This document is a lab project report submitted by three students on an automatic headlight dimmer circuit. It includes an introduction describing the need for automatic headlight dimmers in vehicles. It then describes the circuit components used, including an NE555 timer IC, resistors, capacitors, an LDR light sensor, diode, potentiometer, relay and battery. The components are described in detail. The circuit diagram and working are also explained, indicating how the LDR sensor detects ambient light levels and the timer IC controls the relay to switch the headlights on and off automatically. Applications of the circuit and conclusions from the project are also mentioned.

1. AUTOMATIC HEAD LIGHT DIMMERS
A Lab based project report Submitted in partial fulfilment of the requirements
for the award of degree of
BACHOLER OF TECHNOLOGY
In
Electrical & Electronics Engineering (EEE)
By
S.NO NAME OF THE STUDENT BRNCH SEC.NO ROLL.NO ID.NO
1 N.TEJASWANI EEE A 25 12006119
2 CH.NARESH KUMAR EEE A 26 12006124
3 G.CHIRANJEEVI EEE A 27 12006144
Under the guidance of
Mr. M. SUNEEL
Department of Electrical & Electronics Engineering
K.L.UNIVERSITY, Vaddeswaram, Guntur Dist.
2013-2014

2. K L UNIVERSITY
DEPARTMENT OF eee
CERTIFICATE
This is certified that the work contained in the thesis titled “AUTOMATIC
HEAD LIGHT DIMMERS” by N.Tejaswani (12006119), CH.Nareshkumar
(12006124),G.Chiranjeevi (12006144), in the partial fulfilment for the lab based project, is a
bonafide work carried out by them under my supervision.
SIGNATURE OF THE GUIDE SIGNATURE OF THE HOD
Mr .M. SUNEEL Dr. M. VENUGOPAL RAO
(Asst.Professor) (Professor)

3. ACKNOWLEDGMENT
I have taken efforts in this project. However, it would not have been possible without
possible without the kind, support and help of Mr. M. SUNEEL. I would like to extend my
sincere thanks to all of them.
I am highly indebted to Mr. SUPARSHYA BABU, Mr. M. SUNEEL for their guidance and
constant supervision as well as for providing necessary information regarding the project &
also for his support in completing the project. I would like to express my special gratitude
towards my parents for their kind co-operation and encouragement which help me in
completion of this project.
My thanks and appreciations also go to my friends in developing the project and people who
have willingly helped me out with their abilities.
N. TEJASWANI (12006119)
CH.NARESH KUMAR (12006124)
G. CHIRANJEEVI (12006144)

4. CONTENTS
1. ABSTRACT
2. INTRODUCTION
3. CIRCUIT COMPONENTS
4. COMPONENTS DESCRIPTION
5. CIRCUIT DIAGRAM
6. CIRCUIT WORKING
7. APPLICATIONS
8. CONCLUSION
9. REFERENCES

5. Abstract
Automatic headlamps are the latest convenience in today's cars. These eliminate the need for
the driver to manually switch on or switch off the headlamps in most driving situations. The
automatic headlight system reacts like the human eye to outside light levels and
independently turns the lights on and off when needed. Such a system offers both safety and
convenience. The car‟s headlights will automatically switch on after sensing the poorly lit
tunnel. When the car comes out of the tunnel, the headlights will switch off.
The LDR1 is connected to the trigger input (pin 2) of IC1.The output of IC1 is connected to
the base of relay-driver transistor T1. The 12V supply voltage is connected to the circuit
through switch S1. LDR1 and the 100-kilo-ohm preset constitute a voltage divider
arrangement at pin 2 of IC1. When there is sufficient ambient light, the resistance of LDR1
remains low (a few hundred ohms). The voltage at pin 2 is greater than two-third of 12V. The
output at pin 3 of IC1 remains low stable state for monostable mode of operation and the
headlights of the vehicle connected to the normally-open (N/O) contacts of relay RL1 remain
off. When the ambient light decreases, the resistance of LDR1 shoots up to a few mega-ohms
and the voltage at the trigger input (pin 2) of IC1 decreases to less than one-third of 12V. The
output at pin 3 of IC1 goes high to energise relay RL1 and turn the headlights „on‟. Switch S2
can be used to manually operate the headlights.

6. INTRODUCTION
The requirement of headlight is very common during night travel. The same headlight which
assists the driver for better vision during night travel is also responsible for many accidents
that are being caused. The driver has the control of the headlight which can be switched from
high beam (bright) to low beam (dim). The headlight has to be adjusted according to the light
requirement by the driver. During pitch black conditions where there are no other sources of
light, high beam is used to. On all other cases, low beam is preferred. But in a two-way
traffic, there are vehicles plying on both sides of the road. So when the bright light from the
headlight of a vehicle coming from the opposite direction falls on a person, it glares him for a
certain amount of time. This causes disorientation to that driver. This discomfort will result in
involuntary closing of the driver‟s eyes momentarily. This fraction of distraction is the prime
cause of many road accidents. The prototype that is has been designed, reduces this problem
by actually dimming down the bright headlight of our vehicle to low beam automatically
when it senses a vehicle at close proximity approaching from the other direction. The entire
working of the dimmer is a simple electronic circuitry arrangement which senses and
switches the headlight according to the conditions required.
The number of vehicles on our roads is burgeoning day by day. This is turn forced almost all
this vehicle manufactures to think about the extra safety instruments and electronic controls
to attach with these products for giving the users a safety derived in all road conditions
through a mass flow traffic. If asked, one should always mention that the right driving is very
cumbersome due to the dazzling light problems and the frequent dipping of headlights by
manual means that often causes fatigue to the driver particularly at the time of peak traffic.
So naturally to get rid of this perennial problem, an automatic mechanism has to come up to
dip the headlamp automatically whenever required. For keeping a motor vehicle under
perfect control and reins of the driver, different types of controls and accessories are provided
in an automobile around the driver„¢s seat, on the dashboard and at the footboard. There are
controls like clutch, brake pedal, accelerator pedal, and sharing the same importance, the
dimmer switch is changed with time, and its pace in the field of automobile safety is one of
the uppermost. Simply, an automatic dipper is a unit, which can automatically judge when the

7. headlight beam needs to be lowered, and which dip the headlamp from which beam to a
dipped beam. As the dipper unit is well connected to the lighting system of the vehicle, we
have to look short into the type and construction of a head light before discussing the wiring
diagram or the construction of Automatic dippers.
HEADLAMPS
The modern lighting system consists of switches, lamps, wiring harness, and fuses or circuit
breakers. It may be mentioned that the primary purpose of the headlight design is to produce
illumination over considerable distance ahead of the vehicle and enable the driver to drive at
reasonable speeds at night with safety. But the provision should also be made that the drivers
of other vehicles coming from the opposite direction to not experience a glare. For this
purpose a dipped or meeting beam is also provided for maintaining the reasonable speed with
safety without dazzling the coming driver. To prevent dazzle to the oncoming driver during
particularly misty or hazy conditions the light about the horizontal should be cut off. This is
called dipping of the head light beam. In an average car, the lighting system consumes about
70 to75% of electrical energy when driven at night. In terms of amperage the consumption
may be from 24 to40 A at night for al purposes including the radio, heater, and transmission
controls. Light Source There are two kinds of light sources, namely, the one that emits light
and the other that reflects light. In the case of headlamp used in automobiles, both the things
are combines in A. The filament of the electric lamp is the primary source, while the reflector
is referred to as the secondary source. The intensity, colour and distribution are the important
characteristics of any light source. The headlight is composed of three elements:
1. The light filament that gives off light when a current flows through it.
2. The parabolic reflector that reflect the light in front: and
3. The lens that refracts of distorts the light beam into an illuminating pattern.
Construction
The present day headlights are the outcome of a lot of research and development. Earlier a
single electric bulb of the carbon filament type was employed. The bulb was placed at the
focus of a parabolic silvered reflector in order to give a parallel beam of light. The following
figure shows a parabolic reflector with bulb, the lines showing light rays emitted from the
filament of the bulb in all directions. This type of headlight given a parallel light beam, that
saves greater illumination nearer the axis. It may be mentioned that the bulb itself and the
intensity of light fall off towards the outer portion of the beam block a small amount of light.
From figure it may be seen that if the bulb filament is moved from position, the focus of

8. reflector, to a position, the light beam will no longer be a parallel one but will become
divergent. On the contrary, if the filament is moved to position the beam will take the course
a shown by and will meet at a point on the axis of the bulb. It may be mentioned that by a
single adjustment of the bulb, the beam cane be concentrated at a ore-determined distance
ahead of the vehicle in order to give a spot light effect. Formerly the headlight was provided
with certain means of adjusting the bulb holder with respect to the reflector along with the
bulb axis in order to focus it. It had to be done up every time the bulb was changed. This was
essential, otherwise it would cause increased dazzle to other motorists. The filament is
encased in an airtight bulb in order to prevent burning up of the white-hot filament because of
oxygen in the air. The reflector is generally of polished metal and it throws all the light rays
into a cylindrical beam. The lens is made up of a number of glass prisms moulded together
and they bend the beam of light into an oval pattern which is aimed ahead of the vehicle and
somewhat in the downward direction. A part of the light is spread out in front of the vehicle
for providing local illumination, whereas the rest of it is focused into a hot spot that provides
distant illumination. The first major advancement in headlight design took place with the
introduction of pre-focused bulbs. It has two filaments, one for normal driving and the other
for city driving or for overtaking. These days even two sets of headlights are used for the
above said purposes. Generally a foot selector switch is provided, which enables the driver to
select either the normal driving or the passing beam. The parabolic reflector showing light
rays emitted from the filament of the bulb The effect of changing the position of filament on
the nature of the light beam Sealed-Beam Headlights In a sealed beam headlight, the filament
and the reflector along with the lens are sealed in an airtight unit. The front face of the sealed
beam unit is a lens, which is fused to the reflector after the two filament units, has been
inserted through the center of the reflector and sealed in position. The complete unit is then
evaluated and filled with inert gas. With these units the only service required is to aim them.
This type of headlight introduced in 1940 in the U.S.A. Sealed beam headlights has the
followingadvantages
1. The glass unit is self-contained with accurately focused filaments.
2. Dust, moisture, etc. are prevented from entering from the back of the lens and the
reflector.
3. A greater amount of light is provided in the beam because, of the absence of a filament
bulb
4. The beam of light obtained is greatly improved due to the pre-focused filament and
permanentlybrightreflector.

9. It is essential to have a provision for horizontal and vertical adjustment of the beam by titling
the sealed beam unit in its body housing. Two adjusting screws, as shown in the following
figure, generally do it. Fig. shows an improved version of a selected beam unit with the
provision of a metal mask or shield in front of the upper filament to prevent stray light rays
from escaping upwards and reflecting back to the eyes of the driver. This type of headlights
gives a brighter beam pattern and further reduces the repair and maintenance time. Such type
of units are, however, more expensive or replace than plain or pre-focused type of units.
Recently headlights of 50-60 W for the lower filament and 40-45 for the upper filament have
been used, thus increasing their light output. Control of Headlight Beam The circular beam
can be spread horizontally to any desired extent with the help of prisms, molded in the inside
of the headlight cover glass, as shown in the figure. The horizontal light intensity can be
controlled in any desired way by a suitable design of the headlight. The circular beam can
also be controlled in the vertical direction with the help of prisms molded in front of the
glass, thus redirecting the light rays in the downward direction. This way the downward
reflected beam does not obstruct the vision of the other road users coming from the opposite
side. With these two combinations, any required main beam illumination, combined with side
illumination nearer to the vehicle can be produced.

10. CIRCUIT COMPONENTS
1) IC NE 555
2) Resistors- 4.7k (2)
3) Capacitors- 10 micro Farads , 0.01 micro Farads
4) LDR
5) IN 4007 diode
6) Variable pot—100K
7) Relay -12 v
8) Battery-12v
9) Bulb-12v
DESCRIPTION OF COMPONENTS
555 Timer
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation,
and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and
as a flip-flop element. Derivatives provide up to four timing circuits in one package.
Introduced in 1971 by Signetics, the 555 is still in widespread use due to its ease of use, low
price, and stability. It is now made by many companies in the original bipolar and also in

11. low-power CMOS types. As of 2003, it was estimated that 1 billion units are manufactured
every year.
The IC was designed in 1971 by Hans Camenzind under contract to Signetics, which was
later acquired by Philips (now NXP).
Depending on the manufacturer, the standard 555 package includes 25 transistors,
2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package
(DIP-8) Variants available include the 556 (a 14-pin DIP combining two 555s on one chip),
and the two 558 & 559s (both a 16-pin DIP combining four slightly modified 555s with DIS
& THR connected internally, and TR is falling edge sensitive instead of level sensitive).
The NE555 parts were commercial temperature range, 0 °C to +70 °C, and the SE555 part
number designated the military temperature range, −55 °C to +125 °C. These were available
in both high-reliability metal can (T package) and inexpensive epoxy plastic (V package)
packages. Thus the full part numbers were NE555V, NE555T, SE555V, and SE555T. It has
been hypothesized that the 555 got its name from the three 5 kΩ resistors used within,]
but
Hans Camenzind has stated that the number was arbitrary.
Low-power versions of the 555 are also available, such as the 7555 and CMOS TLC555. The
7555 is designed to cause less supply noise than the classic 555 and the manufacturer claims
that it usually does not require a "control" capacitor and in many cases does not require
a decoupling capacitor on the power supply. Those parts should generally be included,
however, because noise produced by the timer or variation in power supply voltage might
interfere with other parts of a circuit or influence its threshold voltages.
The connection of the pins for a DIP package is as follows:
Pin Name Purpose
1 GND Ground reference voltage, low level (0 V)
2 TRIG
The OUT pin goes high and a timing interval starts when this input falls below
1/2 of CTRL voltage (which is typically 1/3 ofVCC, when CTRL is open).
3 OUT This output is driven to approximately 1.7 V below +VCC or GND.

12. 4 RESET
A timing interval may be reset by driving this input to GND, but the timing does
not begin again until RESET rises above approximately 0.7 volts. Overrides
TRIG which overrides THR.
5 CTRL Provides "control" access to the internal voltage divider (by default, 2/3 VCC).
6 THR
The timing (OUT high) interval ends when the voltage at THR is greater than
that at CTRL (2/3 VCC if CTRL is open).
7 DIS
Open collector output which may discharge a capacitor between intervals. In
phase with output.
8 VCC
Positive supply voltage, which is usually between 3 and 15 V depending on the
variation.
Pin 5 is also sometimes called the CONTROL VOLTAGE pin. By applying a voltage to the
CONTROL VOLTAGE input one can alter the timing characteristics of the device. In most
applications, the CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF
capacitor between pin 5 and 0 V to prevent interference. The CONTROL VOLTAGE input
can be used to build an astable with a frequency modulated output.
Modes
The 555 has three operating modes:
Monostable mode: In this mode, the 555 functions as a "one-shot" pulse generator.
Applications include timers, missing pulse detection, bouncefree switches, touch
switches, frequency divider, capacitance measurement, pulse-width modulation (PWM)
and so on.
Astable (free-running) mode: The 555 can operate as an oscillator. Uses
include LED and lamp flashers, pulse generation, logic clocks, tone generation, security
alarms,pulse position modulation and so on. The 555 can be used as a simple ADC,
converting an analog value to a pulse length. E.g. selecting a thermistor as timing resistor
allows the use of the 555 in a temperature sensor: the period of the output pulse is

13. determined by the temperature. The use of a microprocessor based circuit can then
convert the pulse period to temperature, linearize it and even provide calibration means.
Bistable mode or Schmitt trigger: The 555 can operate as a flip-flop, if the DIS pin is not
connected and no capacitor is used. Uses include bounce-free latched switches.
Specifications
These specifications apply to the NE555. Other 555 timers can have different specifications
depending on the grade (military, medical, etc.).
Light dependent resistor (LDR)
Supply voltage (VCC) 4.5 to 15 V
Supply current (VCC = +5 V) 3 to 6 mA
Supply current (VCC = +15 V) 10 to 15 mA
Output current (maximum) 200 mA
Maximum Power dissipation 600 mW
Power consumption (minimum operating) 30 mW@5V, 225 mW@15V
Operating temperature 0 to 70 °C

14. A photo resistor or light-dependent resistor (LDR) or photocell is a light-controlled
variable resistor. The resistance of a photo resistor decreases with increasing incident light
intensity; in other words, it exhibits photoconductivity. A photo resistor can be applied in
light-sensitive detector circuits, and light- and dark-activated switching circuits.
A photo resistor is made of a high resistance semiconductor. In the dark, a photo resistor can
have a resistance as high as a few mega ohms (MΩ), while in the light, a photo resistor can
have a resistance as low as a few hundred ohms. If incident light on a photo resistor exceeds a
certain frequency, photons absorbed by the semiconductor give bound electrons enough
energy to jump into the conduction band. The resulting free electrons (and
their whole partners) conduct electricity, thereby lowering resistance. The resistance range
and sensitivity of a photo resistor can substantially differ among dissimilar devices.
Moreover, unique photo resistors may react substantially differently to photons within certain
wavelength bands.
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. In 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 band gap. Extrinsic devices have
impurities, also called dopants, added whose ground state energy is closer to the conduction
band; since the electrons do not have as far to jump, lower energy photons (that is, longer has
some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons
available for conduction. This is an example of an extrinsic semiconductor.
IN 4007 diode
The 1N4001 series (or 1N4000 series) is a family of popular 1.0 A (ampere) general
purpose silicon rectifier diodes commonly used in AC adapters for common household

15. appliances. Blocking voltage varies from 50 to 1000 volts. This diode is made in an axial-
lead DO-41plastic package.
The 1N5400 series is a similarly popular series for higher current applications, up to 3 A.
These diodes come in the larger DO-201 axial package.
These are fairly low-speed rectifier diodes, being inefficient for square waves of more than
15 kHz. The series was second sourced by many manufacturers. The 1N4000 series were in
the Motorola Silicon Rectifier Handbook in 1966, as replacements for 1N2609 through
1N2617. The 1N5400 series were announced in Electrical Design News in 1968, along with
the now lesser known 1.5 A 1N5391 series.
These devices are widely used and recommended.
The table below shows the maximum repetitive reverse blocking voltages of each of the
members of the 1N4000 and 1N5400 series.
CAPACITORS
A capacitor (originally known as a condenser) is a passive two-terminal electrical
component used to store energy electro statically in an electric field.
The forms of practical capacitors vary widely, but all contain at least two electrical
conductors (plates) separated by a dielectric (i.e., insulator). The conductors can be thin films
of metal, aluminium foil or disks, etc. The 'non-conducting' dielectric acts to increase the
capacitor's charge capacity. A dielectric can be glass, ceramic, plastic film, air, paper, mica,
etc. Capacitors are widely used as parts of electrical circuits in many common electrical
devices. Unlike resistor, a capacitor does not dissipate energy. Instead, a capacitor
stores energy in the form of an electrostatic field between its plates.

16. When there is a potential difference across the conductors (e.g., when a capacitor is attached
across a battery), an electric field develops across the dielectric, causing positive charge (+Q)
to collect on one plate and negative charge (-Q) to collect on the other plate. If a battery has
been attached to a capacitor for a sufficient amount of time, no current can flow through the
capacitor. However, if an accelerating or alternating voltage is applied across the leads of the
capacitor, a displacement current can flow.
An ideal capacitor is characterized by a single constant value for its capacitance. Capacitance
is expressed as the ratio of the electric (Q) on each conductor to the potential difference (V)
between them. The SI unit of capacitance is the farad (F), which is equal to
one coulomb per volt (1 C/V). Typical capacitance values range from about 1 pF (10−12
F) to
about 1 mF (10−3
F).
The capacitance is greater when there is a narrower separation between conductors and when
the conductors have a larger surface area. In practice, the dielectric between the plates passes
a small amount of leakage current and also has an electric field strength limit, known as
the breakdown voltage. The conductors and leads introduce an undesired inductance and
resistance.
Capacitors are widely used in electronic circuits for blocking direct current while
allowing alternating current to pass. In filter networks, they smooth the output of power
supplies. In resonant circuits they tune radios to particular frequencies. In electric power
transmission systems they stabilize voltage and power flow.
RESISTORS

17. A resistor is a passive two-terminal electrical component that implements electrical
resistance as a circuit element. Resistors act to reduce current flow, and, at the same time, act
to lower voltage levels within circuits. Resistors may have fixed resistances or variable
resistances, such as those found.
in thermistors, varistors, trimmers, photoresistors and potentiometers.
The current through a resistor is in direct proportion to the voltage across the resistor's
terminals. This relationship is represented by Ohm‟s:
where I is the current through the conductor in units of amperes, V is the potential difference
measured across the conductor in units of volts, and R is the resistance of the conductor in
units of ohms (symbol: Ω).
The ratio of the voltage applied across a resistor's terminals to the intensity of current in the
circuit is called its resistance, and this can be assumed to be a constant (independent of the
voltage) for ordinary resistors working within their ratings.
Resistors are common elements of electrical networks and electronic circuits and are
ubiquitous in electronic equipment. Practical resistors can be composed of various
compounds and films, as well as resistance wires (wire made of a high-resistivity alloy, such
as nickel-chrome). Resistors are also implemented within integrated circuits, particularly
analog devices, and can also be integrated into hybrid and printed circuits.
The electrical functionality of a resistor is specified by its resistance: common commercial
resistors are manufactured over a range of more than nine orders of magnitude. When
specifying that resistance in an electronic design, the required precision of the resistance may
require attention to the manufacturing tolerance of the chosen resistor, according to its
specific application. The temperature coefficient of the resistance may also be of concern in
some precision applications. Practical resistors are also specified as having a
maximum power rating which must exceed the anticipated power dissipation of that resistor
in a particular circuit: this is mainly of concern in power electronics applications. Resistors
with higher power ratings are physically larger and may require heat sinks. In a high-voltage
circuit, attention must sometimes be paid to the rated maximum working voltage of the
resistor. While there is no minimum working voltage for a given resistor, failure to account

18. for a resistor's maximum rating may cause the resistor to incinerate when current is run
through it.
Practical resistors have a series inductance and a small parallel capacitance; these
specifications can be important in high-frequency applications. In a low-noise
amplifier or pre-amp, the noise characteristics of a resistor may be an issue. The unwanted
inductance, excess noise, and temperature coefficient are mainly dependent on the technology
used in manufacturing the resistor. They are not normally specified individually for a
particular family of resistors manufactured using a particular technology. A family of discrete
resistors is also characterized according to its form factor, that is, the size of the device and
the position of its leads (or terminals) which is relevant in the practical manufacturing of
circuits using them.
BC 547 Transistor
The BC547 is a general purpose epitaxial silicon NPN bipolar junction transistor found
commonly in European electronic equipment, and part of an historically significant series of
transistors that began in 1966 with Philips' introduction of the BC108 and its high-
voltage BC107and low-noise BC109 variants. The BC107/8/9 devices became the most used
transistors in Australia and Europe, and subsequent members of the series in plastic packages
(such as the BC547, BC548 and BC549) retained the specifications of the metal-cased
BC107/8/9 essentially unchanged except for improvements - in thermal resistance and
reliability for example.
The part number is assigned by Pro Electron, which allows many manufacturers to offer
electrically and physically interchangeable parts under one identification. The BC548 is
commonly available in European Union countries. It is often the first type of bipolar
transistor young hobbyist‟s encounter, and is often featured in circuit diagrams and designs
published in hobby electronics magazines.
BC548 is one of a series of related transistors: BC546 to BC550. These have broadly similar
ratings and the same collector current anther, but their breakdown voltage ratings VCEO and
VCBO vary across the range. -548 have a 30V VCBO, the -547 50V and the -546 80V. The -
549 and -550 variants are low-noise versions.

19. The pin out for the TO-92 package used for the BC546 to BC560 has pin 1 (the leftmost pin
in the diagram above, i.e. when the flat face of the package faces the viewer with leads at the
bottom) attached to the collector, pin 2 connected to the base, and pin 3 connected to the
emitter. Note that not all transistors with TO-92 cases follow this pin out arrangement.
RELAY
A relay is an electrically operated switch. Many relays use an electromagnet to mechanically
operate a switch, but other operating principles are also used, such as solid-state relays.
Relays are used where it is necessary to control a circuit by a low-power signal (with
complete electrical isolation between control and controlled circuits), or where several
circuits must be controlled by one signal. The first relays were used in long
distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit
and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges
and early computers to perform logical operations.
A type of relay that can handle the high power required to directly control an electric motor
or other loads is called a contactor. Solid-state relays control power circuits with no moving
parts, instead using a semiconductor device to perform switching. Relays with calibrated
operating characteristics and sometimes multiple operating coils are used to protect electrical
circuits from overload or faults; in modern electric power systems these functions are
performed by digital instruments still called "protective relays".

20. CIRCUIT DIAGRAM
CIRCUIT DESCRIPTION
The LDR1 is connected to the trigger input (pin 2) of IC1.
The output of IC1 is connected to the base of relay-driver transistor T1.
The 12V supply voltage is connected to the circuit through switch S1.
LDR1 and the 100-kilo-ohm preset constitute a voltage divider arrangement at pin 2 of
IC1.
When there is sufficient ambient light, the resistance of LDR1 remains low (a few
hundred ohms).
The voltage at pin 2 is greater than two-third of 12V.

21. The output at pin 3 of IC1 remains low stable state for mono stable mode of operation—
and the headlights of the vehicle connected to the normally-open (N/O) contacts of relay
RL1 remain off.
When the ambient light decreases, the resistance of LDR1 shoots up to a few mega-ohms
and the voltage at the trigger input (pin 2) of IC1 decreases to less than one-third of 12V.
The output at pin 3 of IC1 goes high to energise relay RL1 and turn the headlights „on‟.
Switch S2 can be used to manually operate the headlights.

22. Applications
1. Used in cars
2. Twilight switch
3. Road way lighting
4. Building perimeters lighting
CONCLUSION
Use of these automated head lights results in convinence to the driver while driving at night.
The same principle can be used is automatic street light so that manual switching is avoided.

23. REFERENCES
http://seminarprojects.com/Thread-automatic-headlight-full-report
http://eprints2.utem.edu.my/5603/
www.scribd.com
www.electronicschematics.in
www.circuiteasy.com
www.kpsec.com
www.buildcircuit.com
www.electronicsforu.com