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566 SMOKE ALARM

The smoke alarm circuit pre-sented here is based on the readilyavailable photon+oupled intenutermodule and timer lC NE 555. Thephoto Jntenupter modulb,is used

' as the smoke detector, while timer555 is wired in astable configura-tion ag an AF oscillator for sound'ing alarm via a loudspeaker.

ln the absence of any smoke, the

gap of photo interrupter module isclear and the light from LED fallson the photo-transistor through theslot. As a result, the collectot otphototransistor is pulled towardsground. This causes reset pin 4 ollC 555 to go low. Acc,grdqtg$-i-t[etimer is reset and hence the alarmdoes not sound. However, whensmoke is present in the gap of the

photo interrupter module,'the lightbeam from led to the phototransistoris obstructed. As a result, thephototransistor stops conductingans pin 4 (reset)ol lC 555 goes highto activate the alarm.

Note l- The unit must be housedinside an enclosure with holes toallow entry of smoke.

DETAILED DESCRIPTION

Appllcation of the circuitMost of the smoke alarms availablein market use one or another

;..3*kind ol smoke sensor which in-'' creases the cost and also requirecomplicated interfacing circuitry'around it. But this smoke alarmcircuit uses readily available phe'ton-coupled interrupter module assmoke detector. The functioning ofthis smoke alarm is to coniinuouslydetect smoke and automaticallyactivate the alarm in case of densesmoke. Loud sound is heardcontinously indbating the presence

of smoke till smoke clears. Afierthis, the circuit is automatically re-set for further detection ol smokeprovided it is kept in power ON con-dition.

Why thls particular circuitPhoto interupter module used hereas smoke detector cosists ol anintemal LED and phototransistor. ln

. normal phototransistors the LEDON condition provided base bias-

- ing to phototransistor in form of lightwhich helps in changing of its statefrom cut-off to saturation. Butthisphoton-coupled interrupter moduleis slightly different as provision ofwindow gap is also provided here.The variation in intensity of smokeatwhich its detection iswantedcaneasily be done using preset. Thecircuit operates normally over a DCvoltage range of 9 to 12V which canbe obtained by and DC supplysource elimination any need ol

bulky transformer. Slight modifica- square wave output at its pin no.3.tion in circuit enabbs variation of The frequency of this wave shouldits frequency response. be in the range of 20-20KHz and is

dependent upon the values of resis-Explained working of circuit tors R3, R4 and capacitor c1. ForThebasicid,eabehindcircuitopera- th values shown in the eircuit it istion istotum ON the alarm in case about 3g0Hz Capacitor C3 etfec-of dense smoke. A smoke detec- tively passes the output from lC totorworking in tandem with oscil- loudspeaker which then con-'.htsl*s.-rlgeded for this purpose. verts electricals signals to sound.Here, photon-t ou@Cflffi rzuptdt-C6F6e[6ng2Tomh6Xpffifpnnb-,7module works as smoke detector provides immunityfrom anykingolwhile NE 555 timer working as os- false triggering.cillator generates loud alarm sound,Resistor R1 and preset VR1 pro- How to buildvide cunent path to light up intemal * First of all read the given manualLED in normal condition, intemal thoroughly and study the circuitLEDof photo-intenupier emits pho- gi"";il figure. Atso have a took attonswhichcontinuouslykeepinter- pCB and component suppliednal phototransistor in saturation along with the kit. Each compo-regionowingwhichitscollectorand nent has to be soldered in itsReset input (pin no.4) of lC1 are position on pCB.putled towards ground. As reset pin . ldenfjcation ol resisfors is doneof lC1 NE555timeris activelow it bycolorcoding. Thecolorbandoncontinuously reset the timer and it each resistor corresponds to itsdoesn't produce any kind of output exact value.in'such a situation. * There are d6lerent methods inNow consider'ihe seeond ---- sce-'WhiChvaluesaredefinedoncapaci..nario, when a room is filled by tors. But usually values are speci-smoke, slot ol photo-intenupter 1ed numerically on them. Refer "gets engulfed in smoke which ob. Basic Electronics" section ofstructs the path of light rays from manuallor details of resistor andLED to phototransistor. capacitoridentificationmethods.Phototransisto/s base being Re- * Can you make out the wholeverse Based causes it to cut-oJf and working ol circuit and are you ableas a result its collector and Reset to ide;tify each componentpin of l0l are held highvia resistor separetely as to where each ofR2. Thus Reset condition ol Timer them has io be placed ?is disabled by smoke. lC1 work- i [, yes only then procced fur-inghereinastablemodegenerated tnei io actually mounting and