Other Alarm Devices


MICROWAVE MOVEMENT DETECTORS

A microwave movement detector is an active movement detector. It works by sending out a uhf (microwave) radio signal, which bounces around the protected area. If there is any major movement of an object in its field of view, the signal being received back from the room shifts frequency slightly (Doppler Effect) and the difference is enough to trigger an alarm.

Microwave detectors are prone to coverage creep. The detector can see through some walls and windows, causing false alarms from movement outside the required protected area, or fluid moving through plastic pipes. Objects moving towards or away from the unit achieve the best detection. They have been known to detect buses moving in the road outside!

MICROWAVE Movement Detectors – Advantages and Disadvantages

MWDs are active devices that can detect movement of water through plastic pipes, and if badly positioned, pass through glass and thin partitions. A large metal object can be detected through glass or partitions. A lorry or bus can be detected at a long range. Leaving a fluorescent light fitting turned on in the detection zone of a MWD may cause a false alarm, as the movement of gasses within the tube may be detected.

The MWD will usually fail safe. It will not allow its zone to set, if a fault developes in the unit.

DUAL TECHNOLOGY

Dual Technology detectors use a combined P.I.R and a microwave movement detector, housed in the same case. Both units are wired in parallel, this means that both units must trigger to cause an alarm signal. As a Microwave movement detector (MWD) is an active device, this gives the detector several advantages over a stand alone P.I.R. Dual-Technology detectors by combining the good and bad properties of PIR/S and Microwave movement detectors cut down the possibilities of false alarms. The disadvantage of Dual-Technology devices are that the response to movement in the protected area can be sluggish compared with the response of individual technology detectors, as both detector systems need to be triggered to create an alarm signal to the control unit. As each of the detectors have different advantages and disadvantages, the combined units cancel out most major problems and make for a reduction in false alarms. The detection range of dual tec units is greater than most P.I.Rs (around 20 metres).

ULTRASONIC MOVEMENT DETECTORS-(UMD's)

Ultrasonic detectors work on the same broad principle as Microwave detectors, using high frequency sound, of a frequency above human hearing range, instead of radio waves. UMDs do not see through solid objects, and the detection area is confined between any solid structure. Ultrasonic detectors are rarely used as the PIR is cheap to buy and usually superior in coverage and more reliable.

Advantages and Disadvantages

Ultrasonic movement detectors are rarely used today. The coverage and detection range is depended on the materials in the surrounding walls etc. The PIR is superior in most respects, including cost.

INFRA-RED RAYS

Infrared rays are simple beam devices. They comprise alight source in one unit and a photo diode type receiver in the other unit. The two units are placed to shine a beam of light, usually infrared across the area to be protected. Walking through the beam triggers an alarm, as walking between the transmitter and receiver interrupts the beam of light, stopping it reaching the receiver. The units are usually placed across roller shutters, or diagonally along corridors, as trap protection devices. By setting the rays to shine diagonally along the length of a corridor, anyone walking along the corridor will at some point interrupt the beam. To stop the bypassing of the ray by shining a torch etc. at the receiver, infrared light and a modulated beam are usually used. Modulation causes the beam to turn on and off at a fixed rate, many hundreds or thousands of times per second. The receiver is tuned to only accept this frequency, and will not respond to unmodulated, or wrongly modulated signals.

Advantages and Disadvantages

Fixed IR rays (beams) and microwave are rarely used internally, as they can be avoided and are easily knocked out of alignment. The main use of rays in various guises is for trap protection (put in a position that should mean that an intruder is likely to walk in front of the beam whilst moving within the protected premises) or for outdoor protection of perimeter fences or walls, where they are mostly used as a local indicator to alert security or turn on CCTV.

Fixed infra red rays are rarely used now, as a PIR will usually give better protection for less outlay. A fixed ray is easily overcome(avoided) if seen, and are easily knocked out of alignment. The projector must look directly at the receiver to allow for correct operation.

In a bad environment, such as a vermin infested shed or warehouse, the use of two fixed infra red rays set a few feet apart, and wired in parallel,ZONE PAIRING can be a solution to false alarms. If the rays are set one above another, an object or intruder will trigger both units together, and trigger the circuit or zone. If a bird or rodent passes through the rays only one ray will trigger at a time, therefore not causing an alarm condition.

A narrow angle (corridor) PIR may be used in many cases, in place of an infra red ray. It has the advantage of not needing accurate alignment and having a less predictable protective beam.

VIBRATION DETECTORS

The crudest form of vibration detector was in the form of a weighted spring contact held lightly against a fixed contact. The unit was fixed to a flat surface to be protected, usually a plate glass window, with the fixed contact nearest to the surface of the glass. If the detector received a substantial blow, the weighted contact would be knocked away from the fixed contact thus breaking the circuit, and operating the alarm. The great flaw in this crude form of detection was that if the contact was set too loose, false alarms would occur if the window was knocked, and if too tight, there would be no false alarms, or genuine alarms either!

The only way to test a contact was to give the protected window a hard knock with the handle of a large screwdriver. If a little too much force was applied, the window has been known to break.

Modern vibration detectors are more sophisticated and work in conjunction with electronic analysers to differentiate between ambient normal vibration, and an attempted forced entry.

Vibration detectors may be fitted to walls, window frames or doorframes; to detect the vibration caused by attempted forced entry. Most are fitted with a L.E.D (light emitting diode) indicator, which remains lit after an activation to indicate the source of the alarm. Useful when more than one detector is fitted to a circuit. With modern units, false alarms are rare, as the electronic analysers can filter out ambient vibration caused by traffic etc. The only circuit not suitable for vibration detectors is the main entrance doors, as post or visitors trying the door, may cause a false alarm.

Advantages and Disadavantages

Modern vibration detectors can give reliable and economic protection to the fabric of a protected building –one unit protecting up to 3 to 6 metres. of wall or roof.

The units can only operate in relatively stable conditions. The presence of nearby railway, roadways used by heavy vehicles, piling, building and roadworks can make the use of vibration detectors unsuitable. Fixing a vibration detector to an external roof is not advised as heavy birds, squirrels or rain and hail can cause a false alarm.

GLASS BREAK DETECTORS

Ultrasonic, or Glass Break Detectors, are tuned to detect the ultrasonic sounds created by breaking glass. They are used to detect the breaking of glass, usually larger areas, used in shops etc. The units are mounted internally in the premises, and operate the alarm if the windows are broken to gain entry. One unit can detect the breaking of large areas of glass.

Advantages and Disadvantages

The advantage of glass break detectors is that the alarm operates immediately the glass is broken, without being attached to the glass. The disadvantage is that the alarm operates only after the perimeter has been breached. The breaking of glass internally e.g. falling glassware, cutlery or exploding beer or lemonade bottles can also cause false alarms, although relatively rare. The units are also difficult to test, as breaking the glass is not an option! Electronic test units are available but one simple method of testing units is to rattle a bunch of keys near the unit, this produces frequencies similar to that of breaking glass, and will operate most units. Remember to check for jangling metal wind chimes and similar items in the protected area after false alarms.

The units are less successful in detecting small panes of glass breaking and are ineffective if the glass is already cracked or safety plastic film is fitted to the protected glass areas. Also wired safety glass may not operate the unit when broken. The same applies to laminated glass.

AUDIO DETECTORS

Audio Detectors are mainly used in soundproof areas, such as safes and strongrooms. They are used to detect the sound of attempted entry by explosive or mechanical means, and are only reliable if used in stable soundproof areas. The proximity of railways, or tunnelling can be a source of false alarms.

MATS

Open circuit mats consist of two plastic sheets fitted inside with metal strips or metallic foil separated by foam rubber with openings pierced at intervals, which are normally held apart, but when stood on they are pushed together to form a closed circuit. They operate like a torch switch or bell push on a front door. These mats are fitted underneath a carpet, but above the underlay in an obvious access area, and rely on not being seen. A narrow type of mat is used on stairs for detection of intruders at night.

O/C MATS IN CLOSED CIRCUIT MODE

Although open circuit type of alarm technology, a mat may be used to protect an item by placing the item on the mat to cause a closed circuit. If the mat is then wired to a standard Closed Circuit zone, any attempt to remove the protected object will operate the alarm system.

As a constant pressure on a mat could cause it to fail in a closed circuit state, the operation of the circuit should be checked regularly and the object should be re sited on the mat periodically to even the wear.

Advantages and Disadvantages

Mats were formally used, in pre PIR (passive infrared movement detector) days as a form of trap protection to be triggered by an intruder moving within the premises. Their advantage was that they were a relatively cheap way to protect an open area. Narrow mats are also produced to protect stair treads. The occupant could leave the mats live on a system when going to bed, and avoid them by walking around their positions. Mats are rarely used now as PIR’S give vastly superior protection at an economical price.

Mats should be fixed down firmly as they have a tendency to move under carpets, ending up some distance from their original position, causing false alarms, as well as the expense of removing and relaying carpets. Being open circuit also means that a failure will not be apparent, unless they are periodically tested.

Older alarm control units are often fitted with mat, or O/C mat terminals. The O/C (open circuit) terminals (2)) need a short put across them to create an alarm condition. If a control unit does not have an O/C alarm provision, mats can still be used if a simple external relay circuit is used.

The mat is used in series with the coil of a 12-volt relay to complete the circuit when it is triggered. The isolated contacts of the relay are used to break the feed and return loop of a spare closed circuit or zone. The relay is in effect an O/C to C/C (open circuit to closed circuit) converter. The converter can also be used in other alarm situations.

Always ensure that any relay used is fitted with a diode across its coil, as the high voltage produced by the relay coil when it is de energised can destroy a modern solid state control unit. The +ve end oif rthe diode (often marked with a coloured band) should be connected to the +ve supply end of the relay.

TIP

If there is a problem with a mat, check that there are no pins or staples stuck through the two conductor layers. It does happen! Also check that it is still located in the position you thought it was!

AIR PRESSURE CHANGE ALARMS

Air Pressure Alarms can only be used in an area suitably airtight. The air pressure in the protected area is constantly monitored, and any entry is detected, in a similar way to a barometer, by the rapid change in air pressure, as the area is breached. Although also used as a domestic alarm system, it use is not recommended. Use in strong rooms is probably more practical and reliable.

Advantages and Disadvantages

Variations in air pressure can cause false alarms, and the system is rarely used. In a secure sealed area, such as a strong room or safe, the system would be more practical. A combined air pressure and PIR detector is currently on the market, but its reliability is yet to be proved.

CLOSED CIRCUIT WIRING and TUBE AND BATTEN-

Before the advent of cheap reliable movement detectors, a mechanical method of intrusion detection was used. To protect external windows a method called tube and batten was installed. The window was covered internally by a frame made from wood battens holding tubing containing snap wire, (thin, hard drawn, insulated wire that snaps easily when stretched) forming part of the alarm closed circuit.

The frame looked like a system of metal bars across the window, but worked by the wires inside the bars snapping if any attempt was made to force the bars aside, or remove the frame. The frame is also anchored to the building fabric to ensure an alarm condition if an attempt is made to remove it.

CLOSED CIRCUIT WIRING

Closed circuit snap wire was also stapled to external doors, and sometimes floors and ceilings, to give similar protection against intrusion. This is covered with hardboard to give it some protection from physical damage.

Wooden cabinets are also fitted to floor safes and wired with snap wire to give alarm protection. Most wiring consisted of parallel wiring of alternate polarity stapled approximately 4 inches --100 mm apart. On very high security installations the wiring was also continued at right angles producing squares of wiring.

Advantages and Disadvantages

The tube and batten on windows had a visual deterrent effect, and gave an alarm condition before entry was gained very far into the premises.

The installation of tube and batten, and snap wire took skill and time, and was prone to mechanical damage, and damp problems. Also if the staples holding the wire to the wood are hammered too tightly, intermittent faults can develop, especially as the wood contracted with the weather etc. The cosmetic appearance also left much to be desired, and was therefore not suitable for private residences.

Tube and batten framing is easily damaged, hinders window cleaning, and is prone to damp damage.

The introduction of cheap reliable movement and vibration detectors, along with glass break detectors has made the use of snap wire systems almost obsolete. Closed Circuit wiring on wooden fire escape doors in commercial buildings is, however making a comeback, to comply with the ACPO 2000 POLICY on alarm confirmation.

LEAD / METAL FOIL

Foil is a strip of lead / aluminium or similar foil stuck to the inner surface of a plate glass window. The foil is fixed around near the edge to allow good visibility through the glass. The foil is stuck to the glass by its own adhesive or by a layer of clear varnish, and then often protected by further layers of varnish. The foil is connected to connector blocks at each end and forms a closed circuit loop. If the window is broken, or cracked across the foil, the loop is broken and the alarm operates.

(If you want a guide to fixing foil try the site below-includes instructions and supplys required--USA site) http://www.alarmfoil.com/install.htm

Advantages and Disadvantages

Foil acts as a visual deterrent. It is easily damaged by window cleaning and is prone to high resistance and intermittent faults, especially where the foil joins the connectors. The visual impact also makes it more suitable for commercial premises. Because of the relative cheapness and reliability of Audible (Glass break) Detectors, lead foil is now rarely used. When fitting lead foil to adhesive connecting blocks, always site the block on the protected glass pane- not touching the glass surrounds.

This is because vibration and the different expansion rates of the frame and glass can cause the connectors to break the foil strip. Even worse, an intermittent fault or high resistance joint can be created causing unexplained false alarms.

Always check the foil as it enters the connector block for cracks and imperfect connection, if a fault occurs.