Passive Infrared (and "Dual-Tech" or "Tri-Tech") Motion Detectors

Infrared sensors rely on the heat radiated by an intruder's body.  There has to be a change in the infrared heat energy reaching the detector lens, and also, the change in the radiated energy has to move across the field of view in order to cause an alarm.  (So-called "Dual-Tech" and "Tri-Tech" detectors are used in environments that are hostile to standard infra-red detectors.  For those, it is also necessary for a microwave element to detect motion toward or away from the detector.)  Check for the following possibilities:

 

Environmental

•Heater or air conditioner running for the first time of the season

•New heating and air conditioning systems that have just been put into service

•New ducts that direct the air near a detector

•New ceiling fans, especially those with large blades.

•Drafts from ceiling tiles or other accesses temporarily opened for building improvement or maintenance

•Thermal excursions from portable heaters

•Heavy winds creating drafts

•Air drafts around windows and doors, especially overhead doors, and especially during a cold snap

•Air drafts entering the detector case through the wiring hole, which is usually in the back of the detector, where it meets the wall

•Doors swinging free

•Lightning storms

•Power losses, either momentary or extended

•Mail being inserted through mail slots

•Snow on the ground outside the protected room.  (Causes reflections and shadows from exterior activity into the protected room, that would not otherwise occur.)

•New windows or doors that now allow the detector to have a view of the outside world.  

•Property owner has relocated detectors without consulting a professional installer.  The worst case we have on record is a detector that not only had a nice new view of the parking lot, it was also rotated 90 degrees onto its side.  This is comparable to turning your television on its side.  The detector is utterly confused by what it sees.

 

Unusual conditions

•Party balloons (especially the shiny polyester ones), mobiles, swinging signs, anything that moves

•New mirrors or reflective wallpaper that change the field of view

•Light arrays that create the appearance of movement

•Objects, posters, papers falling from the walls 

•Curtain tie backs coming loose allowing the curtain to swing down

 

People, Large Critters and Things

•An actual entry may have occurred by a family member, friend, house sitter or service person who is not completely familiar with the system's operation.  They may not have had an opportunity to touch base with you since the event, or they may be too embarrassed to mention it.  You may have to call all the people who have access, and tactfully ask if they were there.

•Check to see if anyone was departing at about the time of the alarm.  Perhaps they exceeded the exit time delay or they set the system in an "instant" mode by mistake.

•Sometimes people with hearing impairments do not hear the system's entry warning beep when they enter.  Note that many systems have entry warning sounders that make noise at one single frequency.  Hearing loss can occur at a specific frequency without creating any noticeable problem in most other day to day activity.  Suggestion:  Conduct a test to verify everybody can hear the entry warning clearly.  Some remedies:  Add sounders that make a different tone or sweeping tones that can be heard clearly by the person with the hearing impairment; add a light, strobe or other visual device. 

•Verify that entry warning sounders are positioned close enough to all of the potential entry doors.  The problem of insufficient warning noise is especially prevalent in large facilities having multiple entry points, and particularly when several far flung entry points share a common, centrally located keypad.  The remedy is to add keypads sounders or entry warning sounders, or make the existing sounders much louder.

•A deer or person may have passed by a window that is not properly "masked" out of the coverage pattern.

•Someone may have shined a flashlight through a window, causing the detector to think something warm was moving around in the room.

•A vehicle's headlights may have shined through a window onto the walls or into a mirror in the protected room during a turn, causing the detector to think something warm was moving around in the room.

•Although we have never confirmed it, some activations were attributed to the gleam from passing airplanes, or the shadows they cast across a room.

 

Medium to small critters

•Dogs and cats:  

•If you have a pet, the protection must be configured to allow for any expected movement of the pet.  Some subscribers make a regular practice of confining the pet to a cage or a certain room or section of the house, and then they do not configure the protection in the remainder of the house to allow for an "escape."  Others let the pet roam free inside the house, but they always disable the motion detectors when they set the system, because they know the pet will trip the alarm.  We advise against these practices, because sooner or later, someone will inadvertently set the system without disabling the motion detectors or there may be an escape.  We always try to design systems that will tolerate these scenarios, which we see frequently.  Ask us to review the coverage and make a recommendation.  

•In a second scenario, a house sitter forgets to confine the pet, can't find the pet when it's time to leave, or does not properly secure the door or gate, permitting an escape.  

•In a third scenario, visiting guests bring their pet to a protected home that is not configured for pets.  Everyone goes out to dinner, leaving the pet behind.  The owner forgets to disable the motion detectors when setting the alarm system.  

•Squirrels, raccoons, groundhogs, etc. can get in.  Squirrels especially like to break in through the eaves and set up housekeeping in the attic.  If you hear them, take corrective action immediately.  Squirrels and other rodents are very destructive and will eat the insulation off your wiring.  Note that squirrels are protected by law.  You should have them trapped by a professional pest remover, who will set them up in a new hometown with a new identity.

•Bats frequently get into homes.  They fly in through open doors and windows, and sneak in through gaps around window air conditioners, flues and ducts.  The most likely way you will realize you have a bat is the flicker you see on your television screen as they fly around the house while Jay Leno is on.  They often land behind curtain rods.  If you can locate the bat at rest, a vacuum cleaner hose... well, you figure it out. 

•Wild Birds commonly get into garages and houses through open doors and windows.  They can enter attics through gaps in the eaves.  

•Pet birds are also known to escape from their cages once in a while, and some have reportedly returned to their cages voluntarily, even closing the door after themselves.

•Mice can enter any time, but we notice a high incidence when the weather first turns cold.  They travel throughout the entire house and will damage your wiring severely if left unchecked.  We recommend that you keep mousetraps set at all times, regardless of any security system.   

•Snakes:  We found numerous copperheads in one basement ceiling, among the pipes and in the spaces at the ends of the floor joists.  (They apparently entered through a network of tunnels leading out under the flagstone patio.)  We are not aware that they caused false alarms.  They just made it very difficult to service the security system.

 

Insects

•Spiders, weevils, ants and moths can get inside the detector case if it is not sealed.  Although they are cold-blooded, if one walks across the face of the optical detector, it would cast shadows that disrupt the ambient room energy reaching the sensor.

•Spider webs in front of the detector are a sign of spiders.  Knock the webs down.  Lightly spray some ant and roach killer on the wall around the detector. (Important!  Don't spray aerosols near your smoke detectors!  Instead, use a paper towel to dab the pesticide where you want it.)

•Large millers have been known to cause alarms by flying close to the detectors.

 

•We recommend that you always have a mouse trap set.

•Make sure basement doors, windows, eaves, attic windows, air conditioners, ducts and other openings to the outside are always secure against rodent entry.

•Notice that many of the above problems are design-related.  If you see yourself in any of these scenarios, ask us to review the coverage and the Control Panel setup for possible improvements that will reduce the chance of a false alarm.

 

Stress  Detectors (also known as "The Pulsor")

Description

Stress detectors have existed for several decades, having been used to detect the stress in airplane wings, among other things.  They have many industrial and residential uses.

"The Pulsor" is a product line we use, marketed by Sure Action Incorporated. Because of the way it works, they call it a "Presence" detector rather than a "Motion" detector.  We have been very impressed and satisfied with the performance and false-alarm resistance of The Pulsor.  (You can see their web page for some general information describing why this is the detector of choice in many applications.  

 http://www.sureaction.com/Pulsor.htm 

A Stress Detector is a crystal enclosed in a plastic case, about the size of a person's index finger.  It is typically epoxied to the bottom of a structural member like a floor joist, beam or stair riser.  When a person walks across a section of flooring, the floor bends a little bit under the weight.  When a crystal is bent or compressed, the internal stress changes its electrical value.  This change is detected in a Processor Unit usually located inside the alarm system cabinet.  It has a circuit to measure the value of the crystal, a "pot" to adjust the trigger threshold and an output relay.  (We do not recommend that the end user attempt to adjust these settings.)

A stress detector is more "Spot" protection than "Space" detection.  It yields a sensitive area roughly three to six feet in diameter.  There may be a "sweet" spot within that zone, too, usually near the middle of the zone, depending on the structure.  We suggest locating stress detectors in major thoroughfares within a protected premise and in the vicinity of any specific burglary targets like safes, gun cabinets, silver chests, etc.

 

Installation Considerations

Epoxy Hardening.  When the crystal is first installed, the epoxy starts out very "green."  Over the first few minutes it hardens rapidly, and over the following week or two it will continue to harden at a decreasing rate.  Green epoxy absorbs some of the energy that is available to flex the crystal unit.  As it hardens, the sensor will more closely track the movement of the floor.  This will cause the detector to trip the alarm under less weight.  

Sensitivity Adjustments.  The installer will try to anticipate this increasing sensitivity curve when making a final setting, but there is no absolute way to predict the ideal final setting. When the installation is delivered, the client usually wants to see the system operate as described.  Partly because of a changing sensitivity curve and partly because the pets will not always cooperate with the installer, it is common for the company to adjust the settings during the actual delivery demonstration.   

•A five pound cat is usually "adjusted out" in the initial setup.  A ninety-pound dog will require more walk testing and a more accurate setting.  You should assume a minor adjustment could be needed a week or two after the initial installation, and possibly again in a few months to a year. 

  1. •We recommend "baking in" stress detectors when there are large pets, because we cannot always anticipate how much the sensitivity will increase over the first week or two.  Ask us about the procedure for baking in various types of detectors.  It will vary from one system to the next.  


Known Issues 

1.  The biggest culprit is trying to achieve more intruder detection than is practical, especially when trying to "adjust out" a heavy pet.  

•The degree of compromise necessary will vary from one installation to the next.  The major factors include the floor plan, the structural configuration, and the size of the pets.  With a small pet, there is virtually no compromise, because most intruders will weigh at least five or ten times a cat's weight.  A compromise with an adequate safety margin has to be made when adjusting for pets that approach or exceed a light human intruder's weight.  

2.  Some basements and crawl spaces are prone to high moisture.  This can affect the epoxy bond and also attack the electrical wires and connections, especially where the wires enter the Pulsor assembly.  This is one of the most common causes for service on Pulsors over a year old.

3.  Lightning has been known to damage security systems, including the processor modules, altering system sensitivity.  You should walk test your detectors (of all types) after any severe electrical storm.

4.  Rickety buildings that flex excessively under wind and thunder may be prone to weather-related incidents.

5.  Machinery such as washing machines can flex the floor noticeably.  A competent installer will recognize this possibility and design around it.

6.  Some buildings are so close to roads that the whole structure flexes when a heavy truck passes or turns the corner.  We discovered one of these through experience.

7.  Sensors may be adjusted to ignore the normal passage of pets, but can trip if a large pet leaps from a nearby table or chair.  

•Furniture may have to be considered in the initial placement of sensors.  

•The homeowner should be alert to this possibility when rearranging furniture.

•Additional compensation may be required in making the sensitivity adjustment.

•A "Sweet" spot within the protection zone may exacerbate the problem.

8.  Placement of heavy furniture near a previously existing sensor may preload the structure, decreasing the sensitivity.  The owner needs to be alert for this possibility.  

9.  Beams or joists that pass out of the protected room over a supporting wall to a porch or unprotected room may flex due to a person on the "outside."  (Cantilever effect)  A competent installer will recognize this possibility and design around it.

10.  Settling of a newly built structure can cause transient activations.  A bake in period is suggested for brand new buildings.

11.  Snow sliding off a roof can cause a structure to "un-flex."  Circumstance:  A knee-wall or partition connected to the roof rests on a floor having a sensor.  A competent installer will recognize this possibility and try to avoid it.  

•It is possible for interior partitions to pass the stress of a heavy snow down through several floor levels.  This will be more difficult for an installer to anticipate.  

12.  Temporary bracing can transmit side wall flexing (as from wind) to the protected floor.

13.  Rodents can damage the wiring, especially if it is in an open exterior crawl space such as you would find beneath a shed.