CONTENTS
1. Objective 1
2. Brief Theory 2-3
3. Circuit Diagram 3
4. Apparatus/ Equipment/ Components Required 3
5. Discussion about above components 4-14
i. The Triac 4-7
a. Triacconstruction 4-5
b. SCR equivalentcircuit of triac 5-6
c. Triacoperation 6-7
d. Triaccharacteristics 7
ii. The Diac 7-8
a. Operation 8
iii. Different Resistor Types 8-10
a. Carbon filmresistors 8
b. Metalfilm resistors 9
c. Reading resistor values from the colored bands 9
d. Single-In-Line (SIL) Resistor network 9
e. Wire wound resistors 10
f. Ceramic(or cement) resistors 10
g. Thermistor resistors 10
h. SMD resistors 10
iv. Capacitor Theory 11-13
a. Different Types of Capacitors 11
b. CeramicCapacitors 11-12
c. ElectrolyticCapacitors 12
d. Tantalum Capacitors 12
e. Polyester Film Capacitors 12
f. Metalizedpolyester Film Capacitors 13
g. SMD Capacitors 13
h. Variable Capacitors 13
v. Multimeters 13
vi. LDR 14
a. How it works 14
b. Uses 14
c. Interfacing with Arduino 14
6. Procedure 15
7. Observations 15
8. Precautions 15
9. Results and discussions 15
10.Conclusion 15

AUTOMATIC STREET LIGHT CONTROL USING LDR
By
AVISHAKE SAHOO
Roll: D127052006 No: 15239
Under the Guidance of
ANUJ KUMAR GHOSH
A Thesis
Submitted in Partial Fulfillment of the Requirements
For the Degree of
DIPLOMA IN ENGINEERING (Electrical)
Department of Electrical Engineering
MALDA POLYTECHNIC, MALDA
Malda – 732101
West Bengal

MALDA POLYTECHNIC, MALDA
MALDA-732101
ACKNOWLEDGEMENT
I take this opportunity to express my sincere thanks to my thesis guide Sir. Anuj Kumar
Ghosh, Department of Electrical Engineering, Malda Polytechnic, Malda for his valuable
advice, guidance, active supervision and constant encouragement without which it would
have been very difficult for me to complete the Thesis.
Date : 18/06/2014
Department of Electrical Engineering (Name: Avishake Sahoo)
Malda Polytechnic, Malda Roll: D127052006 No:15239
Malda, Pin – 732101 (West Bengal).

MALDA POLYTECHNIC, MALDA
MALDA-732101
CERTIFICATE
I hereby forward the Thesis“AUTOMATIC STREET LIGHT CONTROL USING LDR”, submitted by
Avishake Sahoo (Roll: D127052006 No.15239 of 2013-2014) Under my guidance and
supervision in partial fulfilment of the requirements for the Degree of Diploma in
Electrical Engineering .
(Sir. Anuj kumar Ghosh)
Department of Electrical Engineering
Malda Polytechnic, Malda
Malda, Pin-732101

MALDA POLYTECHNIC, MALDA
MALDA-732101
CERTIFICATE OF APPROVAL
We hereby, have approved the Thesis “AUTOMATIC STREET LIGHT CONTROL USING LDR”, prepared by
Avishake Sahoo under the guidance and supervision of sir. Anuj Kumar Ghosh in partial fulfilment of the
requirements for the Degree of Diploma in Electrical Engineering.
Date : 18/06/2014

1.
Objective
1. To understand the construction and working of a light dependent resistor (LDR).
2. To observe the effect of intensity of light falling on the surface of the LDR in a triac triggering
circuit.
3. To verify the working of a light aciviated turn OFF circuit using an LDR , a diac and a triac.
Brief theory
A light dependent resistor (LDR) is a semiconductor device whose resistance changes depending on
the quality of light flux falling on its furace. It is virtually a small photo conductive cell provided
withtwo tinned copper connecting leads. LDRs are generally made in the form of discs of insulating
material having coating of a photo-sensitive material which contain a negligible number of free
electrons when kept in complete darkness. This causes its resistance to be very high. When it absorbs
light, electrons are liberated and the device starts conducting.
Maximum resistance of the device measured in total darkness is in mega ohms whereas the
minimum resistance of the device measured in full intensity of light, say about 1000 lix, varies
between 75 and 200 ohms. The values of maximum and minimum resistance of the device depend
on the surface area of the disc. The rate at which its resistance changes when shifted from
illumination to darkness is called the recovery rate of the device. It is specified in kΩ/s. the value of
maximum resistance of the device also depends on the temperature. It is directly proportional to the
value of temperature. The LDR is mostly used as a switch. Proper heat sinks should be used in the
circuit for protecting the device. LDR is also known as phototransister. It has been observed that
lower the initial conductivity of a semiconductor, higher is its photosensitive poperty. That is why
photoresistor are prepared from lightly doped semiconductors.
Photo resistors are generally prepared either by coating a layer of a powdered photosensitive
material or by depositing a semiconductor film on an insulating base. Silicon is usually preferred for
this purpose. The semiconductor film deposited on an insulating substance constitutes a unit which
is placed in either a metallic or a plastic case whose top surface is covered with glass. The glass cover
of the compact unit allows light rays to reach the semiconductor coated surface and accordingly the
resistance changes. They are usually made in the form of cadmium sulphide discs provided with two
2.

tinned copper connecting leads. Ratting and performance of the device, shown in Fig 11.1 are
characterised by the value of current flowing through the device at a given voltage and the amount
of light flux. The rating and control capabilities of the device will depend upon the amount of
semiconductor film deposited on the insulating substance. More amount more will be the control
capabilities. In other words, the cross-sectional area of the surface to be exposed to the light is one of
the factors related to its ratting. An LDR is specified according to the diameter of its surface. Because
of very large difference in the value of its resistance between the (i) illumination state and (ii) dark
state, the LDR behaves like a switch. In the presence of light it poses a very little (almost negligible)
resistance in the circuit giving ON state, whereas in darkness it poses a very high resistance which
causes almost on no flow of current thus resulting in the OFF state of the switch. Hence the device
acts like an automatic switch whose ON and OFF states are dependent on the total illumination and
dark conditions respectively.
Fig 11.1 Contraction of an LDR.
Light dependent resistors have wide range of applications. Automatic street lighting, OFF at dark
circuit, burglar alarm etc. are some of its important applications.
In this experiment the LDR has been used as a switch for automatic triggering of the triac through a
diac. When light impinges on the LDR surface, resistance of the device becomes low allowing
current flow through it. This prevents the voltage across the 0.1µF, 400V capacitor from increasing
up to the level breakover voltage of the diac, thus, the diac is unable to conduct. This in turn keeps
the triac in OFF position and therefore, the lamp does not glow. When the light source is removed,
the resistance of the LDR becomes very high and practically does not allow any current flow through
it. The voltage across the capacitor now increases up to or than the breakover voltage of the diac,
thus enables the diac to trigger. The diac starts conducting and sends pulses at the gate of the triac.
The triac on the receiving trigger pulses starts conducting and in turn connect the lamp in the
circuit. The lamp immediately stars glowing. The triac continues to conduct till it is dark. As soon as
sufficient amount of light is available the LDR automatically switches OFF the diac as well as the
triac and hence the lamp stops glowing. In other words, the lamp glows only when the surrounding
3.

is dark. It does not glow in the presence of light. Thus it is seen that in this further, by varying the
amount of light falling on the surface of the LDR, the brightness of the lamp can be changed.circuit
the LDR acts as a switch. It triggers the triac in the absence of light and switches it OFF in the
presence of light. The lamp glows only when the tric is conducting.
Circuit diagram :
Apparatus/ Equipment/ Components Required
Serial
No.
Name of the apparatus/equipment/
components
Range/ rating Make Quantity
1. Triac
Diac pair
4A, 400V
BT136 or any other
equivalent
One
2. LDR - One
3. Resistor (i) 1.2K, 1W
(ii) 10K, 10W
One
Two
4. Capacitors 0.1 µF, 400V One
5. Lamp 230V, 40W One
6. Multimeter - Scientific One

4.
Discussions about above components:
The Triac
The major drawback of an SCR is that it can conduct current
in one direction only. Therefore, an SCR
can only control d.c. power or forward biased half-cycles of
a.c. in a load. However, in an a.c.
system, it is often desirable and necessary to exercise control
over both positive and negative halfcycles.
For this purpose, a semiconductor device called triac is used.
A triac is a three-terminal semiconductor switching device
which can control alternating current in a load.
Triac is an abbreviation for triode a.c. switch. ‘Tri’– indicates that the device has three terminals
and ‘ac’ means that the device controls alternating current or can conduct current in either
direction.
The key function of a triac may be understood by referring to the simplified Fig. 21.1. The
**control circuit of triac can be adjusted to pass the desired portions of positive and negative
halfcycle
of a.c. supply through the load RL. Thus referring to Fig. 21.1 (ii), the triac passes the positive
half-cycle of the supply from θ1 to 180° i.e. the shaded portion of positive half-cycle. Similarly, the
shaded portion of negative half-cycle will pass through the load. In this way, the alternating current
and hence a.c. power flowing through the load can be controlled.
Since a triac can control conduction of both positive and negative half-cycles of a.c. supply, it is
sometimes called a bidirectional semi-conductor triode switch. The above action of a triac is
certainly not a rectifying action (as in an *SCR ) so that the triac makes no mention of rectification in
its name.
Triac Construction
A triacis a three-terminal, five-layer semiconductor device whose forward and reverse
characteristics
are indentical to the forward characteristics of the SCR. The three terminals are designated as
main terminal MT1, main terminal MT2 and gate G.

5.
it can conduct current in either direction. This is unlike an SCR which can conduct current only in
one
direction. Fig. 21.2 (iii) shows the schematic symbol of a triac. The symbol consists of two parallel
diodes connected in opposite directions with a single gate lead. It can be seen that even the symbol of
triac indicates that it can conduct current for either polarity of the main terminals (MT1 and MT2)
i.e. it can act as a bidirectional switch. The gate provides control over conduction in either direction.
The following points many be noted about the triac :
(i) The triac can conduct current (of course with proper gate current) regardless of the polarities
of the main terminals MT1 and MT2. Since there is no longer a specific anode or cathode, the
main leads are referred to as MT1 and MT2.
(ii) A triac can be turned on either with a positive or negative voltage at the gate of the device.
(iii) Like the SCR, once the triac is fired into conduction, the gate loses all control. The triac can
be turned off by reducing the circuit current to the value of holding current.
(iv) The main disadvantage of triacs over SCRs is that triacs have considerably lower
currenthandling
capabilities. Most triacs are available in ratings of less than 40A at voltages up to 600V.
SCR Equivalent Circuit of Triac
We shall now see that a
triac is equivalent to two
separate SCRs connected in
inverse parallel (i.e. anode
of each connected to the
cathode of the other) with
gates commoned. Fig. 21.3
(i) shows the basic
structure of a triac. If we
split the basic structure of
a triac into two halves as shown in Fig. 21.3 (ii), it is easy to see that we have two SCRs connected in
inverse parallel. The left half in Fig. 21.3 (ii) consists of a pnpn device (p1n2 p2n4) having three pn
junctions and constitutes SCR1. Similarly, the right half in Fig. 21.3 (ii) consists of pnpn device

6.
(p2n3p1n1) having three pn junctions and constitutes SCR2. The SCR equivalent circuit of the triac
is shown in Fig. 21.4.
Suppose the main terminal MT2 is positive and main terminal
MT1 is negative. If the triac is now fired into conduction by proper
gate current, the triac will conduct current following the path (left
half) shown in Fig. 21.3 (ii). In relation to Fig. 21.4, the SCR1 is
ON and the SCR2 is OFF. Now suppose that MT2 is negative and
MT1 is positive. With proper gate current, the triac will be fired
into conduction. The current through the devices follows the path
(right half) as shown in Fig. 21.3 (ii). In relation to Fig. 21.4, the
SCR2 is ON and the SCR1 is OFF. Note that the triac will conduct
current in the appropriate direction as long as the current through
the device is greater than its holding current.
Triac Operation
Fig. 21.5 shows the simple triac circuit. The a.c. supply to be controlled is connected across the main
terminals of triac through a load resistance RL. The gate circuit
consists of battery, a current limiting resistor R and a switch S.
The
circuit action is as follows :
(i) With switch S open, there will be no gate current and the
triac is cut off. Even with no gate current, the triac can be
turned on
provided the supply voltage becomes equal to the breakover
voltage
of triac. However, the normal way to turn on a triac is by
introducing
a proper gate current.
(ii) When switch S is closed, the gate current starts flowing in
the gate circuit. In a similar manner to SCR, the breakover
voltage of the triac can be varied by making proper gate current to flow. With a few milliamperes
introduced at the gate, the triac will start conducting whether terminal MT2 is positive or negative
w.r.t. MT1.
(iii) If terminal MT2 is positive w.r.t. MT1, the triac turns on and the conventional current will
flow from MT2 to MT1. If the terminal MT2 is negative w.r.t. MT1, the triac is again turned on but
this
time the conventional current flows from MT1 to MT2.
The above action of triac reveals that it can act as an a.c. contactor to switch on or off alternating

7.
current to a load. The additional advantage of triac is that by adjusting the gate current to a proper
value, any portion of both positive and negative half-cycles of a.c. supply can be made to flow
through the load. This permits to adjust the transfer of a.c. power from the source to the load.
Traic Characteristics
Fig. 21.8 shows the V-I characteristics of a triac. Because the triac essentially consists of two
SCRs of opposite orientation fabricated in the same crystal, its operating characteristics in the first
and third quadrants are the same except for the direction of applied voltage and current flow. The
following points may be noted from the triac characteristics :
(i) The V-I characteristics for triac in the Ist and IIIrd quadrants are essentially identical to
those of an SCR in the Ist quadrant.
(ii) The triac can be operated with either positive or negative gate control voltage but in *normal
operation usually the gate voltage is positive in quadrant I and negative in quadrant III.
The Diac
A diac is a two-terminal, three layer bidirectional device which can be switched from its OFF
state to ON state for either polarity of applied voltage.
The diac can be constructed in either npn or pnp form. Fig. 21.17 (i) shows the basic structure of
a diac in pnp form. The two leads are connected to p-regions of silicon separated by an n-region. The
structure of diac is very much similar to that of a transistor. However, there are several imporant
differences:
(i) There is no terminal attached to the base layer.
(ii) The supply voltage at which the triac is turned ON depends upon the gate current. The
greater the gate current, the smaller the supply voltage at which the triac is turned on. This permits to
use a triac to control a.c. power in a load from zero to full power in a smooth and continuous manner

8.
with no loss in the controlling device.
(iii) The three regions are nearly identical in size.
(iv) The doping concentrations are identical (unlike a bipolar transistor) to give the device
symmetrical
properties.
Fig. 21.17 (ii) shows the symbol of a diac.
Operation. When a positive or negative voltage is applied across the terminals of a diac, only a
small leakage current IBO will flow through the device. As the applied voltage is increased, the
leakage current will continue to flow until the voltage reaches the breakover voltage VBO. At this
point, avalanche breakdown of the reverse-biased junction occurs and the device exhibits negative
resistance i.e. current through the device increases with the decreasing values of applied voltage. The
voltage across the device then drops to ‘breakback’ voltage VW.
Fig. 21.18 shows the V-I characteristics of a diac. For applied positive voltage less than + VBO
and negative voltage less than − VBO, a small leakage current (± IBO) flows through the device.
Under
such conditions, the diac blocks the flow of current and effectively behaves as an open circuit. The
voltages + VBO and − VBO are the breakdown voltages and usually have a range of 30 to 50 volts.
When the positive or negative applied voltage is equal to or greater than the breakdown voltage,
diac begins to conduct and the voltage drop across it becomes a few volts. Conduction then
continues until the device current drops below its holding
current. Note that the breakover voltage and holding current
values are identical for the forward and reverse regions of
operation. Diacs are used primarily for triggering of triacs in
adjustable phase control of a.c. mains power. Some of the
circuit applications of diac are
(i) light dimming
(ii) heat control and
(iii) universal motor speed control.
Different Resistor Types:
Carbon film resistors:
The size of the resistor decides its
power rating (i.e., the maximum power
it can dissipate without burning).
Power rating from the top of the graph:
1/8 W
1/4 W
1/2 W

9.
Metal film resistors:
Used when a higher tolerance (more accurate value) is needed.
Power rating from the top of the
graph:
1/8 W (tolerance ±1%)
1/4 W (tolerance ±1%)
1 W (tolerance ±5%)
2 W (tolerance ±5%)
Reading resistor values from the colored bands:
Single-In-Line (SIL) Resistornetwork:
Variable Resistors:

10.
Wirewound resistors:
Ceramic (or cement) resistor:
Thermistor (thermally sensitive resistor):
SMD resistors (Surface-Mount Device):
11.

Capacitor Theory
Any arrangement of two conductors separated by an
electric insulator (i.e., dielectric) is a capacitor. An electric
charge deposited on one of the conductors induces an
equal charge of opposite polarity on the other conductor.
As a result, an electric field exists between the two
conductor surfaces and there is a potential difference
between them. The electric field anywhere between the conductor surfaces is directly proportional to
the magnitude of the charge Q on the conductors. And the potential difference V is also directly
proportional to the charge Q. The ratio Q/V is thus a constant for any electric field distribution as
determined by the shape of the conductors, the distance of separation, and the dielectric in which the
field exists. The ratio Q/V is called the capacitance, C, of a particular arrangement of conductors and
dielectric. Thus, C = Q/V, where Q and V are in units of coulomb and volt. C has the units farad (F).
The simple theoretical expression for the capacitance value of a parallel plate capacitor is where
A = plate area [m2] = cross section of electric field,
d = distance between plates [m],
€o = permittivity of free space = 8.854 x 10-12 F/m and
€r = relative permittivity of the dielectric between the plates [dimension less].
This calculated value is based on the assumption that the charge density on the plates is uniformly
distributed. In practice there is always a concentration of charge along the edges. This charge
concentration is at the sharp corners of the plates. Thus for a given voltage, the actual total
charge is always greater than the theoretical total charge.
Different Capacitor Types:
Ceramic Capacitors:
Limited to quite small values, but have high voltage ratings. They range from 1pF to 0.47μF and are
not polarized.
12.

Example: 102 means 10 (and two zeroes) 00 or 1,000 pF or .001uF.
Electrolytic Capacitors (Electrochemical type capacitors): Used for
all values Above 0.1μF. Electrolytics have lower accuracy and
temperature
stability than most Other types and are almost always polarised. It's
usually
best to only use an electrolytic when no other type can be used, or
for
all values over 100μF.
Tantalum Capacitors: Tantalum capacitors
pack a large capacity into a relatively small and
tough package compared to electrolytics, but
have
much smaller voltage ratings. They are often
polarized and range from 0.1μF to 100μF.
PolyesterFilmCapacitors (Green
Caps): Ranging from 0.01μF to 5μF.
They are
similar to ceramics with some larger
values and a slightly larger
construction. They are not polarized.
13.

Metallized Polyester Film Capacitors:
SMD Capacitors:
Variable Capacitors:
Multimeters (Analog and
Digital):

14.
Light Dependant Resistor
A light dependant resistor also known as a LDR,
photoresistor, photoconductor or photocell, is a
resistor whose resistance increases or decreases
depending on the amount of light intensity. LDRs
(Light Dependant Resistors) are a very useful tool
in a light/dark circuits. LDRs can have a variety
of resistance and functions. For example it can be
used to turn on a light when the LDR is in
darkness or to turn off a light when the LDR
is in light. It can also work the other way around
so when the LDR is in light it turns on the circuit and when it’s in darkness the resistance increase
and disrupts the circuit.
How it Works
The way an LDR works is that they are made of many semi-conductive
materials with high resistance. The reason they have a high resistance is
that are very few electrons that are free and able to move because they are
held in a crystal lattice and are unable to move. When light falls on the
semi conductive material it absorbs the light photons and the energy is
transferred to the electrons, which allow them to break free from the
crystal lattice and conduct electricity and lower the resistance of the LDR.
Uses
Light dependant resistors have many uses, many of the uses have to do with objects that have to
work in certain levels of light. Some of the uses of the LDR are in photographic light meters,
streetlights and various alarms’ light burglar alarms, _re alarms and smoke alarms.
Interfacing with Arduino
A light sensor or LDR can be very easily
interfaced with an Arduino. The light sensor is
connected to the analogue inputs of the
Arduino. One of the pins of the LDR is
connected to the ground while the other is
connected to one of the 5 analogue in pins.
Depending on the function of the LDR it may
need another resistor connected to it. Say for
instance if the LDR is controlling a LED, if it
allows to much current to get through it might
cause the LED to blow up. This is the reason for
another resister being needed. And of course the Arduino will need to be programmed in order for
there to be an output from the input of the LDR.

15.
Procedure
1. Connect the circuit as shown in fig 11.2
2. Switch ON the supply.
3. Observe the glow of the lamp.
4. Cover the LDR with a piece of paper or cloth and observe whether the lamp glows or not.
5. Observe the change in intensity of illumination of the lamp by varying the amount of light
falling on the LDR surface.
Observations
1. Observe the intensity of illumination of the lamp for two conditions.
(i) When sufficient amount of light falls on the LDR surface.
(ii) When no light falls on the LDR surface.
2. Observe the flickering of the lamp by varying frequency the amount of light falling on the LDR
surface.
Precautions
1. A diac – triac matched pair should be used for fabricating the circuit.
2. The three terminals of the diac, viz G, MT1, MT2 should be checked before connecting the
device in the circuit.
3. The LDR should not be touched bare-handed while the supply is ON.
Results and discussions
1. Study the variation of resistance of an LDR with the variation in the light flux.
2. Study and observe the function of an LDR as an automatic switch for controlling
the glow of the lamp.
Conclusion
“AUTOMATICSTREET LIGHT CONTROL USING LDR” is one of the important thinkingin
Our modern day power system. Now a days we aregoing to trend to minimise the
Power consumption by using different modern electronics circuit whichhas low power
consumption and high efficiency. Fromthis inspiration wemade this kind of project.
This project is very usefulto minimise theunnecessarypower losses day by day in street
lighting Purpose. It also has a smooth operation and high efficiency. Themanpower
needed for this purposeis very less as it is operated automatically.