Passive IR Movement Detectors (PIR's) Magic Eyes


PASSIVE INFRA RED Movement Detectors (PIRs) (Including fault finding and fitting tips)

The most common type of movement detector is the passive infrared known as the PIR for short.

The unit is a compact box usually mounted in the corner of a room or storage area. The most usual position of a detector is in a corner, at a height of approximately 7-8 feet, 2.1 Metres. As the name suggests the unit does not radiate a signal to detect movement. It is a complex lens or mirror design that “looks” at the temperature of objects in a room and focuses on an infra red sensor. The very small change in temperature caused by an intruder moving across adjacent detection “lanes” is detected by the infrared sensor, which triggers the alarm. The units are sensitive to mid range infra red in the 7-15 micron range. Which is the wavelength of body heat (98.4 degrees F.) at room temperature. The sensitivity of a PIR falls off rapidly as the ambient temperature approaches body temperature.

Depending on the design of the lens or mirror the detection pattern can be varied for several different purposes. Some units are supplied with interchangeable lens systems.

QUAD PIRs

A QUAD PIR has two identical detector circuits operating in parallel, coupled with the one lens or mirror assembly. To create an alarm condition both detector circuits must trigger. This type of detector although reducing spurious false alarms, is often slower to detect an intruder in its protection area.

CORRIDOR P.I.R

Corridor PIRs have a long thin beam type detection area, designed to give movement detection along corridor areas, or as a form of beam protection, across a shutter or loading area.

CURTAIN PIRs

Have a flat curtain type detection area, especially useful to put an invisible curtain of protection across a window or skylight.

CEILING OR 360 degree PIRs

Ceiling PIRs are designed to be fitted on the ceiling of premises, above the object to be protected. They are ideal for protecting stand alone risk areas such as filing cabinets or safes. They give a 360 degree (shower head type) protective area, looking down on the area to be protected.

PET ZONES

Most PIRs can be set with a pet zone. This means that the ability of the detector to look down to floor level is removed or reduced, usually by masking part of the PIR lens, to allow a dog etc. to move around the protected area. The use of a pet zone will of course lower the security of the protected area, although this does not seem to cause a problem. The pet zone facility is not very effective with cats, as they tend to move at all levels of a room! Care should be taken with caged pets such as birds etc. If they are fairly close to the PIR, their movement can cause a false alarm.

Some more sophisticated PIRs differentiate between animal and human movement without the loss of low level coverage. These work fairly well except in the case of a large dog.

PIR's-- FALSE ALARM PROBLEMS

Early P.I.Rs were prone to false alarms caused by nearby radio transmissions e.g. taxis and police vehicles transmitting close to the unit---RFI (radio frequency interference). Modern detectors should now be protected against this problem, but it is worth checking for this, in the event of unexplained false alarms.

A second problem is the sudden shining of close, bright lights, such as headlights into the unit, or the sun rising or setting and suddenly shining into the unit’s field of view, causing a fast rise in temperature. A spate of early morning or evening alarms could be caused by the sun moving around to illuminate and heat up objects in the detection area at certain times of the year.

A rapid change of temperature well within the detectors range, such as a thermostatic fan heater or air conditioning unit turning on, or draughts from a window left open, could also trigger a false alarm, by rapidly warming or cooling objects in the detectors field of view. Normal slow reacting radiators should not cause problems

Air currents will not trigger the unit, unless blowing directly onto the PIR, but the rapid change in temperature of surfaces in the field of view of the PIR may trigger an alarm.

Try to avoid hanging objects close to detectors. Mobiles, hanging signs and CHRISTMAS DECORATIONS! moving close in front of a PIR can cause a false alarm. Posters falling from walls can also cause false alarms, because they are at slightly different temperatures to the objects in the detection area, and are therefore detected. This is a major problem in schools and shops. Rats and mice can also be a problem, causing false activations, if they can move close to the detector along ledges etc.

An additional hazard is caused by spiders, large moths or an abundance of very small harvest flies moving very close to, or inside the PIR Efficient sealing of the PIR, especially around the cable entry holes will help stop this problem. Although how often insects and spiders are blamed for an unexplained alarm by a desperate alarm engineer is debatable!

The sealing of cable entries will also stop air currents giving a false alarm if they circulate within the detector housing.

Another fault condition noticed by the author, is false alarms caused by very high room temperatures. After a few days of high temperatures,especially at week ends, a PIR will trigger for no apparent reason.

Because PIR'S are cheap and reliable, they are one of the most common detection devices used,

Advantages and Disadvantages

Old PIR detectors are prone to RFI (radio frequency interference) although this fault has been mostly overcome in current units. Bright lights shining into the device can cause false alarms. If stock etc. is stacked in front of the detector, it cannot see the area to be protected.

The advantage of a P.I.R detector is that it is a very cost-effective solution for alarm coverage of medium sized open areas.

The disadvantages of PIR. detection is that generally, as the unit is passive, it is just looking into the protected area and is not sending out any signal- a fault developing in the unit could leave an area unprotected, without the occupier being aware.Always check the areas protected by PIR's for stock or fittings blocking the units view.It has been known for shelves of stock to be built in front of PIR's without being noticed!

The alarm control unit can sometimes not detect damage to the detector and the area will not be protected. Make a point of checking PIR's for physical damage.

The PIR relies on having an unobstructed field of view. It is therefore made inoperative if stock or other items are put in front of it. (Very modern units to the European standard have an ANTI MASKING facility to overcome this fault)

This should be checked, before setting the system.

To counteract this problem most P.I.Rs are fitted with an LED (light emitting diode) this remains lit if the unit is not detecting movement, and extinguishes when movement is detected. The detectors remain active at all times, although obviously do not trigger the alarm when it is not set. This allows a check to be made on the correct operation of the detector, by observing the indicator light, while moving around in the protected area. In a closed premises with no public access this is not a problem. In public areas, potential thieves to check the coverage of the unit during open hours - to be exploited when the system is set could use this facility. To stop this, most detectors have the facility to disable the LED indicator. A WALK TEST facility, programmed into the main alarm control unit, should be used in place of the LED test light.

Some modern PIR detectors can detect objects masking their view and will not set.This is a requirement of the new European Standard.

FAULT FINDING ON PIR DETECTORS

Draughts and air currents do not directly trigger PIR’S. The rapid temperature change of objects in the detection area affected by the air currents does. The same applies to sunlight suddenly warming up objects in the field of view

As a rule of thumb, do not allow a PIR to "see" an external glass window within 50 % of its range. It is better to position the PIR to avoid looking towards windows at all.

Mains spikes (very fast pulses of high voltage) often caused by refrigerators, electric motors and fluorescent lights may be induced into alarm cables causing unexplained false alarms, on most types of detector units. The connecting cables act as aerials and feed the interference to the detector, and the control unit. Therefore don’t run alarm cables in parallel to mains cables. Mains spikes and radio transmission faults can be often eliminated by the fitting of mains spikes filters or radio frequency interference (RFI) filters, at the control unit, or in severe cases, adjacent to the PIR.

Most alarm equipment is fitted with a TAMPER switch. Always check that the lid or cover of a detector is correctly fitted and is operating the tamper switch before any other checking is carried out.

Fitting a PIR in a small space, much less than its normal range, can be a cause of false alarms. although they often work quite happily in this position. This may not apply with the latest European Standard detectors! Some modern PIR detectors can detect objects masking their view and will not set.

DOUBLE KNOCK

Double knock is the term used for a facility available on some control units When set to double knock a detector, such as a PIR, does not create an alarm condition after the first incident triggering the unit, but triggers if a second activation is detected. This is a method of cutting down on false alarms caused by one-off incidents such as falling posters etc. The use of double knock degrades the security of the protected area, but is not usually a problem.

If you really are technology minded look in:

http://home.howstuffworks.com/framed.htm?parent=question238.htm&url=http://www.glolab.com/pirparts/infrared.html

How Burglar Alarms Work - very American, but goes into a lot of detail including circuit diagrams for PIR's !