AUTOMATIC EMERGENCY LIGHT

CONTENTS

Abstract ……………………………………………………………………………………6

Introduction ………………………………………………………………………………...7

Chapter 1

1.1. Block Diagram ………………………………………………………………………...10

1.2. Block diagram with explanation of each block ………………………………………..11

Chapter 2

Design details ……………………………………………………………….12-22

2.1. Transformer ……………………………………………………………………………13

2.2. 7808 voltage regulator ………………………………………………………………….13

2.3. LM324 quad operational amplifier ……………………………………………………..14

2.4. Photo transistor ………………………………………………………………………….15

2.5. 2n2222 npn transistor ……………………………………………………………………19

2.6. 2955 pnp power transistor ………………………………………………………………..20

2.7. BD140 pnp medium power transistor ……………………………………………………21

2.8. SL100 npn transistor ……………………………………………………………………..21

2.9. 3.3v zenor diode ………………………………………………………………………….22

1

Chapter 3

Circuit diagram ……………………………………………………………..24

Circuit diagram with operational details …………………………………25-37

3.1. LM324 quad operational amplifier …………………………………………………25

3.1.1. Introduction

3.1.2. General description

3.1.3. Unique characteristics

3.1.4. Advantages

3.1.5. Features

3.1.6. Representative circuit diagram of LM324

3.1.7. Circuit description

3.1.8. Performance characteristics of LM324

3.2.7808 voltage regulator ………………………………………………………………..31

3.2.1. General description

3.2.2. 78xx series of regulators

3.2.3. Ripple voltage

3. 3.SL100 npn transistor ………………………………………………………………..33

3.4. 2n2222 npn transistor ………………………………………………………………33

3.4.1. General description

3.4.2. NPN characteristics curves

3.4.3. Photo sensor

3.5. Photo transistor ………………………………………………………………………36

3.5.1. General characteristics and features

2

3.5.2. Applications

3.5.3. Opto isolator

3.5.4. Optical switch

3.5.5. Retro sensor

3.6. RESULT ……………………………………………………………………………….38

3.7. CONCLUSION ………………………………………………………………………..38

3.8. FUTURE SCOPE ……………………………………………………………………...38

3.9. REFERENCES ………………………………………………………………………...38

3

ABSTRACT

Today, in industries and as well as in household applications an emergency light is employed

where there is frequent non uniform voltage distribution occurs. Many types of emergency lights

from rechargeable torches to systems like generators are available in market. All of them require

a switch to operate them when frequent power failure occurs. The present one deals with a model

which senses the mains as well as daylight to switch on the emergency light. This emergency

light holds requirements of domestic purposes also. There is no need to search the switch in the

dark as it switches on /off automatically.

There are some special features in this project which are as follows:

When mains power is available, it senses and switches off the lamp instantly. This may

be a common feature in any of the emergency power systems.

It incorporates an opto-eye which senses the ambient light and when the ambient light

reaches a present low level when there is no power, it switches on the emergency light

automatically. The switching is instantaneous.

In most of the emergency lights there is a drawback. The discharge level of the battery is

not being controlled to a safe level. The batteries get discharged completely and lose their

life rapidly. This is a very serious complaint from the users. In this one, cut-off is

provided at predefined manufacturers minimum discharge level which gives the specified

life of the battery.

The opto-eye mentioned in the above features is a special one in this automatic emergency light

and opto-eye consists of a photo transistor and some specific arrangement of resistors. Here

instead of photo transistor a photo diode is also used and we can also increase the sensitivity of

4

Photo diode in reverse configuration. We also come across different operational amplifiers,

diodes, light emitting diode. In addition to these we also employ step down transformer and we

also make use of one switch for proper operation of the devise. For different batteries, charging

voltage and lamp can be choosen accordingly to be more illuminated.

INTRODUCTION:

The present mini project deals with the recent requirements of emergency light both for

industries and domestic purposes. Many types of emergency lights from rechargeable torches to

systems like generators are available in the market. When compared to all other emergency lights

this one is quite efficient one as we make use of integrated circuits, voltage regulator and an

excellent feature of opto eye is included in this present mini project. The present one deals with a

model which senses the mains as well as daylight to switch on the emergency light.

The circuit consists of an LM324 quad operational amplifier which

consists of fourteen pins. A logic function is performed by using this type of IC. There are four

ICs involved in this circuitry. Also photo device is also included which makes the use of light

intensity according to which voltage is produced across its terminals. When the wired AND logic

function is fulfilled from the three ICs which work as comparators and the remaining another IC

respond to it according to which the bulb will be glown . It also consists of a Darlington pair of

pnp transistors whose set is capable of delivering the fifteen amperes to the load.

This present one has one on/off switch on operating which the emergency

light glows. In most of the emergency light there exists a drawback. The discharge level of the

battery is not being controlled to a safe level. The batteries get discharged completely and lose

their life rapidly. This is a very serious aspect in order to overcome this a cutoff is provided and

there exists a minimum discharge level which ensures the long life of battery used in the circuit.

5

It incorporates an opto-eye which senses the ambient light when the ambient light reaches a

preset level when there is no power and it helps to add some voltage due to the fact that a photo

device like photo transistor is involved.

6

CHAPTER-1

BLOCK DIAGRAM WITH EXPLANATION OF EACH BLOCK

7

1.1. BLOCK DIAGRAM:

8

1.2. EXPLANATION OF EACH BLOCK:

INPUT SUPPLY:- Input supply of 230 volts ac is fed to the main comparator-1 where it will be

rectified. This input supply is not only fed to the mains comparator-1 but also to another block

called mains comparator-2.

MAINS COMPARATOR-1: The function of the this block is to convert ac into dc i.e,

rectification is done in this process by having some basic elements like capacitor and a

unidirectional device. The output delivered from this block will be used for different blocks.

MAINS COMPARATOR-2: The function of this block is to compare the ac mains and the

output delivered from the mains comparator-1. Depending on the status of the ac mains the

output of the mains comparator-2 will be decided for the further process in the circuit.

AMBIENT LIGHT COMPARATOR:-This block includes a unique feature of having an opto

eye which is formed by photo transistor and some resistor chain. This comparator will make use

sense of the photo device and also the output delivered from the mains comparator-1.

CUT OFF LOGIC:- This block plays a major role in lightning up the lamp by deciding the

status of different other blocks and according to which the voltage will be given to the lamp

driver circuit. This block will not enable the battery to discharge in a quick time and hence

improving the life of the battery.

LAMP DRIVER:-This consists of basic circuitry which contains some transistors and other

basic elements which constitute the illumination of the lamp of a specified rating. It also consists

of warming circuit to avoid frequent failure of lamps and this warming circuit will have some

basic elements like diode and a resistor. It also control sudden inrush of current during switching

times.

9

CHAPTER-2

DESIGN DETAILS

10

DESIGN DETAILS:

2.1. TRANSFORMER:- In this circuit the transformer used is of step down type which

consumes 230 volts as input(primary side) and produces output of 10volts,1 amp. This can be

termed as 230votls primary,10v 1amp secondary step down transformer.

2.2. 7808 VOLTAGE REGULATOR:- In the name itself indicates that it is a 8 volt regulator

i.e., 78 indicates series and 08 indicates voltage. This regulator has three pins for three different

purposes and we can name these as pin1, pin2 and pin3. Pin1 is for input , and pin2 is for ground

and finally pin3 is for output. The below diagram indicates the voltage regulator.

If we consider about the pin configuration of this voltage regulator as already stated first two

digits indicate the series and the last two for indicating voltage. The following figures are the

front view and bottom view of the device

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2.3. LM324 QUAD OPERATIONAL AMPLIFIER: This device consists of 14 pins. It

consists of four independent, high gain, internally frequency compensated operational amplifiers

which were designed specifically to operate from a single power supply over a wide range of

voltages. Operation from split power supplies is also possible and the low power supply current

drain is independent of the magnitude of the power supply voltage.

Application areas include transducer amplifiers, dc gain blocks and

all the conventional operation amplifier circuits which now can be more easily implemented in

single power supply systems. For example, the lm324 series can be directly operated off of the

standard +5v power supply voltage which is used in digital systems and will easily provide the

required interface electronics without requiring the additional +15v power supplies.

The below is the figure of LM 324 quad operational device showing the function of each pin

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2.4. PHOTO-TRANSISTOR:

Like diodes, all transistors are light-sensitive. Phototransistors are designed specifically to take

advantage of this fact. The most-common variant is an npn bipolar transistor with an exposed

base region. Here, light striking the base replaces what would ordinarily be voltage applied at the

base. So, a photo transistor amplifies variations in the light striking it. Note that photo transistors

may or may not have a base lead(if they do the base lead allows to bias the photo transistor light

response).

Fig(a) Fig(b)

Like other transistors it has three terminals namely collector, base and emitter. But in this photo

transistor we don’t give much difference for base i.e., diagrammatically there is no presence of

base. Light is injected through the base region and arrow in the above figure indicates that the

light is being injected in to the photo device called photo transistor. In the figure (a) a photo

diode is connected across the photo transistor and operational concept of photo diode is same as

that of photo transistor. We can replace photo diode with photo transistor.

13

Design fundamentals for Photo Transistor circuits:

The common-emitter amplifier circuit (Fig. 1) generates an output which transitions from a high

state to a low state when light in the near-infrared range is detected by the phototransistor. The

wavelength range for light in the near infrared region is about 700 nanometers (nm) to 1100 nm.

The output is created by connecting a resistor between the voltage supply and the collector pin of

the component. The output voltage is read at the terminal of the collector. It is called an amplifier

circuit because the current generated in the component when light is detected is very small.

However, the component has an internal amplifier (in this case a phototransistor) which

magnifies this current to useful levels.

The common-collector amplifier (Fig. 2) generates an output which transitions from a low state

to a high state when IR light is detected by the phototransistor. The output is created by

connecting a resistor between the emitter pin of the component and ground. The output is read at

the emitter terminal.

14

In both circuits the phototransistor can be used in two modes, an active mode and a switch mode.

Operating in the active mode means that the phototransistor generates a response proportional to

the light received by the component up to a certain light level. When the amount of light

surpasses that level, the phototransistor becomes saturated and the output will not increase even

as the light level increases. This mode is useful in applications where it is desired to detect two

levels of inputs for comparison. Operating in the switch mode means that the phototransistor will

either be ”off” (cut-off) or ”on” (saturated) in response to the light.

This mode is useful when a digital output is required for object detection or encoder sensing.

By adjusting the load resistor in the amplifier circuit one can set the mode of operation. The

correct value for the resistor can be determined by the following equations:

Active Mode: VCC> RLx ICC

Switch Mode: VCC< RLx ICC

15

Typically a resistor value of 5kΩ or higher is adequate to operate the phototransistor in the

switch mode. The high level output voltage in the switching mode should equal the supply

voltage. The low level output voltage in the switching mode should be less than 0.8 Volts.

The circuits just described can be applied to all two pin IR phototransistor components that

Fairchild Semiconductor offers. They can also be applied to three pin phototransistor

components that have a base lead. A third phototransistor circuit (Fig. 3) involves only the three

leaded components that have a base connection. Access to the base allows a base-emitter resistor

to be connected. A high RBE value will prevent low levels of light from triggering the

phototransistor and help provide a more digital output. The collector and emitter terminals can be

connected in the same way as described above. Fairchild Semiconductor offers the three leaded

component in a hermetic (metal can) package only.

16

Finally, the phototransistor should be biased (voltage applied to VCC) with 5 Volts. The

maximum bias is 16 V, however the performance of the component doesn’t change with a

greater bias except when the phototransistor is used as a switch -- the high level output will equal

the higher setting.

Note: Fairchild’s photo Darlington products can be used in the same manner as the

phototransistors. The photodarlingtons will provide greater outputs for the same light level

because they have a greater internal gain, but will have a higher saturation voltage and slower

turnoff than the phototransistor devices.

2.5. 2N2222 NPN TRANSISTOR:

In designing purpose these 2n222 transistor are specified for high current and low voltage values.

And these are used for linear amplification and switching.

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2.6. 2955 PNP POWER TRANSISTOR:

This single-in-line package consists of a circuit mounted on a lead frame and encapsulated within

a plastic compound. The compound will withstand soldering temperature with no deformation,

and circuit performance characteristics will remain stable when operated in high humidity

conditions. Leads require no additional cleaning or processing when used in soldered assembly.

In the figure it is observed that there are three terminals as obvious as a normal transistor. The

three terminals can be named as emitter, base and collector. There are some ratings which are

defined for this specific 2955 transistor and some of the ratings are as follows

1. Collector base voltage

2. Collector emitter voltage

3. Base emitter voltage

4. Continuous collector current

5. Continuous base current

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2.7. BD140 PNP MEDIUM POWER TRANSISTOR:

It is designed for use as audio amplifiers and drivers utilizing complementary or quasi

Complementary circuits.

• DC Current Gain — hFE = 40 (Min) @ IC = 0.15 Adc

• BD 136, 138, 140 are complementary with BD 135, 137, 139

The below figure shows the pin configuration of BD140 pnp medium power transistor. Here the

pin 1 shows that emitter, pin 2 shows collector and pin 3 shows the base.

2.8. SL100 NPN TRANSISTOR:

Here the transistor that is used to be npn type and it is general purpose medium power transistor.

The design of this transistor is as same as the normal transistor .

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2.9. 3.3 VOLT ZENER DIODE:

The use of a 3.3-V device in a 5-V system has become a common theme with today’s denser,

faster microcontrollers. Using these processors in a 5-V design, without redesigning the end

system for 3.3 V.

In recent years there have been numerous voltage-regulation solutions introduced specifically to

generate 3.3-V supply power from 5-V supplies. These are broken into three main groups:

passive zener diode, active switching and active linear regulators. The zener diode is the simplest

and least expensive solution, but has a slower response than an active supply. It also requires

strong current biasing, and may dissipate a good deal of power, as the difference voltage is

dropped across the diode and bias resistor.

The nature of this solution is that the current supplied by the 5-V power supply is nearly

constant; this means that as the load current decreases the current is shunted through the zener

diode. The increased current through the zener diode causes a corresponding increase in the

voltage across the zener diode. Because of this, when using this solution, care must be given to

insure that variations in the load do not result in excessive variation in the regulated voltage. A

complete analysis of system current requirements must be completed to properly specify the bias

resistor and zener diode. Care must be taken to include all devices that draw current from the

3.3-V power plane. It is also very important that the zener diode be able to handle the maximum

current shunted from the load. If the zener diode is over driven to the point of failure, the only

protection left for the microcontroller is the series bias resistor which will typically result in the

microcontroller being driven with a voltage over the maximum rated Vcc of 3.6 V.

20

CHAPTER-3

CIRCUIT DIAGRAM WITH EXPLANATION OF EACH BLOCK

21

CIRCUIT DIAGRAM:

22

CIRCUIT OPERATION

In comparator IC1a, the non inverting input is connected to the potential divider formed by R1 and VR1.

VR1 is used to set the offset voltage proportional to the ambient light intensity. This will create a

threshold or datum level for the room light. The inverting input is connected to a DC voltage derived

from mains through diode D3 and RC network. When there is power v- will be higher than v+ and output

of IC2a goes low which pulls down the input voltage of the follower IC2d. This will switch off the lamp

driver. When there is no power, V+ will be more positive than V- and output goes high. Now the

voltage level at V+ of IC2 depends up on the output of comparators IC2b and IC2c. The combinations of

D4, D5, D7 and R8 form the wired AND logic. When outputs of all comparators are high, the V+ at the

follower will be high which in turn switches on the lamp driver.

In the second comparator IC2b, the non-inverting input is connected to the threshold level setting of the

ambient light intensity. The inverting input is connected to an opto-eye. The opto-eye is formed by a

photo transistor T5 and terminating resistor chain R2 and R3. Instead of photo transistor, photo diode

can be used with slight changes in R2 and R3 to increase the sensitivity of photo diode D10, in reverse

bias configuration. When light falls on the photo transistor, it conducts and a voltage will be developed

across the resistors due to the low opto-current. The conductivity of the photo transistor varies with the

intensity of the ambient light. So when the light intensity of the room is more than the set level, the

voltage at V- will be more than that of V+. The output of comparator IC2b will be low and it pulls down

the V+ of the follower IC2d to low value such that it switches off the lamp driver. When room is dark

enough, the comparator output will be high. Now if output of the comparator IC2a is also high, the

voltage at V+ will depend up on the output of IC2c.

23

Comparator IC2c controls the threshold of the battery to increase its life. In the charged condition the

voltage at V+ will be more than at V- and the comparator output will be high. Assuming the outputs of

IC2b and IC2a are high, the voltage at V+ of IC2d will switch on transistor T4 which keeps the opto eye

insensitive to the emergency light switched on. The inverting input of IC2c is connected to a voltage

reference of 3.3v. When the battery is getting discharged, the voltage at V+ will also come down. When

the voltage reaches 5.5v the V+ will be adjusted by preset VR2 to just below 3.3v. Now the output of

IC2d will go low which in turn switches off the lamp. When the lamp is switched off the voltage may go

up and there is a chance of oscillation in the comparator. To avoid this, enough hysterisis is provided by

resistors R5 and R6.

The lamp driver is designed to deliver safely an output of 75w from a 6v lamp. When the output of IC2d

is high, transistor T1 will be on and the collector will be pulled down to vsat. Now T2 and T3 will be on

and the lamp will be switched on. Diode D9 and resistor R16 form a warming circuit to avoid frequent

failure of lamps. This will keep the filament a little warm and hence control the inrush current during

switching. This will increase the life of the battery also.

24

CIRCUIT DIAGRAM WITH OPERATIONAL DETAILS:

3.1. LM324 QUAD OPERATIONAL AMPLIFIER:

3.1.1. Introduction:

The LM324 contains four independent high gain operational amplifiers with internal frequency

compensation. The four op-amps operate over a wide voltage range from a single

Power supply. Also use a split power supply. The device has low power supply current drain,

regardless of the power supply voltage. The low power drain also makes the LM324 a good

choice for battery operation.

The LM324 series are low-cost, quad operational amplifiers with true differential inputs. They

have several distinct advantages over standard operational amplifier types in single supply

applications.

3.1.2. General Description

The LM124 series consists of four independent, high gain, internally frequency compensated

operational amplifiers which were designed specifically to operate from a single power supply

over a wide range of voltages. Operation from split power supplies is also possible and the low

power supply current drain is independent of the magnitude of the power supply voltage.

Application areas include transducer amplifiers, DC gain blocks and all the conventional op amp

circuits which now can be more easily implemented in single power supply systems. For

example, the LM124 series can be directly operated off of the standard a5V power supply

voltage which is used in digital systems and will easily provide the required interface electronics

without requiring the additional g15V power supplies.

3.1.3. Unique Characteristics

In the linear mode the input common-mode voltage range includes ground and the output voltage can also swing to ground, even though operated from only a single power supply voltage

The unity gain cross frequency is temperature compensated

25

The input bias current is also temperature compensated

3.1.4. Advantages

Eliminates need for dual supplies Four internally compensated op amps in a single package

Allows directly sensing near GND and VOUT also goes to GND

Compatible with all forms of logic

Power drain suitable for battery operation

3.1.5. Features

Internally frequency-compensated for unity gain

Large DC voltage gain:100 dB

Wide bandwidth(unity gain):1 MHz(temperature-compensated) Wide power supply

range:

Single supply:3VDC to 32 VDC

Dual supplies:±1.5VDC to ±16VDC

Differential input voltage range equal to the power supply voltage

Power drain suitable for battery operation

Large output voltage swing:0VDC to VCC-1.5VDC

3.1.6. Representative circuit diagram of LM324:

26

3.1.7. Circuit Description:

The LM324 series is made using four internally compensated, two stage operational amplifiers.

The first stage of each consists of differential input devices Q20 and Q18 with input buffer

transistors Q21 and Q17 and the differential to single ended converter Q3 and Q4. The first

stage performs not only the first stage gain function but also performs the level shifting and Tran

conductance reduction functions. By reducing the Tran conductance, a smaller compensation

capacitor (only 5.0 pF) can be employed, thus saving chip area. The Tran conductance reduction

is accomplished by splitting the collectors of Q20 and Q18. Another feature of this input stage is

that the input common mode range can include the negative supply or ground, in single supply

operation, without saturating either the input devices or the differential to single ended converter.

The second stage consists of a standard current source load amplifier stage.

3.1.8. Some performance characteristics of LM324 quad operational amplifier:

27

28

29

30

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3.2. 7808 +8V REGULATOR

3.2.1. General description:

The linear regulator is the basic building block of nearly every power supply used in electronics.

The IC linear regulator is so easy to use that it is virtually foolproof, and so inexpensive that it is

usually one of the cheapest components in an electronic assembly.

This paper will present information that gives the user greater understanding of how a linear

regulator works, and will help to de-mystify regulator specifications and applications. Some

typical circuits will be presented to highlight the commercial regulators that are currently

available.

The primary focus of the new product examples is in the area of Low-dropout regulators, which

offer great advantages over standard regulators in many applications.

Every electronic circuit is designed to operate off of some supply voltage, which is usually

assumed to be constant. A voltage regulator provides this constant DC output voltage and

contains circuitry that continuously holds the output voltage at the design value regardless of

changes in load current or input voltage (this assumes that the load current and input voltage are

within the specified operating range fourth part).

A voltage regulator is a constant voltage source that adjusts its internal resistance to any occurring

changes of load resistance to provide a constant voltage at the regulator output.

3.2.2. The 78xx Series of Regulators

There are many types of regulator IC and each type will have different pin-outs and will need to

be connected up slightly differently.  Therefore, this article will only look at one of the common

ranges of regulator, the 78xx series.

There are seven regulators in the 78xx series, and each can pass up to 1A to any connected

circuit.  There are also regulators with similar type numbers that can pass a higher or lower

current, as shown in the table below.  In addition, variable regulators are available, as are

regulators that can provide negative regulation voltages for circuits that require them.

32

Type Number    Regulation Voltage    Maximum Current    Minimum Input Voltage

78L05 +5V 0.1A +7V

78L12 +12V 0.1A +14.5V

78L15 +15V 0.1A +17.5V

78M05 +5V 0.5A +7V

78M12 +12V 0.5A +14.5V

78M15 +15V 0.5A +17.5V

7805 +5V 1A +7V

7806 +6V 1A +8V

7808 +8V 1A +10.5V

7812 +12V 1A +14.5V

7815 +15V 1A +17.5V

7824 +24V 1A +26V

78S05 +5V 2A +8V

78S09 +9V 2A +12V

78S12 +12V 2A +15V

78S15 +15V 2A +18V

 

3.2.3. Ripple Voltage

If you are using a regulator after the smoothing block of the power supply, then you shouldn't

need to worry about the ripple voltage, since the whole point of using a regulator is to get a

stable, accurate, known voltage for your circuits!  However, if the ripple voltage is too large and

the input voltage to the regulator falls below the regulated voltage of the regulator, then of course

the regulator will not be able to produce the correct regulated voltage.  In fact, the input voltage

to a regulator should usually be at least 2V above the regulated voltage.  In our power supply

circuit, the input to the 7805 regulator is around 12V, and the regulation voltage is 5V, so there is

plenty of headroom.  The maximum input voltage to any 78xx regulator is 30V.

33

3.3. SL100 NPN TRANSISTOR:

The device specification of SL100 transistor is as follows:

TYPE: SL100

POLARITY: NPN

APPLICATION: General purpose medium power transistor

PACKAGE: TO-39

Some of the important characteristics of this transistor are as given below:

Collector-Emitter voltage

Collector-Base voltage

Emitter-Base voltage

Total power dissipation @ Ta=25deg Celsius

Collector current

Operating & storage junction temperature

3.4. 2n2222 NPN TRANSISTOR:

3.4.1. General description:

In general depends upon their operation transistor are two types and those are pnp and npn types.

These can be shown in the below forms:

34

A bipolar junction transistor consists of three regions of doped semiconductors. A small current

in the center or base region can be used to control a larger current flowing between the end

regions (emitter and collector). The device can be characterized as a current amplifier, having

many applications for amplification and switching.

3.4.2. NPN Characteristic Curves:

35

For IB = 10 µA, 20 µA, 30 µA, ... , 80 µA

3.4.3. PHOTO SENSOR

The photo sensor circuit operates in the following way: When light shines on the Cadmium

Sulfide photocell, the resistance across the leads of the photocell decreases. This decrease in

resistance brings the base of the 2N2222 transistor "closer" to ground and, therefore, starts to

turn off the transistor. If enough light illuminates the photocell, the transistor will shut off and

the input pin to the PIC16C84 will be pulled "high" by the 10K resistor. Otherwise, as long as

the transistor is on, the input pin of the PIC16C84 will be pulled "low" through the collector-

emitter junction of the transistor. The 47K ohm variable resistor is used to set the threshold level

of the light detection circuit. The 150 ohm resistor is used to keep from frying the transistor in

the event that the 47K ohm variable resistor is adjusted such that there is NO resistance across

the 47K ohm variable resistor (the base of an NPN transistor is shorted to the emitter via a diode

- if you were to put a regular diode in the forward bias position across a battery or power supply,

you would see exactly what would happen to the transistor if the 150 ohm resistor was left out

and the 47K ohm variable resistor was adjusted to "NO" resistance).

36

3.5. PHOTO TRANSISTOR:

3.5.1. General characteristics and features:

Phototransistors are solid state light detectors that possess internal gain. This makes them much

more sensitive than photodiodes of comparably sized area. These devices can be used to provide

either an analog or digital output signal. This family of detectors offers the following general

characteristics and features:

• Low cost visible and near-IR photo detection

• Available with gains from 100 to over 1500

• Moderately fast response times

• Available in a wide range of packages including epoxy coated, transfer molded, cast, hermetic

packages, and in chip form

• Usable with almost any visible or near infrared light source such as IREDs; neon, fluorescent,

incandescent bulbs; lasers; flame

Sources; sunlight; etc.

• Same general electrical characteristics as familiar signal transistors (except that incident light

replaces base drive current)

37

3.5.2. Applications

Phototransistors can be used as ambient light detectors. When used with a controllable light

source, typically an IRED, they are often employed as the detector element for optoisolators and

Tranmissive or reflective optical switches. Typical configurations include:

3.5.3. Optoisolator

The optoisolator is similar to a transformer in that the output is electronically isolated from the

input.

3.5.4. Optical Switch

An object is detected when it enters the gap of the optical switch and blocks the light path

between the emitter and detector.

3.5.5. Retro Sensor

The retro sensor detects the presence of an object by generating light and then looking for its

reflectance off of the object

to be sensed.

38

3.6. RESULT:

Hence the IC controlled emergency light can be constructed from the components

mentioned above in which automatic feature is involved by making use of the photo devices like

photo diode, photo transistor, photo sensor, e.t.c.

3.7. CONCLUSION:

This circuit can also be extended to a higher output voltage for which the charging voltage and

the load are to be chosen accordingly. All the other components involved in the circuit are of

same value.

3.8. FUTURE SCOPE: The emergency light which uses IC is a reliable one comparing to other

non IC emergency lights and there is an automatic feature by which itself get glows.

3.9. REFERENCES:

Websites:

www.google.com

www.wikipedia.org

www.datasheetcatalog.com

Text books:

Linear and Digital circuit IC applications by Roy Choudary

Electronic devices and circuits by Jocab Milliman

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