1. AUTOMATIC ROOM POWER CONTROLLER
TONY GLADVINGEORGE
An ordinaryautomaticroompowercontrol circuithas onlyone lightsensor.Sowhenaperson
enters,the roomitgetsone pulse andthe lightscome ‘on.’Whenthe persongoesoutitgetsanother
pulse andthe lightsgo‘off.’Butwhat happenswhentwopersonsenterthe room, one afterthe
other?It getstwopulsesandthe lightsremainin‘off’state.
The circuit describedhere overcomesthe above-mentionedproblem.Ithasa small memorywhich
enablesittoautomaticallyswitch‘on’andswitch‘off’the lightsinadesiredfashion.The circuituses
twoLDRs whichare placedone afteranother(separatedbya distance of sayhalf a metre) sothat
theymay separatelysenseapersongoingintothe roomor comingout of the room.
Outputsof the twoLDR sensors,afterprocessing,are usedinconjunctionwithabicolourLEDinsuch
a fashionthatwhena persongetsintothe room it emitsgreenlightandwhenapersongoesoutof
the room itemitsredlight,andvice versa.These outputsare simultaneouslyappliedtotwo
counters.
One of the counterswill countas+1,+2, +3 etc whenpersonsare comingintothe room andthe
otherwill countas -1, -2, -3 etcwhenpersonsare goingout of the room.These countersmake use
of Johnsondecade counterCD4017 ICs. The nextstage comprisestwologicICswhichcan combine
the outputsof the twocountersand determine if thereisanypersonstill leftinthe roomor not.
2. Since inthe circuitLDRs have beenused,care shouldbe takentoprotectthemfrom ambientlight.If
desired,one mayuse readilyavailable IRsensormodulestoreplace the LDRs.The sensorsare
installed insuchaway that whenapersonentersor leavesthe room, he interceptsthe lightfalling
on themsequentially—oneafterthe other.
Whena personentersthe room,firsthe wouldobstructthe lightfallingonLDR1, followedbythat
fallingonLDR2. When a personleavesthe roomitwill be the otherwayround.
In the normal case lightkeepsfallingonboththe LDRs, and as such theirresistance islow (about5
kilo-ohms).Asaresult,pin2 of both timers(IC1and IC2),whichhave beenconfiguredasmonostable
flip-flops,are heldnearthe supplyvoltage (+9V).
Whenthe lightfallingonthe LDRs isobstructed,theirresistance becomesveryhighandpin2
voltagesdropto neargroundpotential,therebytriggeringthe flip-flops.Capacitorsacrosspin2 and
groundhave beenaddedtoavoidfalse triggeringdue toelectrical noise.
Whena personentersthe room,LDR1 istriggeredfirstandit resultsintriggeringof monostable IC1.
The short outputpulse immediatelychargesupcapacitorC5, forwardbiasing transistorpairT1-T2.
But at thisinstantthe collectorsof transistorsT1 and T2 are inhighimpedance state asIC2 pin3 isat
lowpotential anddiode D4isnot conducting.
But whenthe same personpassesLDR2, IC2 monostable flip-flopistriggered. Itspin3 goeshighand
thispotential iscoupledtotransistorpairT1-T2 viadiode D4. As a resulttransistorpairT1-T2
conductsbecause capacitorC5 retainsthe charge for some time asits discharge time iscontrolledby
resistorR5 (andR7 to an extent).ThusgreenLEDportionof bi-colourLED islitmomentarily.
The same outputisalsocoupledtoIC3 forwhichit acts as a clock.With entryof each personIC3
output(highstate) keepsadvancing.Atthisstage transistorpairT3-T4 cannotconduct because
outputpin3 of IC1 isno longerpositiveasitsoutputpulse durationisquite shortandhence
transistorcollectorsare inhighimpedance state.
Whenpersonsleave the room,LDR2 istriggeredfirst,followedbyLDR1. Since the bottomhalf
portionof circuitis identical totophalf,thistime,withthe departure of eachperson,redportionof
bicolourLED islitmomentarilyandoutputof IC4 advancesinthe same fashionasin case of IC3.
The outputsof IC3 and those of IC4 (afterinversionbyinvertergatesN1throughN4) are ANDedby
ANDgates (A1throughA4) and thenwire ORed(usingdiodesD5throughD8). The neteffectisthat
whenpersonsare entering,the outputof atleastone of the ANDgatesishigh,causingtransistorT5
to conductand energise relayRL1.The bulbconnectedtothe supplyviaN/Ocontact of relayRL1
alsolightsup.
Whenpersonsare leavingthe room,andtill all the personswhoenteredthe roomhave left,the
wiredORoutputcontinuestoremainhigh,i.e.the bulbcontinuestoremains‘on,’until all persons
whoenteredthe roomhave left.
The maximumnumberof personsthatthiscircuitcan handle islimitedtofoursince onreceiptof
3. fifthclockpulse the countersare reset.The capacityof the circuitcan be easilyextendedtohandle
up to nine personsbyremovingthe connectionof pin1 fromresetpin(15) and utilisingQ1to Q9
outputsof CD4017 counters.Additionalinverters,ANDgateSanddiodeswill,however,be required.
