Acknowledgements

I would like to take this time to express our

greatest thanks to my Project supervisor,

Mrs.Ajita Selvapandi for her time to give her

invaluable advice and help during the course of

the project. I would like to thank my school for

providing me with the equipment and other

material, which helped me in my project. To all

those who have helped in one way or another

during the course of the project, Thank you.

1

Table of Contents

Title Page

Abstract 41 Introduction 5

1.1 Back ground 51.2 Specification1.3 Statement of problem 51.4 Functional block diagram 61.41 Description of the block 71.42 Overall operation of the block 81.5 Objectives 91.6 Justification of the project 101.7 Scope and Limitation

2 Theory of components and diagram 11 2.1 Light 11 2.2 Optical detectors 12 2.21 Design of sensory unit 19 2.3 Voltage comparator 21 2.4 Power supply 27 2.41 Transformer 30 2.42 Rectifier 34

2

2.43 Filters and regulation 41 2.44 Voltage regulator 43 2.45 Design of power supply 45 2.5 Relays 48 2.51 Design of the output section 53 3 Complete circuit 55

3

List of Figures

Title Page

1 Functional block diagram 62 Light spectrum 113 Photo transistor 124 Photo diode 145 Photo conductor cell 176 Photo valtic cell 177 Photo multiplier 188 Design of sensory unit 199 Hex inverter Schmitt trigger 2310 Inverting Schmitt trigger 2511 design of control unit 2712 Transformer principle operation 3113 Half wave rectifier 3514 Positive half cycle 3615 Negative half cycle 3716 Bridge full wave rectifier 3817 Negative cycle 3918 Bridge output voltage 3919 Fixed positive linear voltage regulator 4320 Adjustable positive linear regulator 4421 Fixed negative linear regulator 4522 Stepdown transformer 4523 Rectification and filtering 4724 Voltage regulator 4825 Relay connection 5026 Design of output section 5327 Complete circuit diagram 55

4

1.0 INTRODUCTION

The automatic light activated switch refers to a

circuit that employs a photo detector that senses the

amount of light intensity. It will automatically switch

ON or OFF the lamps depending with the amount of

light intensity.

A light activate switch circuit is an electronic circuit

that using light to control the relay for open or close

circuit home appliances. When photo transistor

detects light, relay is working by starting those

connected electric home appliance.

When photo transistor does not detect light, the

relay is not working and immediately stopping those

electric appliances. Light activate switch is Simple

and inexpensive circuit, suitable for many

applications like the automatic switching of the

lights in a shop-window or a room, according to the

ambient light level. Suitable for alarm systems,

production control, remote controls etc.

The circuit uses a light dependent resistor that

changes its value according to the amount of light it

receives and three transistors which are used to

5

amplify the signal from the LDR and operate a mains

rated relay.

1.1 BACKGROUND

Whenever the sun goes down, many people are

gripped with fear. The fear of being robbed even in

the early hours of the night will make people leave

their work stations early just to save their lives and

property. Thus in the major busy towns and other

small trading centers upcountry, the need for

security lights is crucial. Such security lights will be

installed in residential areas for safety purposes.

Therefore vices and other incidents that occur at

night are curbed completely from the society.

The automatic light activated switch refers to a

circuit that employs a photo detector that senses the

amount of light intensity. It will automatically switch

ON or OFF the lamps depending with the amount of

light intensity.

6

1.2 SPECIFICATIONS

Supply voltage Vcc is 12V DC

Mains supply voltage is 240V to 250V, 50Hz

frequency

Vref is 5V DC

1.3 STATEMENT OF THE PROBLEMDue to the rising cases of insecurity, it is paramount

that people take security measures to protect

themselves from crime. One of these measures is

installation of security lights of which this project is

meant to achieve.

1.4 FUNCTIONAL BLOCK DIAGRAM

7

240VA.C.

INPUT

LDRSCHMITT TRIGGER

RELAYSECURITY

LIGHT

240V TO 12VSTEP-DOWN

TRANSFORMER

RECTIFIER

FILTER

REGULATOR

12V DC OUTPUT

240VA.C.

INPUT

Fig1: Functional block diagram of automatic light activated switch

1.41 DESCRIPTION OF THE BLOCKS

AC input power from the mains supply is of about

220V to 240V. It supplies power to the powering

unit of the circuit and the security lights.

Powering unit

8

It consists of a 240V to 12V step down transformer,

bridge rectifier, filter capacitor and regulator. It

provides the necessary positive voltage Vcc to power

the Schmitt trigger, voltage regulator and the relay.

Sensory unit

It comprises of the voltage divider and photo

resistor. The photo resistor senses the light or

darkness falling on it. The voltage divider that has

two resistors will set the sensitivity of the sensory

unit.

Control unit

The control unit comprises of the Schmitt trigger

and a PNP transistor. The Schmitt trigger is an

electronic circuit whose output switches suddenly to

either positive saturation or negative saturation

depending on the input voltage.

Output section

It comprises of the relay and the security lights. The

relay is an electronic circuit that opens and closes

9

under the control of the Schmitt trigger. If there is

flow of current through the relay windings then its

contacts close hence lights ON the security lamps

and the lights will be OFF if there’s no current flow.

1.42 OVERALL OPERATION OF THE BLOCK

The input section supplies power to the powering

unit of the circuit and the security lights which is

around 220-240V. This is stepped down by a

transformer to 12V i.e. a low AC voltage that is

rectified to DC by the bridge rectifier. Its then

filtered by a filter capacitor and regulated by a 3

terminal voltage regulator. The powering unit

provides the necessary positive Vcc to power the

Schmitt trigger, voltage regulator and the relay.

This then is inputted to the sensory unit that has a

photo detector and voltage divider R1 and R2.

The photo resistor senses either light or darkness

and falling on it. When light falls on it, its resistance

decreases and will increase with darkness. Resistors

R1 and R2 set the sensitivity of the sensory circuit.

10

Resistor R2 is a preset resistor that is used to just

turn OFF the security lights at dawn or turn them

OFF at sunset by adjusting its movable arm.

At the control unit and output section, Schmitt

trigger produces two voltages; upper threshold

voltage VUT and lower threshold voltage VLT. The

output of the Schmitt trigger switches to positive

saturation when its voltage at its input is greater

than VUT and switches negatively when its voltage at

its input is less than VLT.

Output of the Schmitt trigger is connected to the

base of a PNP transistor driving it to cut off thereby

cutting off current and making the relay contacts

remain open. This makes the security lights remain

OFF during the day. During the night, the Schmitt

trigger switches to negative saturation driving

transistor to near saturation. Current therefore

flows through the relay winding closing its contacts

and allowing current to flow hence lighting ON the

security lights.

11

1.5 OBJECTIVES

The broad objective is to design and construct an

automatic light activated switch that controls the

switching of on and off the security lights.

The specific objectives will be;

1. To design the various blocks an come up with a

schematic diagram

2. To construct the automatic light activated

switch

3. To test the working of the blocks at each input

and output and the overall operation of the

block

1.6 JUSTIFICATION OF THE PROJECT

In order to curb the rising insecurity in our society,

it is necessary that people take precautions of their

own safety. This can be achieved by installing such

security measures as security lights in their homes

which is what the project is meant to achieve.

1.7 SCOPE AND LIMITATIONS

12

1.71 SCOPE

The main aim of designing this project is to come up

with a workable project employing a photo detector

that will sense the amount of light intensity and

automatically switch on or off the security lights.

1.72 LIMITATIONS

1)Lack of some components used in the design

2)High cost of some of the components

3)Time allocated was minimal

2.0 THEORY OF COMPONENTS AND DESIGN

2.1 LIGHT

The Quantum theory states that light consists of

discrete packets of energy called photons. The

energy contained in a photon depends on the

frequency of light and is expressed as

13

ω = hf where ω = energy

h = planks constant i.e.

6.624 × 10-34 Joules/ second

f = frequency

As frequency increases so does the energy and vice

versa. Light is usually referred in terms of

wavelength.

= V

F

Any light source emits light over a limited range of

wavelength. When the amount of energy is plotted

against a wavelength, the graph is the emission

spectrum.

Visible light

Decreasing wavelength

Fig 2: Light spectrum

14

Invisible light

Infrared Red Orange Yellow Green Blue Indigo Violet UltraViolet

Invisible light

2.2 OPTICAL DETECTORS

Optical detectors are devices that convert optical

energy or light into electrical energy. There are two

types of conversions:

Photo voltaic effectThis is where optical energy is converted to electrical voltage

Photoconductive effectThis is where light is converted into an electrical voltage.

a) Photo transistor

The photo transistor consists of a normal bi-polar

transistor that is packaged in a transparent casing

to enable light reach the base of the transistor. The

base is an open circuit and a normal biasing is

provided between the collector and the emitter as

shown below.

Symbol

Base

Emitter

Collector

15

Ic

VCE

Fig 3a.

With no illumination, only the leakage current (ICEO)

flows in the collector-emitter junction. When the

base/collector junction is illuminated, holes –

electrons pairs are generated and a minority photo

current flows across the junction. This increases the

forward bias of collector. Thus the collector current

is the sum of the photo current and electron current

from the emitter. This amplified current makes the

phototransistor very sensitive.

16

Light

b) Photodiode

A photo diode is a P-N junction silicon diode that is

packaged with a transparent window that allows

light to pass through. In operation, the P-N junction

diode is reverse biased whereby in this mode the

value of reverse current will depend on the amount

of illumination on the junction. Only a small reverse

current will flow. In dark conditions, it is near zero

and under bright, it’s in tens or hundreds of M

Amps.

Symbol

The magnitude of a photocurrent depends on the

number of charge carriers generated.

Below is a series of characteristics obtained from

various levels of illumination.

Increasing illumination

17

Ie (mA)

Fig 3bBelow is a spectral response for silicon and Germanium diodes

Fig 3c

c) Photo conductive cell/ LDR/ photo resistor

18

Vce (V)

Response

Germanium

Silicon

1.0

A Light Dependent Resistor also known as the

Photoconductive cell consists of a semiconductor

material above which a transparent window whose

surface is exposed to light allows light to pass

through. Light allows more current to flow within

the semiconductor material.

When the light photons are absorbed by the

semiconductor, the electrons acquire enough energy

to break the bonds that hold them in a covalent

structure. This is by moving from valence band to

the conduction band.

The higher the light intensity, the more the free

electrons in the conduction band. Since the

conductivity of the material increases as number of

free electrons increases, the electrical resistance of

the semiconductor decreases with increase in light

intensity.

Electrical conduction occurs when free charge

carriers are available when an electric field is

applied. In certain semiconductors light energy

falling on them is of the correct order of magnitude

to release charge carriers which increase the flow of

current produced by an applied voltage.

19

The increase in current with increase in light

intensity with the applied voltage remaining

constant means that the resistance of

semiconductors decreases with increase in light

intensity.

The most commonly used photoconductive

semiconductor materials are Cadmium Sulphide

(CdS) with a band gap of 2.42eV and Cadmium

Selenide (CdSe) with a band gap of 1.74eV. Both

have a very high resistivity at ambient temperature

which gives a high value of resistance.

When the cell is in darkness, its resistance is known

as dark resistance. It may be as high as 10 × 1012 Ω.

Resistance depends on the physical character of

photoconductive layer, dimensions of the cell and its

geometric configurations.

The figure below shows a symbol for an LDR;

Symbol

20

Light intensity

ResistanceFig 3d

d) Photovoltaic cell / Solar cell

It’s a device that converts light energy to electrical

energy by photovoltaic effect. Photons of light

produce electrons and holes in a PN junction diode.

Light

V

Fig 3e

21

P

N

A solar cell consists of a P- semiconductor region

and N semiconductor region. The photons of light

cause electrons to move from valence to conduction

band creating a hole as a result. Holes move to the P

region and electrons move to the N region resulting

in accumulation of positive charge in the P region

and accumulation of negative charge in the N

region. This creates a pd/voltage.

e) Photomultiplier

These are devices constructed from vacuum tubes.

They contain a photocathode anodes and dynodes.

Fig 3f

Light photons strike the photocathode and electrons

are produced as a result of the photoelectric effect.

These electrons are directed by the focusing

22

Cathode

Focusing electrodes

Dynodes

Anode

electrodes towards the electron multiplier that

consists of electrodes known as dynodes. Each

dynode is held at a more positive voltage than the

previous one.

As the electrons move towards the first dynode, they

are accelerated by the electrical field. On striking

the dynode, more electrons are emitted and

accelerated. This process goes on and the overall

effect is that a large number of electrons accumulate

at the anode compared with those at the

photocathode. This constitutes amplification of light.

Extremely low levels of luminous intensity can be

measured or detected by means of photomultiplier

tubes which utilize many successive stages of

secondary emission to boost up the output current

from its initial very low value.

2.21 DESIGN OF THE SENSORY UNIT

23

To Schmitt trigger

50k

Vcc

LDR

R2

R147k

It comprises of the voltage divider R1, R2 and LDR1.

The LDR senses the amount of light intensity falling

upon it.

Resistors R1 andR2 set the sensitivity of the sensory

unit.

The preset resistor R2 is used to just turn off the

security lights at dawn or dusk by adjusting its

movable arm.

During the day, light falls on LDR1 which makes its

resistance to reduce. Since it is in a voltage divider

24

network, the voltage across it reduces according to

the

Voltage Divider Rule.

During the night, the reverse happens. The

resistance across LDR1 increases proportionate to

the darkness falling upon it. The voltage developed

across LDR1 thus increases according to the Voltage

Divider Rule

The voltage across the LDR1, denoted by Vldr, is given by:

For LDR, part number VT33N3, the specifications are as follows:

From equation (i), when there is light of 10 Lux, the

resistance of LDR1 as seen in the specifications

above is 160,000kΩ.

25

10 Lux DarkMin Typical Max Min80kΩ 160kΩ 240kΩ 2MΩ

………………………….. (i)

(NB: The Typical value is usually taken as the

default value.)

Thus

And when there is darkness, the minimum value of

LDR1 resistance is 2MΩ, from the specifications

above.

Hence the voltage across LDR1 when there is

darkness is given by:

2.3 VOLTAGE COMPARATORS

A voltage comparator is a device used to compare

two voltage levels. The output of the comparator will

show which of the outputs is larger. Therefore it

acts as a switching device, producing a high output

when one input is larger than the other and

26

switching to a low output if the other input becomes

larger.

An operational amplifier is used as a voltage

comparator by operating with no feedback and by

connecting two voltages to be compared to the

inverting and non inverting inputs. The amplifier

output is driven to one of its output voltage limits

when there is a very small difference between its

input levels due to a very large open loop gain.

Properties of operational amplifiers are;

1)Infinite input impedance

2)Zero output impedance

3)Infinite voltage gain

4)Infinite bandwidth

5)Characteristics that do not change with

temperatures

Uses of op amps

Scale changing

Analog computer operations

Instrumentation and Control systems

In phase and oscillator circuits

27

SCHMITT TRIGGER

The Schmitt trigger is a regenerative comparator.

The output can only take up either one of two

possible values. The Schmitt trigger is employed to

convert an arbitrary input signal into a square wave

output. The circuit can either be obtained in either

inverting or non inverting versions and may be made

using either discrete components or an op amp.

V2

V1

Vref

Vout

R2

R1

Fig 4a: A 7414 hex inverter Schmitt trigger

The output voltage can have only one of two possible

values. The output voltage will be higher e.g. +3.3V

for a TTL version, when the input is greater than a

positive going threshold voltage and will remain in

this value until such time as the input voltage falls

below the negative going threshold voltage.

28

The difference between the two input voltages V1

and V2 at which switching takes place is known as

the hysteresis of the circuit.

From the voltage characteristics, when the

sinusoidal input voltage becomes more positive than

the upper threshold V1, the circuit switches to give

an output voltage of +Vout. The output voltage will

remain at this value until the input voltage falls

below the lower threshold voltage V2 and at this

point the output voltage suddenly switches to -Vout.

The output voltage will now remain at –Vout until the

input voltage exceeds the upper threshold voltage

V1.

Hysteresis provides some noise protection to a

circuit. The larger the hysteresis, the greater the

noise protection afforded.

Transfer characteristics

29

+

-

V1 Vin

It shows how the output voltage changes from one

value to another as the input voltage is increased to

the upper threshold voltage V1.

It shows how the output voltage suddenly reverts to

its original value when the input voltage is reduced

below the lower threshold voltage V2.

Shows the complete transfer characteristics

Inverting Schmitt Trigger

30

+

-

V2 Vin

Vin

V2 V1

VoutVin

R2

R1

Fig 4b: Inverting Schmitt trigger

For an inverting Schmitt Trigger:

When Vref is 0, we find that

Therefore when the output is at its negative limit (V0

= -Vmax)

Then Lower threshold level is

Similarly, when V0 = + Vmax, Vin must rise to;

31

Where +Vmax is the maximum positive output voltage -Vmax is the maximum negative output voltage

The hysteresis of the Schmitt trigger is defined as the difference between the input trigger levels.

Hysteresis = Upper threshold level – Lower threshold level

=

If the maximum output voltages are equal, we have

Hysteresis =

For non inverting Schmitt trigger, the lower and upper trigger levels are;

Lower threshold level =

LTL =

Upper threshold level =

UTL =

32

33

2.31 DESIGN OF THE CONTROL UNIT

SCHMITT TRIGGER

The 12V that has been regulated is now fed to the

Schmitt trigger. The 3-terminal 5v voltage regulator

U3 and resistors R3 and R4 set the upper threshold

voltage, Vut and lower threshold voltage, Vlt.

The Schmitt trigger has 2 voltages associated with

it;

Vut and Vlt

The output of the Schmitt Trigger switches to

positive saturation whenever the voltage at its input

34

Input From Sensory Unit

Output to Base of PNP transistor

+U2

LM324

R4

10k

R3

1kIN

COM

OUT

U3LM7805

VCC

VCCVREF

is greater than the Upper Threshold Voltage VUT.

Similarly, it switches to negative saturation

whenever the voltage at its input is less than the

Lower Threshold Voltage VLT.

The Upper Threshold Voltage VUT is given by;

……………………………………… (i)

And the lower Threshold voltage VLT is;

……………….. (ii)

Where V(+) is the positive saturation at the output of

the Schmitt trigger

Vref is the reference voltage

R3 and R4 set Vut and Vlt

V(-) is the negative saturation voltage at the

output of the Schmitt trigger

35

Since in this circuit the Schmitt trigger is connected

only to one positive voltage, Vcc, the negative

saturation voltage V (-) is zero volts. Thus the lower

threshold voltage Vlt is given as;

…… (iii)

The hysteresis voltage, VH , is the voltage difference

between the upper threshold voltage and lower

threshold voltage.

……………………… (iv)

Thus hysteresis voltage VH, becomes;

……………………. (v)

Now since the negative saturation, V (-) is 0Volts, the hysteresis voltage becomes;

…………………….……. (vi)

Taking VH to be 3 volts and V (+) to be 11v to

account for the drops across the transistors in the

operational amplifier, then;

36

………………………………. (vii)

After simplifying this becomes;

……………………..……………… (viii)

Thus taking R3 to be 10k , then R4 becomes 27k .

Substituting the values of R3 and R4 into equations

(i) and (iii) for VUT and VLT, they result into;

……………………………………. (ix)And

……………………………………… (x)

2.4 THE POWER SUPPLY

The essence of having a power supply is to have a

voltage that will be able to power the circuit

according to the requirements of either filtered or

unfiltered, regulated or unregulated which varies

according to different circuits to provide the

necessary dc operating voltages and currents. The

main requirements of a power supply are:

To provide the rated voltages for the circuit to

be supplied with a specified tolerance.

37

To be able to supply the rated maximum current

to be supplied to the circuit without the supply

voltage falling outside the specified limits.

To maintain the supply voltage constant within

specified limits as the load changes or the mains

supply input voltage varies or the ambient

temperature changes.

The power supply consists of a step down

transformer, rectifier circuit, smoothening

capacitors and voltage stabilizer.

2.41 TRANSFORMERS

A transformer is a device that uses mutual induction

to change values of alternating currents and

voltages.

Transformer principle of operation

38

Pry wdgsN1 turns

Sec wdgsN2 turns V2V1

I2I1

Ferro magnetic core

Fig 5

When the secondary is an open circuit and an

alternating voltage V1 is applied to the primary

winding, a small current called the no load current,

Io flows which sets up a magnetic flux in the core.

The alternating flux links with both primary and

secondary coils and induces in them Emfs E1 and E2

respectively by mutual induction.

E = - N volts where = rate of change

of flux

Therefore E1 / N1 = E2 / N2

If there are no losses for an ideal transformer;

E1 = V1 and E2 = V2

Where: Is the voltage ratio

Is the transformation ratio

39

If N2 is less than N1 then V2 is less than V1 thus the

device is a step down transformer and vice versa for

a step up transformer. For an ideal transformer,

primary and secondary ampere turns are equal.

EMF equation of a transformer

The magnetic flux Φ set up in the core of a

transformer when alternating voltage is applied to

its primary winding is also alternating and

sinusoidal.

If Φ m is the magnetic flux value and f the frequency

of the supply, the time for one cycle for the

alternating flux is periodic time T where T = 1/F

The flux rises sinusoidally from zero to maximum

value in a quarter cycle.

Change of flux = Φ m / T

= 4F Φ m wb/sThis is the average induced EMF in each turn.

Form factor = Rms value = 1.11 Average value

Rms value = form factor × average value = 1.11 × 4F Φ m

= 4.44 F Φ m volts

40

EMF induced in the primary winding E1 and E2;

E1 = 4.44 F Φ mN1 volts E2 = 4.44 F Φ m N2 volts

Dividing the two equations:

E1 / E2 = N1 / N2

Transformer efficiency

η = output power ×100 input power

Transformer regulation

Secondary voltage drops with the loading of the

transformer. As the power factor decreases, this

voltage drop increases. This is called regulation of

the transformer i.e.

This is the change in secondary terminal voltage

between no load and full load at a given power

factor.

Regulation = × 100

Transformer losses

41

1. Copper losses

It occurs because the winding have a resistance

causing a power loss in them which can be

accounted for by a resistor in series with each

winding.

2. Iron losses

a) Hysteresis losses

This is the heating of the core due to internal

molecular structure reversals that occur as the

magnetic flux alternates.

b) Eddy current losses

This is the heating of the core due to EMFs being

induced in the transformer windings and core.

Alternating magnetic flux induces Emf causing eddy

currents. Eddy currents can be reduced by

increasing the resistivity of the core material or

laminating the core to increase eddy current path

resistance and reduce the value of eddy currents.

2.42 RECTIFIERS

42

A rectifier converts the A.C. input voltage to a

pulsating D.C. voltage. A rectifier could be a half

wave or a full wave rectifier.

Modes of rectification

There are three typical methods of rectification:

1. Half wave rectification

2. Center tapped full wave rectification

3. Bridge rectification.

a) Half wave rectifier

Half wave rectified voltage Fig 6a

OPERATION

43

-

+

-+

Vin RL

I

Fig 6b

When Vin goes positive, the diode is forward biased

and conducts current through the load resistor. The

current produces an output voltage across RL that’s

the same shape as the input voltage.

Fig 6c

When Vin goes negative during the second half cycle,

diode is reverse- biased. No current flows so RL

voltage is 0V. Thus only positive half cycles of the

A.C. input voltage will appear across the load.

b) Full wave rectifier

A full wave rectifier allows only unidirectional

current through the load during the entire 360

44

-

+

- +

Vin RL

degrees of the input cycle. This results to an output

voltage with a frequency twice the input frequency

that pulsates every half cycle of the input.

1) Centre tapped full wave rectifier

It use two diodes connected to the secondary of a

centre tapped transformer. The input voltage is

coupled through the transformer to the centre

tapped secondary.

Half of the total secondary voltages appear between

the centre tap and each end of the secondary

winding

Positive half cycle

Fig 6d

Negative half cycle

45

I

RL

-+

-

+

+-

+

+

-

D2

D1

-

- +

-

+

+ -

+

-

D2

D1

RL

I

Fig 6e

During the positive half cycle, D1 is forward biased

and D2 is reverse biased and during the negative

half cycle, D1 is reverse biased and D2 is forward

biased. Because the output current during both the

positive and negative portions of the input cycle is in

the same direction through the load, the output

voltage developed across the load resistor is a full

wave rectified D.C. voltage.

2) Bridge full wave rectifier

It uses four diodes.

46

Positive cycle

Fig 7a

During the positive half cycle, diodes D1 and D2 are

forward biased and conduct current in the direction

shown above. A voltage is developed across the load

resistor similar to the one at the output. D3 and D4

are reverse biased.

Negative cycle

47

Vin

D4

D3

D2

D1

RL

I

RL

D4

D3

D2

D1

Vin

I

Fig 7b

During the negative half cycle, D3 and D4 are

forward biased and conduct current through the

load resistor in the same direction as the positive

half cycle and D1 and D2 are reverse biased. As a

result, a full wave rectified output voltage appears

across RL.

Bridge output voltage

Fig 7c

During the positive half cycle of the total secondary

voltage D1 and D2 are forward biased. The secondary

48

-

+ +

-

(0.7v)

(0.7v)D1

D4

D3

D2

Vpri Vsec

RL

voltage appears across the load resistor (neglecting

voltage diode drops).

Same is true when D3 and D4 are forward biased

during negative half cycle.

VP = VP (sec)

If diode drops are now considered i.e. two diodes

are always in series with RL the output voltage is

VP (out) = VP (sec) - 1.4V

Peak inverse voltage

Assuming diodes D1 and D2 are forward biased and

D3 and D4 reverse biased. D1 and D2 are shorts and

therefore D3 and D4 have a peak inverse voltage

equal to the peak secondary voltage as the output

voltage is ideally equal to the secondary voltage.

P.I.V. = VP (out)

If diode drops are included;

P.I.V. = VP (out) + 0.7V

Advantages of full wave over half wave

49

Its more efficient

Little D.C. magnetization of transformer occurs

Ripple voltage is twice the supply frequency

making it easier to reduce the percentage

ripples to the desired level.

2.43 FILTERS AND REGULATORS

A filter is a capacitor connected from the rectifier

output to ground. It eliminates the fluctuations in

the rectified voltage and produces a relatively

smooth D.C. voltage. For a half wave rectifier a

capacitor input is used.

During the first quarter cycle of the input, diode is

forward biased allowing capacitor to charge to 0.7V

of input peak. When the input begins to decrease

below its peak, capacitor retains its charge and the

diode becomes reverse biased because cathode is

more positive than the anode.

During the other part of the cycle, capacitor

discharges through the load resistor at time

constant determined by RLC.

50

During the next cycle, diode becomes forward

biased and input voltage exceeds capacitor voltage

by approximately 0.7V.

A full wave is easier to filter because of the shorter

time between peaks. When filtered, full wave

rectification has smaller ripples than half wave for

same RL and capacitor values. Capacitor discharges

less during shorter intervals between full wave

pulses.

Less effective filtering More effective filtering

Half wave Full waveComparison of ripple voltages for half wave and full

wave rectified voltages with the same filter

51

capacitor and load with same Vin with both having

the same capacitor discharge rate.

Full wave

Fig 8

Ripple factor (r)

This is the indication of effectiveness of the filter

r =

Where Vr (pp) is the peak to peak ripple voltage

Vdc is the average DC value of filters output

voltage

The lower the ripple factor the better the filter. It

can be lowered by increasing the value of filter

capacitor or load resistance.

For full wave rectifier with capacitor input filter;

52

Half wave

Vr (pp) = × Vp (rect)

Where Vp (rect) is the unfiltered peak rectified voltage

Surge current in the capacitor input filter is

prevented by connecting a surge- limiting resistor

that has smaller value than RL.

Diodes should also have a forward surge current

rating that can withstand momentary surge of

current.

2.44 VOLTAGE REGULATORS

A voltage regulator provides a constant D.C. output

voltage that’s independent of the input voltage,

output load current and temperature. The input

voltage comes from the filtered output of the

rectifier derived from an A.C. voltage.

Fixed positive linear voltage regulators

Positive input

Positive output

+

C2

+

C1

IN

COM

OUT

78XX

53

Fig 9a

The input capacitor is used to prevent unwanted

oscillations when the regulator is some distance fro

the power supply filter. The output acts as a line

filter to improve transient response. The input

voltage must be at least 2V above the output voltage

so as to maintain regulation. The circuit has internal

thermal overload protection and short circuit

current limiting features. Thermal overload occurs

when internal power dissipation becomes excessive

and temperature of the device exceeds a certain

value thus a heat sink is used.

Adjustable Positive linear voltage regulator

Adjustment

Positive input

Positive output

R2

R1

+

C2

+

C3

+

C1

IN

COM

OUT

LM317

Fig 9b

54

15V A.C. Output

240V A.C. Input

To fullwave bridge rectifier

F1

100mAT1

Capacitors are used for decoupling and they do not

affect the D.C. operation. Fixed resistor R1 and

variable resistor R2 provide output voltage

adjustment. It acts as a floating regulator because

its adjustments not connected to ground but floats to

whatever voltage is across R2. This allows the

output voltage to be much higher than that of a fixed

regulator.

Fixed negative linear voltage regulators

It’s a 7900 series 3-teminal I.C regulator that provides a fixed negative output voltage.

Negative output

Negative input

+

C3

+

C1

IN

COM

OUT

79XX

Fig 9c

2.45 DESIGN OF THE POWER SUPPLY

STEP DOWN TRANSFORMER

55

From the mains supply 240V AC input is obtained.

This voltage is stepped down to 12V by a step down

transformer rated at least one ampere. A step down

transformer rated at 12V, 50Hz, 300mA is readily

available in the market thus the reason for opting

for it.

Since a regulator is used in the power supply, it is

paramount that at least 3V is available from the

supply voltage apart from the regulator voltage.

This voltage is referred to as drop out voltage and is

the voltage across the input and output terminals of

the voltage regulator that enables it to operate

effectively and maintain a constant output voltage.

By using a step down transformer with an output of

12V after filtering, a peak voltage of 17V is obtained

from the rectified voltage as shown below.

Vpeak = √2 Vrms

56

From Stepdown Transformer

D4 D3

D2D1Vout

+ C1

1000uF, 25V

= √2 (12V)

= 17V

At the primary side, a fuse has been connected to

prevent short circuits and blowing of components.

For an ideal transformer;

VpIp = VsIs

240Ip = 12×1

Therefore Ip = 12/240

= 50mA

Therefore the fuse should be rated at 250V, 100mA to accommodate the Ip= 50mA.

RECTIFICATION AND FILTERING

57

The 12Vrms voltage is rectified by a full wave bridge

rectifier formed by diodes D1 through D4. The

diodes Part Number is 1N4007. These diodes were

chosen because of the following characteristics;

i. Forward current, If = 1A

ii. Reverse breakdown voltage, Vbr = 1000V

iii. Forward voltage, Vf = 0.7V

The filter capacitor C1 rated 1000µF, 25V was

chosen because the higher the value of capacitance,

the better the filtering thus less ripples.

Capacitor C1will therefore charge to Vpeak of Vsec.

From Vpeak = √2Vrms

= √2 (12V)

= 17V

Due to diode drops when the load is connected

Vout = Vpeak – 2 diode drops (each 0.7V)

= 17V - 1.4V

= 15.6V

58

Since the filter capacitor C1 charges to 17V, its

rated voltage was chosen to be 25V so as to

accommodate Vpeak without blowing.

VOLTAGE REGULATION

From filter capacitor

Vcc

+C2 10uF,16V

IN

COM

OUT

U1LM7812

Vout (15.6V) is now regulated to 12V by a voltage regulator LM7812. This regulator was chosen because it requires fewer external components and has an internal overload protection.

2.5 RELAYS

A relay is a device consisting of a coil wound on soft

iron core. It is an electrically operated switch. When

current flows through the coil of the relay, a

magnetic field is set up which attracts the iron arm

of the armature of the core of the magnet.

The set of contacts of the armature and relay frame

close completing a circuit across its terminals. When

59

the magnet is de-energized, the return spring

returns the armature to the open position and the

contacts open breaking the circuit across the

terminals.

The coil current can be on or off so relays have to

switch and they are double throw or change over

switches. Relays allow one circuit to switch a second

circuit which can be completely separate from the

first.

Current flowing through the relay coil creates a

magnetic field that suddenly collapses when the

current is switched off. The sudden collapse of the

magnetic field causes or induces a brief high voltage

across the relay coil. These high voltage spikes can

destroy transistors and ICs in the circuit.

To prevent damage a protection diode is connected

across the relay coil. It allows the induced voltage to

drive a brief current through the coil and diode so

the magnetic field dies away quickly rather than

instantly. This prevents the induced voltage from

60

becoming high enough to cause damage to

transistors and ICs.

Transistor

Relay coil

Protection diode

Relay contactsInput

NO

NO

COM

Normally open

0V

12V

Fig 10: Relay connections

Factors to consider while choosing a relay

1. Physical size and pin arrangement

2. Coil voltage i.e. the relays coil voltage rating and

resistance must suit the circuit powering the

relay coil.

3. Coil resistance – the circuit must be able to supply

the current required by the relay coil.

4. Voltage and current switch ratings

5. Switch contact arrangement (SPDT, DPDT)

Advantages of relays

61

1) Relays can switch A.C. and D.C. while transistors

can only switch D.C

2) Relays can switch high voltages while transistors

can’t.

3) Relays are a better choice for switching large

currents (>5A)

4) Relays can switch many contacts at once.

Disadvantages of relays

1) Relays are bulkier than transistors for switching

small currents.

2) Relays cannot switch rapidly (except reed relays)

but transistors can switch many times per second.

3) Relays use more power due to the current

flowing through their coil.

4) Relays require more current than many ICs can

provide, so a low power transistor may be needed to

switch the current for the relays coil.

Relay switchesA relay will switch one or more poles, each of whose

contacts can be thrown by energizing the coil in one

of these ways.

62

Normally open contacts

Normally open contacts connect the circuit when the

relay is activated and the circuit is disconnected

when the relay is inactive. It is also called a make

contact.

Normally closed contacts

Normally closed contacts disconnect the circuit

when the relay is activated and the circuit is

connected when the relay is inactive. It’s also called

a break contact.

Change over contacts

It’s a double throw contact controlling two circuits;

one normally open contact and normally closed

contact with a common terminal. It’s also called

transfer contact.

Types of switch contacts

Normally closed timed open

This type is normally closed when the coil is

unpowered or de-energized. The contact is opened

with the application of power to the relay coil but

only after the coil has been continuously powered

63

for the specified period of time. The direction of the

contacts motion either to close or to open is

identical to a regular normally closed contact but

there is a delay in the opening direction. Because

the delay occurs in direction of the coil energization,

the contact is called normally closed on delay.

Normally closed timed open

This type of contact is normally closed when the coil

is unpowered and opened by the application of

power to the relay coil. However unlike the normally

closed timed open contact, the timing action occurs

upon de-energization rather than upon energization.

Because the delay occurs in the direction of coil de-

energization, the contacts are also called normally

closed off delay.

Normally open timed closed contact

This type of contact is normally open when the coil

is unpowered. The contact is closed by the

application of power to the relay coil but only after

the coil has been continuously been powered for a

64

BD712Q1

1N4007D5

RLY1

From mains supply

240V A.C. supply to security lights

From output of Schmitt trigger

specified amount of time. The direction of the

contacts motion is identical to a regular normally

open contact but there is a delay in closing

direction.

Because the delay occurs in the direction of coil

energization, its also called normally open on delay.

Normally open timed open contact

This contact is normally open when the coil is

unpowered and closed by the application of power to

the relay coil. Unlike the normally open timed closed

contact, the timing action occurs upon de-

energization. Because the delay occurs in the

direction of the coil de-energization, the contact is

also called normally open off delay.

2.51 DESIGN OF THE OUTPUT SECTION

65

Now, the voltage developed across LDR1 is fed to the

input of the Schmitt Trigger. During the day, the

resistance of LDR1 decreases leading to the voltage

across it reducing. The moment the voltage across

LDR1 reduces to a value below the Lower Threshold

Voltage VLT, the output of the Schmitt Trigger

switches to positive saturation. The output of the

Schmitt Trigger is connected to the base of the PNP

transistor, Q1. This drives it to cut-off.

With transistor Q1 into cut-off, no current flows

through it and in effect the relay winding. This

makes the relay, RLY1 contacts remain open. The

mains power supply to the security lights is

connected through the relay contacts which act as a

switch.

Thus, with the contacts open, no power flows to the

security lights and they remain off during the day.

During the night, the voltage across LDR1 increases

due to the increase in its resistance. Any moment

this voltage increases to a value greater than the

66

Upper Threshold Voltage VUT, the Schmitt Trigger

switches to negative saturation. Transistor Q1 is

driven into near-saturation and current flows

through the relay winding making it close its

contacts. This allows current to flow to the security

lights and they light up.

67

LM324

25V1000uF

240V TO 12V, 300mA STEPDOWN TRANSFORMER

Vcc

Vcc

U1LM7812

LM7805

Vcc

C2C1

D4 D3

D2D1

1N4007

VccVccD5

U2

LDR1

R4

R3

R2

R1

U3

RLY1

Q1BD712

240V A.C. OUTPUT TO SECURITY LIGHT

4, 1N4007

240V A.C.INPUT

16V10uF

50k

47k

1k10k

T1

E

D

CB

A

+

IN

COM

OUT

+

+

IN

COM

OUT

E

D

CB

A

3.0 COMPLETE CIRCUIT DIAGRAM SHOWING TEST POINTS

68

Conclusion

In this project, I learnt a lot about the Light activate switch system of the electric appliances using intelligent sensors. It gave me an opportunity to explore the various sensors and different automation technique differently used in the electronic product environment. After all the efforts, Light activate switch system has been developed to meet the specification defined in the proposal. Due to time constraint the project has been completed with Prototype model. But, it works well to the level, which was mentioned in the proposal.

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