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5.7 LIGHT LIGHT EMITTING DIODES EMITTING DIODES (LED)(LED)

 Figure

 Figure 5.7 5.7 LEDsLEDs

A

A light-emittinlight-emitting g diodediode ((LEDLED) ) is is a a semsemiconiconductductor or liglight ht sousource. rce. LEDLEDs s are are useused d asas indicat

indicator lamps in or lamps in many devices, and many devices, and are increasingare increasingly used ly used for lighting. Introducefor lighting. Introduced d asas a practical electronic component in 1962,

a practical electronic component in 1962, early LEDs emitted low-intensity red light,early LEDs emitted low-intensity red light,  but

 but modern modern versionversions s are are availablavailable e across across the visithe visible, ultrble, ultraviolet aaviolet and infrarnd infrareded wavelengths, with very high brightness.

wavelengths, with very high brightness.

Whe

When n a a liglight-ht-emiemittitting ng dioddiode e is is forforwarward d biasbiased ed (sw(switcitched hed on), on), eleelectrctrons ons are are ableable to

to recrecombiombine ne witwith h eleelectrctron on holeholes s witwithin hin the the devdevice, ice, relreleaseasing ing eneenergy rgy in in the the forformm of

of phphototonsons. . ThThis is efeffefect ct is is calcalleled d elelecectrtrololumiuminesnescecence nce and and the the colcolor or of of ththe e lilightght (corresponding to the energy of the photon) is determined by the energy gap of the (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is often small in area (less than 1 mm

semiconductor. An LED is often small in area (less than 1 mm22_{), and integrated optical), and integrated optical}

ccoommppononeentnts s mmaay y be be uusseed d tto o sshahappe e iitts s rraadidiatatiion on ppaatttteerrn.n. LELEDs Ds prpresesentent many advantages over incandescent light sources including lower energy consumption, many advantages over incandescent light sources including lower energy consumption, long

longer er liflifetietime, me, impimprovroved ed roburobuststnessness, , smasmalleller r sizesize, , fasfaster ter swswitchitchinging, , and and gregreater ater  dur

durabilability ity and and relreliabiiabilitlity. y. LEDLEDs s powpowerferful ul enouenough gh for for room room liglightinhting g are are relrelatiativelyvely ex

expepensnsivive e aand nd rreqequiuire re mmorore e prprececisise e cucurrrrenent t aand nd heheat at mamananaggememeent nt ththaann compact fluorescent lamp sources of comparable output.

compact fluorescent lamp sources of comparable output.

33 33

Light-emitting diodes are used in applications as diverse as replacements for aviation Light-emitting diodes are used in applications as diverse as replacements for aviation lighti

lighting, ng, automotautomotive ive lightilighting (parting (particularlcularly y brake lamps, brake lamps, turn signals turn signals and indicatand indicators) asors) as wel

well l as as in in tratraffic ffic sigsignalsnals. . The The comcompacpact t sizsize, e, the the pospossibsibiliility ty of of narnarrow row bandbandwidtwidth,h, switching speed, and extreme reliability of LEDs has allowed new text and video switching speed, and extreme reliability of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are also useful in displays and sensors to be developed, while their high switching rates are also useful in adva

advanced nced comcommunimunicatications ons tectechnolhnologyogy. . InfrInfrareared d LEDLEDs s are are alsalso o useused d in in the the remremoteote control units of many commercial products including televisions, DVD players, and control units of many commercial products including televisions, DVD players, and other domestic appliances.

other domestic appliances.

5.8 SENSORS

5.8 SENSORS

5.8.a

5.8.a FIRE FIRE ALARMS ALARMS 

An automatic fire alarm system is designed to detect the unwanted presence of fire by An automatic fire alarm system is designed to detect the unwanted presence of fire by monitoring environmental changes associated with combustion. In general, a fire alarm monitoring environmental changes associated with combustion. In general, a fire alarm sys

system tem is is claclassissified fied as as eiteither her autautomatomaticalically ly actactuateuated, d, manumanuallally y actactuateuated, d, or or botboth.h. Automatic fire alarm systems are intended to notify the building occupants to evacuate Automatic fire alarm systems are intended to notify the building occupants to evacuate in the event of a fire or other emergency, report the event to an off-premises location in in the event of a fire or other emergency, report the event to an off-premises location in order to summon emergency services, and to prepare the structure and associated order to summon emergency services, and to prepare the structure and associated systems to control the spread of fire and smoke.

systems to control the spread of fire and smoke.

After the fire protection goals are established - usually by referencing the minimum After the fire protection goals are established - usually by referencing the minimum leve

levels ls of of proprotecttection ion manmandatdated ed by by the the appappropropriariate te modmodel el builbuilding code, ding code, insinsuraurancence agencies, and other authorities - the fire alarm designer undertakes to detail specific agencies, and other authorities - the fire alarm designer undertakes to detail specific com

componeponents, nts, arrarrangeangementments, s, and and intinterferfaceaces s necenecessassary ry to to accoaccomplmplish ish thethese se goalgoals.s. Equipment specifically manufactured for these purposes are selected and standardized Equipment specifically manufactured for these purposes are selected and standardized installation methods are anticipated during the design.

5.8.b

5.8.b

Burglar (or intrusion), fire, and safety alarms are electronic alarms designed to alert the Burglar (or intrusion), fire, and safety alarms are electronic alarms designed to alert the user to a specific danger. Sensors are connected to a control unit via low-voltage user to a specific danger. Sensors are connected to a control unit via low-voltage wiring or a narrowband RF signal which is used to interact with a response device. The wiring or a narrowband RF signal which is used to interact with a response device. The most common security sensors are used to indicate the opening of a door or window or  most common security sensors are used to indicate the opening of a door or window or  detect motion via passive infrared (PIR). New construction systems are predominately detect motion via passive infrared (PIR). New construction systems are predominately hardwired for economy.

hardwired for economy.

The trigger signal from each sensor is transmitted to one or more control unit(s) either  The trigger signal from each sensor is transmitted to one or more control unit(s) either  thr

through ough wirwires es or or wirwireleeless ss meameans ns (rad(radio, io, line line carcarrierier, r, infrinfrareared). d). WiWired red syssystemtems s areare convenient when sensors (such as PIRs, smoke detectors etc.) require power to operate convenient when sensors (such as PIRs, smoke detectors etc.) require power to operate corr

correctlectly, y, howehowever, ver, they may they may be be mormore e coscostly tly to to insinstaltall. l. EntrEntry-ly-leveevel l wirwired ed syssystemtemss utilize a Star network topology, where the panel is at the center logically, and all utilize a Star network topology, where the panel is at the center logically, and all devices "home run" its wire back to the panel. More complex panels use a Bus devices "home run" its wire back to the panel. More complex panels use a Bus network topology where the wire basically is a data loop around the perimeter of the network topology where the wire basically is a data loop around the perimeter of the facility, and has "drops" for the sensor devices which must include a unique device facility, and has "drops" for the sensor devices which must include a unique device identifier integrated into the sensor device itself (e.g. iD biscuit). Wired systems also identifier integrated into the sensor device itself (e.g. iD biscuit). Wired systems also have the advantage, if wired properly, of being tamper-evident. Wireless systems, on have the advantage, if wired properly, of being tamper-evident. Wireless systems, on the other hand, often use battery-powered transmitters which are easier to install, but the other hand, often use battery-powered transmitters which are easier to install, but may reduce the reliability of the system if the sensors are not supervised, or if the may reduce the reliability of the system if the sensors are not supervised, or if the  batter

 batteries are not ies are not maintaimaintained.ned.

Depending on distance and construction materials, one or more wireless repeaters may Depending on distance and construction materials, one or more wireless repeaters may  be

 be requirerequired d to to get get the the signal reliably signal reliably back back to to the the alarm panel. alarm panel. Hybrid systems utilizeHybrid systems utilize  both

 both wirewired d and and wirelwireless ess sensorsensors s to to achieve achieve the the benefits benefits of of both. both. TransmTransmittersitters, , or or  sensors can also be connected through the premises electrical circuits to transmit coded sensors can also be connected through the premises electrical circuits to transmit coded signals to the control unit (line carrier).

5.8.c G 

5.8.c G AA S SEN S SENSING ELESING ELECTROCTRODES DES 

Gas-sensing electrodes respond to dissolved gases in solution. They have a plastic Gas-sensing electrodes respond to dissolved gases in solution. They have a plastic  body

 body that that is is usuallusually y made made of of polytepolytetrafluortrafluorethylene ethylene (PTFE(PTFE). ). With With gas-segas-sensingnsing electrodes, the dissolved gas diffuses across the membrane into a small volume of  electrodes, the dissolved gas diffuses across the membrane into a small volume of   buffer

 buffer. The reaction of . The reaction of the gas with the buffer causes a pH change which is the gas with the buffer causes a pH change which is sensesensed byd by an internal-glass pH electrode. Because the reference element is built-in, a separate an internal-glass pH electrode. Because the reference element is built-in, a separate reference electrode is not necessary. Carbon dioxide and ammonia are among the reference electrode is not necessary. Carbon dioxide and ammonia are among the species measured by gas-sensing electrodes.

species measured by gas-sensing electrodes.