This document is a physics investigatory project report submitted by Nikhil Dwivedi, a class 12 student. The project studies the variation in current flow in a circuit containing a light dependent resistor (LDR) due to variation in light. The 3-page report includes an introduction to LDRs and how they work, the apparatus used, experimental procedure followed, observations made by varying the light source and interceptor material, and conclusions. The aim was to design a simple intruder alarm circuit using an LDR that triggers a buzzer when light is interrupted.

1. NIKHIL DWIVEIDI
KENDRIYA VIDYAIAYA
xil (sEco*D;llTl
MUZAFFA?RUR
(sEcoND sHtFr)
?HYSISCS INVESTIQ/.rORY PR OJ ECT
Season:- 2A:L6-17
PROJEff: TO STUDY THE VARAfl ON,IN CURRENT
FLOW'Nq, N A CIRCUIT CONTAININ4 AN LPR, BECAUSE OF A
VARIATION.
REPOTED BY: N,KHIL DWtvEDt
CI.,A,SS: aZ (9ECOND SHIFO
REPORTED TO : Mr. VTJAY KUMAR
e.Crr. PHYS,d;S)
PH}SI6 INVESNGATORY PROfi CT
Page 1 of 19

2. NIKHIL DWIVE]DI xil (sEcoND sHtFT)
201.6-17
KflNDenYA VuuYmumYm NdUz&f,f,A&PUB
Physics I nvestigatory Project
{,efitficste
This is to certiSr that NIKHIT DWIYEDI, a student of class
KI-SCIENCE (SECOND SHIFTLof KENDRIYA
VI DYALAYA M UZAFFARPUR has successfu lly completed
the research on the below mentioned project under the
guidance of Mr. VIIAY I(UMAR (P.G.T. PHYSICS).
Teacher's Signature
Page 2 of 1.9
PHYSICS INVESTIGATORY PROJECT

3. NIKHIL DWIVEIDI xil (sEcoND sHtFT)
20]6-L7
t would like to express v^g special thanlcs to trng teacher as well
as our principal who gave vwe the gold.en opportunitg to d.o this
wonderful project in PHYstcs, which also helped vne in d.oing a
lot of Research. I cav^e to know ab;out r^a^g
^ew
things.
rhis proiect couldn't have been sdtisfactorilg cornpleted without
the support and guidance of My parents.
rnaking
I would like to thank v^g parents who hetped vne in gathering
diverse inforvvtation, collecting data and guid.ing me frolv. tivne to
tivne in vnaking this project, despite their busg sched.ule. Theg
gave vne d"ifferemt ideas in this project unique.
PHYSICS INVESTIGATORY PROJECT
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4. 1
2
3
4
5
6
7
I
I
10
11
ffi
Certificate of Excellence
Acknowledgement
Aim of ploject
lntroduction
Theory
Apparatus required
Procedure followed
Obseruation
Conclusions
Precautions
Bibliography
xil (sEcoND sHrFr)
2016-t7
Page 4 of,19
PHYSICS t NVESTIGATORY PROJ ECT

5. NIKHIL DWIVEIDI xil (sEcoND sHtFT)
2016-L7
Phm
mA resistor photoresist or or tight-d.epend.ent resistir(LDR)
is a tight-comtrolled. variable. The resistance of a
photor|sist ot" decreases with incneasimg incident light
intevtsitg; in other words, it exhibits photocomductivitg. A
. photoresist or can be applied. in tight-sensitive detector
circuits, and" tiglnt and d.ark activated. switching circuits.
These resistot"s use pure sevwicond.uctot s like siticon or
gery^a^iuvn. When the light falls on the LDR, then the
electroms get excited bg the incident photons and vnove
frovn the valence band to the conduction band. amd"
therefore'the nuvnber of ch,arge carriers increases. tn other
wonds, the cond.uctivitg goes up.
Distinction need"s to be vwade
'here
between photocells and.
LDRs. ln a photocell, whem it is excited bg light (photons),
electricitg is generated". Unlike photocells, LDRs, d"o not
generate electricitg but onlg change their cond.uctivitg.
page 5 of 19
PHYSICS INVESTIGATORY PROJECT
MG

6. ,-:4
t
NIKHIL DWIVEIDI xil (sEcoND sHrFT)
20L6-17
Alight dependent resistor worlcs on the principle of photo
conductivitg. . Photo comductivitg E a^ electro -optical
phenovvrenon 'in which the vwaterial's comductivitg rs imcreased
when light is absorbed bg the vnaterial. Modern light dependent
resisiors are rnade of vnaterialS such as
selenide, indiurn antivnonite and vvtost
supptied (Cods) o:nd. cadvvriuwt selenide.
lead supptied, lead
cornvwonlg cadvniuvn
w
circuifi aynbol
PHYSICS INVESTIGATORY PROJECT
Page 6 of X.9

7. NIKHIL DWIVEIDI xil (sEcoND sHrFT)
20L6-L7
When light falls i.e. when the photoms fall on the ynaterial, the
electrons in the valence barid of the sevniconductoy. vwaterial are
excite:d to the comduction band.. These photons in the incid.ent
light should have e^ergA greater than the band" gap of the
sevwicomductor vwatenial to .vrnake the electrons juvvrp fror,n the
valence band. to the conduction band. Hence when liqht having
enough ,^rrg1g strikes oh the d"evice, y^ore and.
^i, electrons
are excited to the conduction band which results in large muvvtber
of' charae carriers. The result of this process is v^ore and. v^ore
current stants f{owing through the device when the circuit is
closed and hence it is said. that the resistance of the
device has been decreased. This is the v^ost covvry^o^
working principle of LDR.
This car. be clear[g seen frovu the graph. The resistance of the
LDR falls rapidlg with the increasing intercitg of the incident
light,
The converse is also
off.
true when light intensitg rs reduced or cut
PHySTCS TNVESTTGATORY PROJ ECT
Page 7 of 19

8. xFfiEesfiiq$wl
,., ,mt6-17
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9. NIKHIL DWIVEIDI xil (sEcoND sHtFT)
2076-L7
An euvtptg cardboard box was used to vwivnic a house. An opening
was cut on the box to present a door. Behind. the d"oor., a ligttt
source is placed and in the'line of vision of the light source, a^
LDR is also placed,.
A circuit is connected. to the LDR which switches o^ a relag when
the light beann is intercepted. A schevwatic diagray^ of the set-.up
As seem in the above diagravvr, a light source is positioned behind,
the door on the right-hand sid.e and an LDR is .placed, in the
sarwe line of vision at the opposite end of the door. When the
light bearn falls on the LDR, it lowers the resistance of the sav^e
and this activates the switching transistor circuft. rhe fu.ansistor
PHYSICS INVESTIGATORY PROJ ECT
Page 9 of 19

10. I
I
NIKHIL DWIVEIDI xil (sEcoND sHtFT)
2016-L7
cit"cuit (described later) is connected to a relag which is in turm
comnected to am external buz.er.
The relag is a special tgpe of switch which rs driven bg its
rwagnetic coil. The relag
'has
two positions. ln de-energized
condition, position-l- will be a,ctive and in energized conditiom,
wlnen the circuit is powered, the relag goes to position-1- to
wh,ich the buzze,r is comnected.. Thus, the bur.er wiil start ringing.
But when the tight source is switched" on, the resistance of UOa
falls and this drives the transistor switching circuit and the relag
gets energizbd.. This puts the relag switch in position-z, which
cuts .off the buzzer. tn this iond,itiom, whenever the light beavw is
intercepted bg an opaque object, the LDR stops receiving the
incid.ent light and" its rosistance becotnes high. Thk d"e-energizes
the relag and puts it to position-7. and the buzer starts ringing.
Thus, to suvnvvtarize, we ca^ sag that the if the light source is om
and. the LpR is illuvv,inated., the buzzer will not ring. But the
rnovvrent, the figlr$ source is interrupted, the buzz.er witt
start ringing
we can luvtagine the intercuption of the light beavvt, to be
caused bg an intnuder and hence thls arramgevnent can
autowaticallg detect ang intruder.bg turni^g o^ the buzer.
page 10 of 19
PHYSICS INVESTIGATORY PROJ ECT

11. NIKHIL DWIVEIDI
flte buzz.er is just
re{ag. As such we
x, (sECoND;,[:rr]
devices we have connected to the
v^a^g devices to the relag such as
one of t;he
can connect
sirens, flashing tights or eve^ am autom,atic dialer to the nearest
police station. rhis intruder alarrn can be easilg set up in houses,
bamlcs, schools etc.
The savwe circuit can be rew.ired to reverse the effect of light. Viz.
we can rwake the buzzer ring whemever, light is present and
interestingtg ihis set up ean be used as a rvtormimg alarrn.
Now, let u,s understand how the switching takes place when the
LDR is illuvvtinated" bg light. For the autornatic switchina, wo have
used the following circuit consisting of Transistors. One of the
vvtost covruvruoy. uses for tramsistors im'an electronic circuit is as
sivwple switches. lm short,
"9V
a transistor conducts
current 'across the
collector-evwitter path
onlg when sufficient
voltage is applied. to its
base.
tlv ' When no sufficient base
PHYSTCS ! NVESTIGATORY PROJ ECT
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12. NIKHIL DWIVEIDI xil (sEcoND sHtFT)
2016-L7
voltage is present, the switch is off. When sufficient base voltage
is present, the switch is on.
flte circuit uses two transistors, comnec:ted in series. The base
voltage of the first tramsistor is ad,justed carefullg through the
variable resistor (potentiovvreter) so that the slightest imcrease in
the base voltage can fire the transistor TL. Thus, when there is
no light on the LDR, the transistor rernaims un-fired. But when
Iight falls on the LDR, the LDR starts conducting and the net
base voltage . of transistor TL crosses the threshold. voltage,
causing TL to fire. Therefore, the collector current of ft d.rives
transistor T2 and T2 begins to fire. flte collector current of rz is
^Q,w
large enough to energize thd relag. The relag gets energized"
and changes the contact position frovw 1- to 2. when tight is cut
off, the base voltage of rt fatts back to less than the threshold,
voltage of its base -evvtitter junction and. therefore, the relag
gets d.e-energized".
As stated. earlier, the relag is conmected to a buzz.er through a
batterg. so, whenever, light is interrupted"., the relag goes to
position-L whero, the buzz.er is conmected. and. the buzz.er starts
nvlgt^9.
fue ringing buzzer wards off the intrud.er and, alevts the
neighbors about the intrusion. tn addition to the buzz.er, o^e can
PHYSICS INVESTIGATORY PROJ ECT
Page 12 of 19

13. NIKHIL DWVEIDI xil (sEcoND sHtFT)
20L6-L7
also conmect a high intensitg flash light to warm the people in the
neighbo rho o i7 about the intrusion.
One has also to install a stealth switch that is
owner so that the owmer ca^ d.isable it for his own entrg.
ln'the set up described, the following obseruatioms were vnade bg
(a) varging the nqture.of tight source, (b) varging the
transluce^cA of the interceptor object.
knowm onlg to the
Tab[e of obse
S, NO TgPE
Source
of Light Material of
lnterceptor
Result
L White tight Opaque Buzzer Rings
Transparent Buzzer does
not ring.
2 Near ultra-
violet
Opaque Buzzer Rings
Transparent Buzzer does
not ring.
3 Low intensitg
red laser
Opaque Buzzer Rings
Transparemt Buzzer does
not ring.
Extension of scope in future
PHYSTCS I NVESTIGATORY PROJ ECT
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14. -
NIKHIL DWIVEIDI xil (sEcoND sHtFT)
2016-L7
The d'evice was also tested under invisible light using a -tY revnote
control as an infrared light .source. The sarne obsentations were
vwade. as above. rhis extends the scope of the project to the
invisible lig4t sources such as ultraviolet and infrared. The
advantage is that; we can d,upe the intruder bg using invisible
light as the light rags will not be visible eve^ in the night.
Another interesting application that ca^ be thought of is a^
autovvratic 'people countet' that car- be used to y^easure the
nuvnber of people traversing a certain passage or entrance in an
auditoriuv^ or theatre. lt can also be used as a 'traffic countet' in
eheck. posts and toll gates. For this, we v^ust sivwplg replace the
buzz.er with a digital counter. A tgpicald.igital counter is shown
in the accot/^panging picture.
PHYSICS I NVESTIGATORY PROJ ECT
Page 14 of 19

15. NIKHIL DWIVEIDI xil (sECoND SHTFT)
20L6-L7
VAR.IATION
'N
LDR
A photoelectric device can be either intrinsic or extrinsic. An
intrinsic semiconductor has its own charge carriers and is not an
efficient semiconductor, for example, silicon. ln intrinsic devices
the only available electrons' are in the valence band, and hence
the photon must . have enough energy to excite the electron
across the entire bandqap. Extrinsic devices have impurities, atso
called dopants, added whose ground state energy is closer to the
conduction band; since the electrons do not have as far to jump,
Iower energy photons (that is, Ionger wavelengths and lower
frequencies) are sufficient to trigger the device. lf a sample of
silicon has some of its atoms replaced by phosphorus atoms (im
Photoresistors are less light-sensitive devices
than photodiodes or phototransistois: the two latter components
are true semiconductor devices, while a photoresistor is a passive
component and does not have a PN-junction. The photoresistivity
of any photoresistor may vary widely depending on ambient
temperature, making them unsuitable for applications requiring
precise measurement of or sensitivity to light photons.purities),
there will be extra electrons available for conduction.
PHYSICS I NVESTIGATORY PROJ ECT
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16. CONCLUSION
Based on the obsewation above, it eam be cometuded that
the LDR based electrqnic switch cam be successfullg
t t. ..
deploged for the fotlowing applicatioms:
L. {ntruder deteetion / burgtar's' alarvq for horore uses.
2. Avti*theft alarvvr for Banlc and offices.
S.People Counter at Aud,itoriuvns
4.Vehicle eounter at tr*ffie check posts.
NIKHIT DWIVEIDI
PHYSICS INVESTIGATORY PROJECf
xil (sEcoND sHrF0
20L6-17
Page 16 of 19

17. NIKHILDWIVEIDI
PHVsrcs rNvEsnGAToRY PRoJ Ecr
p&f;fieuTT$r[$
The project uses certain devices that can be harvvtful to huvvtans
and" anivnals if proper ca,utiom amd, care are not obsewed..
,t. ttser diode: Although we have used a low intensitg laser,
this can be harvvtfu( if aivned accidentlg at the eges or if the
wear protective sun-glasses while assevnbling / positioning
the tight source.
2. Ultravio,let LED: UV LED with wavelengths betwe'en 3Ls
and 4oo
^vA,
which evvrits the least energetic. and harynful
tgpe of uv tight. However, prolonged. exposure should be
avoided. which can d,avnaae the skin and, eges.
3. Buzzer: This device evwits high frequencg aud,io sounds that
oam davvrage the ears when put close to the ears or heard
for pro (.onged, period.
4.'The LDR: should a need arise to destrog the project; care
should
'be
taken to dispose of the LDR properlg as it
xil (sEcoND sHtFT)
20L6-L7
Page 17 of 19

18. NIKHIL DWIVEIDI
co'ntai^s . hat'vwful
enviromrnemt
chev,nicals that ca^
xil (sEcoND sHrFT)
20L6-77
davwage the
ffiWffiW WffiffiffiWffiWffiffiWffiW&Mtr rye.$ffiffiWMffi*ffi.ffi #k& &
1. www.google.com
2. https ://en.wi ki ped ia.ofq/
3. www.electrical4u.com
PHYSICS INVESTIGATORY PROJ ECT
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httPm
Page 18 of 19

19. NIKHIL DWIVEIDI
PHYSICS INVESTIGATORY PROJ ECT
xil (sEcoND sHrFT)
2016-L7
Page 19 of 19