family dog could be the reason the alarm was falsely activating. I know that most of you reading this are nodding with a complete sense of understanding. Most of my customers swore that their pets never climbed the furniture or moved around much during the day. We all knew better than that, but convincing them otherwise was always a challenge. Cats for example, roam everywhere and will jump up on anything that suits their fancy. Dogs love sofas and soft chairs and can't wait for their owners to leave the home so they can enjoy a few hours of comfort and leisure.

Regardless of whether the detection devices your company uses to protect the customer run on vacuum tubes or transistors, take the time to read the instruction booklets that are often included with the product. The best set of instructions will include a section on the operation as well as the wiring requirements for the product. Corby, one of my favorite companies in the industry, provided with each device a piece of candy that the installer could enjoy on while reading its literature. Gimmicky or not, Corby can find peace knowing its ploy was successful on at least one installer; the documentation was always read by me.