DECLARATION

I declare that this project is done by me and has never been

submitted the best of my knowledge, any where for any

certificate. All literature sighted have been acknowledged

through the references.

…………………..

……………………..

Students sign

Date

1

CERTIFICATION

This is to certify that this project titled “Design and

construction of an aquarium probe” meets the standard

set for a project in Physics department in partial fulfillment

for the award of B.Sc. Hons

……………………….

……………………….

Project supervisor

Date

……………………….

………………………..

Head of Department

Date

2

DEDICATION

This project work is dedicated to my entire family especially my

parents for their maximum cooperation and support throughout

the period of writing this research work, and those who

contributed to the success of this work. God bless you all

(ameen).

3

ACKNOWLEDGEMENT

I wish to give my profound thanks to God Almighty who

made it possible to reach this level of life and give me power

to produce this piece of work. My sincere thanks also goes to

my helpful lecturer in person of Mal. Muazu and Mal

Samaela of electrical electronics department sokoto state

ply technic, who helped me greatly in doing this work. I am

also grateful to the Head of department and all other

lecturers for helping me in various ways during my stay as a

student in the department. I also say thank you to all my

friends and colleagues in the department, your assistances

are well appreciated.

Finally, I thank anybody that assisted indirectly to

accomplish this work.

4

ABSTRACT

The circuit of aquatic probe described here can monitor the

temperature of water and indicate the rise in temperature

through audiovisual indicators. A readily available signal diode

1N34 is used in the circuit as the temperature sensing probe. The

resistance of the diode depends on the temperature in its vicinity.

Typically, the diode can generate around 600 mV when a

potential difference is applied to its terminals. For each degree

centigrade rise in temperature, the diode generates 2 mVoutput

voltage. That is, at 5°C, it is 10 mV, which rises to 70 mV when

the temperature is 35°C.

5

CHAPTER ONE

INTRODUCTION

1.1 AIM OF THE PROJECT

The aim of this project is to design, construct and test an

Aquarium Probe that can be used in detecting the temperature

level in an aquarium and generating a warning alarm when the

temperature is very high.

1.2 PROJECT MOTIVATION

The main motivation for carrying out this work comes up with a

simple device that can assist in knowing the temperature of

Aquarium at any time. The temperature of water has profound

effect because fish cannot breed above or below the critical

temperature limits.

Temperature between 24°C and 33°C is found to be the best to

induce spawning in fishes. This particular temperature range is

also necessary for the healthy growth of nursery fish fries (young

fishes). Rise of water temperature due to sunlight may adversely

affect the fish rearing process.

1.3 SCOPE OF THE PROJECT

6

The scope of this project is the design of the circuit of the

aquarium probe, constructing the designed circuit and testing the

ability of the constructed circuit to work. The designed circuit

should be able to work as an approximate indicator of the

aquarium temperature by rising alarm when the temperature is

high.

1.4 PROBLEM DEFINATION AND METHODOLOGY

The actual problem that this project is expected to solve is that of

designing an aquarium probe circuit that could be used for

helping us to know the level of temperature. The methodology

that would be used for achieving the goals is as follows;

• Designing the comparator circuit.

• Designing the LM3915 circuit

• Constructing the whole circuit

• Packaging the whole system

1.5 LITERATURE REVIEW

Some people carried out projects in this area of sound

generation as well as switching due to trigger signal from a given

source. In1996, Baba Hassan (ABU) designed and constructed

temperature sensor circuit that can produce an audio alarm to

show rise in body temperature. Also in the year 2001, Jonathan

Benjamin (ABU) designed and constructed a comparator circuit

that compares a variable signal from the conductivity of material

and a fixed voltage to produce an output that switch on a siren.

7

Furthermore, Ibrahim Umar designed and constructed a fire and

burglar alarm which gives an alarm as a result of change in

temperature or opening of door by a burglar.

Therefore, this work of design and construction of stress meter is

another version of comparator circuit that can help us have an

idea about the temperature of aquarium.

1.6 PROJECT OUTLINE

This project report should be arranged in five different chapters,

which are as follows;

Chapter one would deal with basic introduction on the project.

Chapter two would cover the general theoretical background on

which the project lies. Chapter three will deal with the design

procedure of the whole circuit. Chapter four covers the

construction and testing of the circuit. Chapter five gives the

conclusion, recommendation for further work and references.

8

CHAPTEWR TWO

THEORITICAL BACKGROUND

2.0 INTRODUCTION

This chapter is going to discuss the basic theoretical aspects that

are the foundation to this project work of design and construction

of Aquarium Probe. We would see the basic principles of the

operation of the circuit’s parts as well as the components used

and their characteristics.

2.1 GENERAL OVERVIEW OF OPERATIONAL AMPLIFIERS

(COMPARATOR)

An operational amplifier or op-amp is an electronic circuit module,

which has a non-inverting input (+), an inverting input (-) and one

output.

Originally, op-amps were so named because they were used to

model the basic mathematical operations addition, subtraction,

integration, differentiation etc in electronic analog computers. In

9

this sense a true operational amplifier is an ideal circuit element

(Stout D.F 1976).

The particular op-amp requires either two supply-voltage sources

of never more than (18 volts(V) for 741C), (22V for 741, 741A and

741E) each, or a center/centre tapped equivalent voltage source

with each half supplying never more than the same voltage.

The maximum permitted power dissipation is 500 milli-Watt

(mW). The maximum input voltage must never be more than 15V,

with the maximum permitted differential voltage being 30 V. The

maximum storage temperature range permitted for all these op-

amps is (-65Oc to +150Oc) . The actual operating ambient

temperature for 741C and 741E is (OOc to 70Oc), and it is (-55Oc to

+125Oc) for 741 and 741A.

A typical circuit symbol for an op-amp looks like this:

Fig: 2.1; operational amplifier

Its terminals are: V+: non-inverting input V−: inverting input Vout:

output VS+: positive power supply VS−: negative power supply

10

Often these pins are left out of the diagram for clarity, and the

power configuration is described or assumed from the circuit.

The input pin polarity is often reversed in diagrams for clarity. In

this case, the power supply pins remain in the same position; the

more positive power pin is always on the top, and the more

negative on the bottom. The entire symbol is not flipped; just the

inputs (Malvino A.P 1979).

2.2 OP-AMP INVERTING AND NON-INVERTING

CONFIGURATIONS

All the more complicated Op Amp configurations are based on two

basic ones the Inverting and Non Inverting configurations. An

understanding of these two configurations, makes it much simpler

to understand the more advanced configurations (Stout D.F

1976).

Inverting Op Amp:

11

Fig:2.2; inverting op-amp

The closed loop gain of an Inverting Op Amp is

R1 = Rin

Af = -Rf/R1 (1)

Input Impedance of this configuration is Zin = Rin (because V− is a

virtual ground, no current flows into the Op Amp ideally.)

To get formula (1) we take a KVL loop with Vin, R1 and the inputs

of the Op Amp. This gives

Vin =iinR 1 + id (2)

Where vd is v + − v − the voltage between the non-inverting and

inverting inputs. But for ideal Op Amps vd is approximately zero.

vd is zero because the input impedance is infinite, which means

the current through the impedance must be zero by Ohms law.

The zero current means that there is no voltage drop across the

impedance. This gives:

iin = vin/R1 (3)

Using this idea.

if = Vout/Rf (4)

If we take KCL (kirchoff’s voltage law) at the inverting input then

12

iin = id - if (5)

For an ideal Op Amp there is no input current because there is

infinite resistance. So using equations 3 and 4.

-Vin/R1=vout/Rf (6)

Since

Af=Vout/Vin=-Rf/R1 (7)

Non-Inverting Op Amp:

The closed loop gain of an Non Inverting Op Amp is

Af =1+R2/R1 (8)

The input impedance of this configuration is Zin = ∞ (realistically,

the input impedance of the Op-Amp itself, 1 MΩ to 1012 Ω).

Fig: 2.3; non inverting op-amp

Ideal Op Amp Derivation

Take a KVL with the inputs of the Op Amp and R1.

13

Vin =Vd + VR1

(9)

But Vd is zero since the Op Amp is ideal. Therefore

Vin =VR1 (10)

According to voltage divider rule

VR1 =VoutR1/R1+R2

(11)

Substitute equation 11 into 10.

Vin = Vout R1/R1+R2 (12)

Thus

Af =Vout/Vin=1+R2/R1

(13)

2.3 LM 3915 INTEGRATED CIRCUIT

The LM3915 is a monolithic integrated circuit that senses analog

voltage levels and drives ten LEDs, LCDs or vacuum fluorescent

displays, providing a logarithmic 3 dB/step analog display. One

pin changes the display from a bar graph to a moving dot display.

LED current drive is regulated and programmable, eliminating the

need for current limiting resistors. The whole display system can

operate from a single supply as low as 3V or as high as 25V. The

IC contains an adjustable voltage reference and an accurate ten-

14

step voltage divider. The high-impedance input buffer accepts

signals down to ground and up to within 1.5V of the positive

supply. Further, it needs no protection against inputs of ±35V.

The input buffer drives 10 individual comparators referenced to

the precision divider. Accuracy is typically better than 1 dB. The

LM3915’s 3 dB/step display is suited for signals with wide

dynamic range, such as audio level, power, light intensity or

vibration. Audio applications include average or peak level

indicators, power meters and RF signal strength meters.

Replacing conventional meters with an LED bar graph results in a

faster responding, more rugged display with high visibility that

retains the ease of interpretation of an analog display.The

LM3915 is extremely easy to apply. A 1.2V full-scale meter

requires only one resistor in addition to the ten LEDs. One more

resistor programs the full-scale anywhere from 1.2V to 12V

independent of supply voltage. LED brightness is easily controlled

with a single pot. The LM3915 is very versatile. The outputs can

drive LCDs, vacuum fluorescents and incandescent bulbs as well

as LEDs of any color. Multiple devices can be cascaded for a dot

or bar mode display with a range of 60 or 90 dB. LM3915s can

also be cascaded with LM3914s for a linear/log display or with

LM3916s for an extended-range VU meter

(datasheetcatalog.com).

Fig 2. 4 gives the pin-out of the Lm 3915.

15

Fig: 2.4; pin-out LM3915

Features

3 dB/step, 30 dB range

Drives LEDs, LCDs, or vacuum fluorescents

Bar or dot display mode externally selectable by user

Expandable to displays of 90 dB

Internal voltage reference from 1.2V to 12V

Operates with single supply of 3V to 25V

Inputs operate down to ground

Output current programmable from 1 mA to 30 mA

Input withstands ±35V without damage or false outputs

Outputs are current regulated, open collectors

Directly drives TTL or CMOS

16

The internal 10-step divider is floating and can be referenced

to a wide range of voltages

The LM3915 is rated for operation from 0°C to +70°C. The

LM3915N-1 is available in an 18-lead molded DIP package.

2.4 DIODE ( )

A two-terminal semiconductor (rectifying) device that exhibits a

nonlinear current-voltage characteristic. The function of a diode is

to allow current in one direction and to block current in the

opposite direction. The terminals of a diode are called the anode

and cathode. There are two kinds of semiconductor diodes: a P-N

junction diode, which forms an electrical barrier at the interface

between N- and P-type semiconductor layers, and a Schottky

diode, whose barrier is formed between metal and semiconductor

regions (Jun J.L and Jians S.Y 1998).

Semiconductors are crystals that, in their pure state, are resistive

(that is, their electrical properties lie between those of conductors

and insulators) -- but when the proper impurities are added (this

process is called doping) in trace amounts (often measured in

parts per billion), display interesting and useful properties.

2.5 TYPES OF DIODES

Basically there are so many types of diodes.but listed below are

the common types used in electronic circuit.

i. Silicon Diodes

ii. Germanium Diodes

17

iii. Photodiodes

iv. Light-Emitting Diodes (LEDs)

v. Zener diodes

Note that the scope of this project is limited to the use of light

emitting diodes(LEDs.)

2.6 LIGHT-EMITTING DIODES (LEDs)

A light-emitting diode (LED) is an electronic light source.

Luminescence from an electrically stimulated crystal had been

observed as early as 1907.

All early devices emitted low-intensity red light, but modern LEDs

are available across the visible, ultraviolet and infra red

wavelengths, with very high brightness.

LEDs are based on the semiconductor diode. When the diode is

forward biased (switched on), electrons are able to recombine

with holes and energy is released in the form of light. This effect

is called electroluminescence and the color of the light is

determined by the energy gap of the semiconductor. The LED is

usually small in area (less than 1 mm2) with integrated optical

components to shape its radiation pattern and assist in reflection

(Ivan. M 2008).

LEDs present many advantages over traditional light sources

including lower energy consumption, longer lifetime, improved

robustness, smaller size and faster switching. However, they are

18

relatively expensive and require more precise current and heat

management than traditional light sources.

Applications of LEDs are diverse. They are used as low-energy

indicators but also for replacements for traditional light sources in

general lighting, automotive lighting and traffic signals. The

compact size of LEDs has allowed new text and video displays

and sensors to be developed, while their high switching rates are

useful in communications technology.

2.7 VIEW OF PHYSICS IN LIGHT EMITTING DIODES

Like a normal diode, the LED consists of a chip of semiconducting

material impregnated, or doped, with impurities to create a p-n

junction. As in other diodes, current flows easily from the p-side,

or anode, to the n-side, or cathode, but not in the reverse

direction. Charge-carriers—electrons and holes—flow into the

junction from electrodes with different voltages. When an electron

meets a hole, it falls into a lower energy level, and releases

energy in the form of a photon.

The wavelength of the light emitted, and therefore its color,

depends on the band gap energy of the materials forming the p-n

junction. In silicon or germanium diodes, the electrons and holes

recombine by a non-radiative transition which produces no optical

19

emission, because these are indirect band gap materials. The

materials used for the LED have a direct band gap with energies

corresponding to near-infrared, visible or near-ultraviolet light.

LED development began with infrared and red devices made with

gallium arsenide. Advances in materials science have made

possible the production of devices with ever-shorter wavelengths,

producing light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode

attached to the p-type layer deposited on its surface. P-type

substrates, while less common, occur as well. Most materials used

for LED production have very high refractive indices. This means

that much light will be reflected back in to the material at the

material/air surface interface. Therefore Light extraction in LEDs

is an important aspect of LED production, subject to much

research and development.

2.8 ADVANTAGES OF LIGHT EMITTING DIODE

2.8.1 Efficiency: LEDs produce more light per watt than

incandescent bulbs (Schubert E. F 2005).

2.8.2 Size: LEDs can be very small (smaller than 2 mm2) and are

easily populated onto printed circuit boards (598 SERIES

Datasheet).

2.8.3 Cycling: LEDs are ideal for use in applications that are

subject to frequent on-off cycling, unlike fluorescent lamps

that burn out more quickly when cycled frequently, or HID

lamps that require a long time before restarting.

20

2.8.4 Cool light: In contrast to most light sources, LEDs radiate

very little heat in the form of IR that can cause damage to

sensitive objects or fabrics. Wasted energy is dispersed as heat

through the base of the LED.

2.8.5 Lifetime: LEDs can have a relatively long useful life. One

report estimates 35,000 to 50,000 hours of useful life, though

time to complete failure may be longer. Fluorescent tubes

typically are rated at about 10,000 to 15,000 hours, depending

partly on the conditions of use, and incandescent light bulbs at

1,000–2,000 hours.

2.8.6 Shock resistance: LEDs, being solid state components,

are difficult to damage with external shock, unlike fluorescent and

incandescent bulbs which are fragile.

2.9 Disadvantages

2.9.1 Temperature dependence: LED performance largely

depends on the ambient temperature of the operating

environment. Over-driving the LED in high ambient

temperatures may result in overheating of the LED package,

eventually leading to device failure. Adequate heat-sinking is

required to maintain long life. This is especially important

when considering automotive, medical, and military

applications where the device must operate over a large

range of temperatures, and is required to have a low failure

rate.

2.9.2 Voltage sensitivity: LEDs must be supplied with the

voltage above the threshold and a current below the rating.

21

This can involve series resistors or current-regulated power

supplies.

2.9.3 Light quality: Most cool-white LEDs have spectra that

differ significantly from a black body radiator like the sun or

an incandescent light. The spike at 460 nm and dip at

500 nm can cause the color of objects to be perceived

differently under cool-white LED illumination than sunlight or

incandescent sources, due to metamerism,( James A.W 2007)

red surfaces being rendered particularly badly by typical

phosphor based cool-white LEDs. However, the color

rendering properties of common fluorescent lamps are often

inferior to what is now available in state-of-art white LEDs.

2.9.4 Area light source: LEDs do not approximate a “point

source” of light, but rather a lambertian distribution. So LEDs

are difficult to use in applications requiring a spherical light

field. LEDs are not capable of providing divergence below a

few degrees. This is contrasted with lasers, which can

produce beams with divergences of 0.2 degrees or less

(Hecht E. 2002).

2.9.5 Blue Hazard: There is increasing concern that blue

LEDs and cool-white LEDs are now capable of exceeding safe

limits of the so-called blue-light hazard as defined in eye

safety specifications such as ANSI/IESNA RP-27.1-05:

Recommended Practice for Photobiological Safety for Lamp

and Lamp Systems (Sciencenews.org. 2006 and Texyt.com. 2007).

22

2.10Applications

2.10.1 Indicators and signs

The low energy consumption, low maintenance and small size of

modern LEDs has led to applications as status indicators and

displays on a variety of equipment and installations. Large area

LED displays are used as stadium displays and as dynamic

decorative displays. Thin, lightweight message displays are used

at airports and railway stations, and as destination displays for

trains, buses, trams, and ferries.

The single color light is well suited for traffic lights and signals,

exit signs, emergency vehicle lighting, ships' lanterns and LED-

based Christmas lights. Red or yellow LEDs are used in indicator

and alphanumeric displays in environments where night vision

must be retained: aircraft cockpits, submarine and ship bridges,

astronomy observatories, and in the field, e.g. night time animal

watching and military field use.

Because of their long life and fast switching times, LEDs have

been used for automotive high-mounted brake lights and truck

and bus brake lights and turn signals for some time, but many

vehicles now use LEDs for their rear light clusters. The use of

LEDs also has styling advantages because LEDs are capable of

forming much thinner lights than incandescent lamps with

parabolic reflectors. The significant improvement in the time

taken to light up (perhaps 0.5s faster than an incandescent bulb)

improves safety by giving drivers more time to react. It has been

23

reported that at normal highway speeds this equals one car

length increased reaction time for the car behind. White LED

headlamps are beginning to make an appearance.

Due to the relative cheapness of low output LEDs, they are also

used in many temporary applications such as glowsticks,

throwies, and the photonic textile Lumalive. Artists have also

used LEDs for LED art.

Weather/all-hazards radio receivers with Specific Area Message

Encoding (SAME) have three LEDs: red for warnings, orange for

watches, and yellow for advisories & statements whenever

issued.

2.10.2 Lighting

With the development of high efficiency and high power LEDs it

has become possible to incorporate LEDs in lighting and

illumination. Replacement light bulbs have been made as well as

dedicated fixtures and LED lamps. LEDs are used as street lights

and in other architectural lighting where color changing is used.

The mechanical robustness and long lifetime is used in

automotive lighting on cars, motorcycles and on bicycle lights.

LEDs have been used for lighting of streets and of parking

garages. In 2007, the Italian village Torraca was the first place to

convert its entire illumination system to LEDs (Sciencenews.org.

2006.)

LEDs are also suitable for backlighting for LCD televisions and

lightweight laptop displays and light source for DLP projectors

(See LED TV). RGB LEDs increase the color gamut by as much as

24

45%. Screens for TV and computer displays can be made

increasingly thin using LEDs for backlighting (New York Times

2007).

The lack of IR/heat radiation makes LEDs ideal for stage lights

using banks of RGB LEDs that can easily change color and

decrease heating from traditional stage lighting, as well as

medical lighting where IR-radiation can be harmful.

Since LEDs are small, durable and require little power they are

used in hand held devices such as flashlights. LED strobe lights or

camera flashes operate at a safe, low voltage, as opposed to the

250+ volts commonly found in xenon flashlamp-based lighting.

This is particularly applicable to cameras on mobile phones,

where space is at a premium and bulky voltage-increasing

circuitry is undesirable. LEDs are used for infrared illumination in

night vision applications including security cameras. A ring of

LEDs around a video camera, aimed forward into a retroreflective

background, allows chroma keying in video productions.

LEDs are used for decorative lighting as well. Uses include but are

not limited to indoor/outdoor decor, limousines, cargo trailers,

conversion vans, cruise ships, RVs, boats, automobiles, and utility

trucks. Decorative LED lighting can also come in the form of

lighted company signage and step and aisle lighting in theaters

and auditoriums.

2.11 TRANSISTORS

25

A transistor is a semiconductor device commonly used to

amplify or switch electronic signals. A transistor is made of a solid

piece of a semiconductor material, with at least three terminals

for connection to an external circuit. A voltage or current applied

to one pair of the transistor's terminals changes the current

flowing through another pair of terminals. Because the controlled

(output) power can be much more than the controlling (input)

power, the transistor provides amplification of a signal. Some

transistors are packaged individually but most are found in

integrated circuits.Thus, transistor is the fundamental building

block of modern electronic devices, and its presence is ubiquitous

in modern electronic systems (Ediger L.J 1925).

2.12 TYPES OF TRANSISTORS

Basically there are two types of transistor namely;

i. Bipolar Junction Transistors (BJT)

ii. Field Effect Transistors (FET)

2.12.1 Bipolar Junction Transistors

A bipolar (junction) transistor (BJT) is a three-terminal electronic

device constructed of doped semiconductor material and may be

used in amplifying or switching applications. Bipolar transistors

are so named because their operation involves both electrons and

holes. Charge flow in a BJT is due to bidirectional diffusion of

charge carriers across a junction between two regions of different

charge concentrations. This mode of operation is contrasted with

unipolar transistors, such as field-effect transistors, in which only

26

one carrier type is involved in charge flow due to drift. By design,

most of the BJT collector current is due to the flow of charges

injected from a high-concentration emitter into the base where

they are minority carriers that diffuse toward the collector, and so

BJTs are classified as minority-carrier devices (Gummel H.K and

R.C Poon 1970).

A BJT consists of three differently doped semiconductor regions,

the emitter region, the base region and the collector region.

These regions are, respectively, p type, n type and p type in a

PNP, and n type, p type and n type in a NPN transistor. Each

semiconductor region is connected to a terminal, appropriately

labeled: emitter (E), base (B) and collector (C) (Gummel H.K and

R.C Poon 1970).

The base is physically located between the emitter and the

collector and is made from lightly doped, high resistivity material.

The collector surrounds the emitter region, making it almost

impossible for the electrons injected into the base region to

escape being collected, thus making the resulting value of α very

close to unity, and so, giving the transistor a large β. A cross

section view of a BJT indicates that the collector–base junction has

a much larger area than the emitter–base junction (Gummel H.K

and R.C Poon 1970).

2.12.1.1 Structure

The bipolar junction transistor, unlike other transistors, is usually

not a symmetrical device. This means that interchanging the

collector and the emitter makes the transistor leave the forward

27

active mode and start to operate in reverse mode. Because the

transistor's internal structure is usually optimized for forward-

mode operation, interchanging the collector and the emitter

makes the values of α and β in reverse operation much smaller

than those in forward operation; often the α of the reverse mode

is lower than 0.5. The lack of symmetry is primarily due to the

doping ratios of the emitter and the collector. The emitter is

heavily doped, while the collector is lightly doped, allowing a

large reverse bias voltage to be applied before the collector–base

junction breaks down. The collector–base junction is reverse

biased in normal operation. The reason the emitter is heavily

doped is to increase the emitter injection efficiency: the ratio of

carriers injected by the emitter to those injected by the base. For

high current gain, most of the carriers injected into the emitter–

base junction must come from the emitter.

The low-performance "lateral" bipolar transistors sometimes used

in CMOS processes are sometimes designed symmetrically, that

is, with no difference between forward and backward operation.

Small changes in the voltage applied across the base–emitter

terminals causes the current that flows between the emitter and

the collector to change significantly. This effect can be used to

amplify the input voltage or current. BJTs can be thought of as

voltage-controlled current sources, but are more simply

characterized as current-controlled current sources, or current

amplifiers, due to the low impedance at the base.

28

Fig: 2.6; structure of transistor

2.12.1.2 Transistor 'alpha' and 'beta'

The proportion of electrons able to cross the base and reach the

collector is a measure of the BJT efficiency. The heavy doping of

the emitter region and light doping of the base region cause

many more electrons to be injected from the emitter into the base

than holes to be injected from the base into the emitter. The base

current is the sum of the holes injected into the emitter and the

electrons that recombine in the base—both small proportions of

the emitter to collector current. Hence, a small change of the

base current can translate to a large change in electron flow

between emitter and collector. The ratio of these currents Ic/Ib,

called the current gain, and represented by β or hfe, is typically

greater than 100 for transistors. Another important parameter is

the base transport factor, αT-.The base transport factor is the

proportion of minority carriers injected from the emitter that

diffuse across the base and are swept across the base–collector

junction without recombining. This has values usually between

0.98 and 0.998. Alpha and beta are related by the following

identities (Gummel H.K and R.C Poon 1970):

29

(pnp device)

Since the majority and minority current carriers are different for

N-type and P-type materials, it stands to reason that the internal

operation of the NPN and PNP transistors will also be different.

These two basic types of transistors along with their circuit

symbols are shown here;

2.13 NPN

NPN is one of the two types of bipolar transistors, in which the

letters "N" and "P" refer to the majority charge carriers inside the

different regions of the transistor. Most bipolar transistors used

today are NPN, because electron mobility is higher than hole

mobility in semiconductors, allowing greater currents and faster

operation.

NPN transistors consist of a layer of P-doped semiconductor (the

"base") between two N-doped layers. A small current entering the

base in common-emitter mode is amplified in the collector output.

In other terms, an NPN transistor is "on" when its base is pulled

high relative to the emitter (Gummel H.K and R.C Poon 1970).

30

The arrow in the NPN transistor symbol is on the emitter leg and

points in the direction of the conventional current flow when the

device is in forward active mode.

Fig: 2.7; the symbol and picture of NPN BipolarJunctionTransistor.

2.12.14 PNP

The other type of BJT is the PNP with the letters "P" and "N"

referring to the majority charge carriers inside the different

regions of the transistor.PNP transistors consist of a layer of N-

doped semiconductor between two layers of P-doped material. A

small current leaving the base in common-emitter mode is

amplified in the collector output.

In other terms, a PNP transistor is "on" when its base is pulled low

relative to the emitter (Gummel H.K and R.C Poon 1970).

The arrow in the PNP transistor symbol is on the emitter leg and

points in the direction of the conventional current flow when the

device is in forward active mode.

31

Fig: 2.8; The symbol and picture of a PNP

BipolarJunctionTransistor.

2.12.2 Field-Effect Transistor

The field-effect transistor (FET), sometimes called a

unipolar transistor, uses either electrons (in N-channel FET) or

holes (in P-channel FET) for conduction. The three terminals of the

FET are named source, gate, and drain .

’

Fig: 2.9;Junction field effect transistor: (a) discrete device cross

sectioschematic symbol (c) integrated circuit device cross-section

On most FETs, the body is connected to the source inside the

package, and this will be assumed for the following description.

32

In FETs, the drain-to-source current flows via a conducting

channel that connects the source region to the drain region. The

conductivity is varied by the electric field that is produced when a

voltage is applied between the gate and source terminals; hence

the current flowing between the drain and source is controlled by

the voltage applied between the gate and source. As the gate–

source voltage (Vgs) is increased, the drain–source current (Ids)

increases exponentially for Vgs below threshold, and then at a

roughly quadratic rate (where VT is the threshold voltage at which

drain current begins) (Horowitz P and Hill. W 1989)in the "space-

charge-limited" region above threshold. A quadratic behavior is

not observed in modern devices, for example, at the 65 nm

technology node (Sansen W.M. C. 2006).

For low noise at narrow bandwidth the higher input resistance of

the FET is advantageous.

FETs are divided into two families: junction FET (JFET) and

insulated gate FET (IGFET). The IGFET is more commonly known

as a metal–oxide–semiconductor FET (MOSFET), reflecting its

original construction from layers of metal (the gate), oxide (the

insulation), and semiconductor. Unlike IGFETs, the JFET gate

forms a PN diode with the channel which lies between the source

and drain. Functionally, this makes the N-channel JFET the solid

state equivalent of the vacuum tube triode which, similarly, forms

a diode between its grid and cathode. Also, both devices operate

in the depletion mode, they both have a high input impedance,

33

and they both conduct current under the control of an input

voltage (Sansen W.M. C. 2006).

Metal–semiconductor FETs (MESFETs) are JFETs in which the

reverse biased PN junction is replaced by a metal–semiconductor

Schottky-junction. These, and the HEMTs (high electron mobility

transistors, or HFETs), in which a two-dimensional electron gas

with very high carrier mobility is used for charge transport, are

especially suitable for use at very high frequencies (microwave

frequencies; several GHz).

Unlike bipolar transistors, FETs do not inherently amplify a

photocurrent. Nevertheless, there are ways to use them,

especially JFETs, as light-sensitive devices, by exploiting the

photocurrents in channel–gate or channel–body junctions (Sansen

W.M. C. 2006).

FETs are further divided into depletion-mode and enhancement-

mode types, depending on whether the channel is turned on or off

with zero gate-to-source voltage. For enhancement mode, the

channel is off at zero bias, and a gate potential can "enhance" the

conduction. For depletion mode, the channel is on at zero bias,

and a gate potential (of the opposite polarity) can "deplete" the

channel, reducing conduction. For either mode, a more positive

gate voltage corresponds to a higher current for N-channel

devices and a lower current for P-channel devices. Nearly all JFETs

are depletion-mode as the diode junctions would forward bias and

conduct if they were enhancement mode devices; most IGFETs

are enhancement-mode types (Sansen W.M. C. 2006).

34

2.13 IMPORTANCE OF TRANSISTORS

The transistor is considered by many to be one of the greatest

inventions of the twentieth century (Robert W.P 2004). The transistor

is the key active component in practically all modern electronics.

Its importance in today's society rests on its ability to be mass

produced using a highly automated process (fabrication) that

achieves astonishingly low per-transistor costs.

Although several companies each produce over a billion

individually-packaged (known as discrete) transistors every year,

the vast majority of transistors produced are in integrated circuits

(often shortened to IC, microchips or simply chips) along with

diodes, resistors, capacitors and other electronic components to

produce complete electronic circuits. A logic gate consists of up to

about twenty transistors whereas an advanced microprocessor, as

of 2006, can use as many as 1.7 billion transistors (MOSFETs)

(Turley J. 2002).

The transistor's low cost, flexibility, and reliability have made it a

ubiquitous device. Transistorized mechatronic circuits have

replaced electromechanical devices in controlling appliances and

machinery. It is often easier and cheaper to use a standard

microcontroller and write a computer program to carry out a

control function than to design an equivalent mechanical control

function (Turley J. 2002).

2.14 SIMPLIFIED OPERATION OF TRANSISTOR

The essential usefulness of a transistor comes from its ability to

use a small signal applied between one pair of its terminals to

35

control a much larger signal at another pair of terminals. This

property is called gain. A transistor can control its output in

proportion to the input signal, that is, can act as an amplifier. Or,

the transistor can be used to turn current on or off in a circuit as

an electrically controlled switch, where the amount of current is

determined by other circuit elements.

The two types of transistors have slight differences in how they

are used in a circuit. A bipolar transistor has terminals labeled

base, collector, and emitter. A small current at the base terminal

(that is, flowing from the base to the emitter) can control or

switch a much larger current between the collector and emitter

terminals. For a field-effect transistor, the terminals are labeled

gate, source, and drain, and a voltage at the gate can control a

current between source and drain. Charge will flow between

emitter and collector terminals depending on the current in the

base. Since internally the base and emitter connections behave

like a semiconductor diode, a voltage drop develops between

base and emitter while the base current exists. The size of this

voltage depends on the material the transistor is made from, and

is referred to as VBE (Ediger L.J 1925).

36

CHAPTER THREE

GENERAL DESIGN OF THE CIRCUIT

3.0 INTRODUCTION

In this chapter the basic design of the circuit is carried out. The

general circuit diagram of the circuit is presented together with its

principle of operation. Then the basic procedure for the design of

the circuit follows.

3.1 GENERAL CIRCUIT DIAGRAM

The circuit of aquatic probe described here can monitor the

temperature of water and indicate the rise in temperature

37

through audiovisual indicators. A readily available signal diode

1N34 is used in the circuit as the temperature sensing probe. The

resistance of the diode depends on the temperature in its vicinity.

Typically, the diode can generate around 600 mV when a

potential difference is applied to its terminals. For each degree

centigrade rise in temperature, the diode generates 2 mVoutput

voltage. That is, at 5°C, it is 10 mV, which rises to 70 mV when

the temperature is 35°C. This property is exploited in the circuit

to sense the temperature variation in aquarium water.

Fig. 3.1 shows the circuit diagram of the aquarium probe. Since

the output from the diode sensor is too low, a high-gain inverting

DC amplifier is used to amplify the voltage. CA3140 (IC1) is the

CMOS version op-amp that can operate down to zero-volt output.

The highest output available from IC1 is 2.25 V less than the input

voltage at pin 7. With resistorR4 andVR2, the variation in diode

volt a g e c a n be amplified to the required level. ResistorR1

restricts current flow through diode D1 and preset VR1 (1-kilo-

ohm) sets the input voltage at pin 3. IC3 (7805) provides

regulated 5 volts to the inputs of IC1, so that the input voltage is

stable for accurate measurement of temperature. The output

from IC1 is fed to display driver LM3915 (IC2) through preset VR3

(50-kilo-ohm). With careful adjustments, the wiper ofVR3 can

provide 0-400 millivolts to the input of IC2. The highly sensitive

input of IC2 accepts as low as50 mV if the reference voltage at its

pin 7 is adjusted using a variable resistor. To increase the

sensitivity of IC2, preset VR4 is connected atone end to ‘reference

38

voltage end’ pin 7 and its wiper is connected to ‘high end’ pin 6 of

the internal resistor chain. When approximately 70 mV is provided

to the input of IC2 by adjusting preset VR3, LED1 (green) lights up

to indicate that the temperature is approximately 35°C, which is

the crossing point. When the input receives 100 mV, LED2 (red)

lights up to indicate approximately 50°C.Finally, the buzzer starts

beeping if the input receives 130 mV corresponding to a

temperature of 65°C.In short, LED sand the buzzer remain stand

by when the temperature of the water is below 35°C(normal).

With each step increase of 30mV in the input (corresponding to

15°Crise in temperature), LEDs and the buzzer become active. Pin

16 of IC2 is used to drive the piezo buzzer through transistor T1.

When pin 16 of IC2 becomes low, T1 conducts to beep the piezo

buzzer. Resistor R7 keeps the base of transistor T1 high to avoid

false alarm. IC4 provides regulated 9 V DC to the circuit.

3.2 DESIGN OF THE COMPARATOR STAGE

The comparator section was designed base on the circuit

configuration shown in fig. 3.2.

39

The resistor R1 supplies the biasing current of 0.1mA to the diode

1N34. The biasing voltage is from the regulator IC3 and is 5 V. Its

value is calculated as follows;

R1 = V/ID1 = 5/0.1mA = 50kΩ

A 47kΩ resistor was used as the R1 to allow the current go a bit

above the minimum value.

The IC CA3140 is given a reference voltage through a voltage

divider network made by resistors R2 and R3 together with a

variable resistor VR1. The voltage is calculated using the relation;

V = VCC R3/(R2 +R3)

Where the VCC = 5V, R2 = 47kΩ, R3 = 470Ω.

V = 5 x 470 / (470+470000) = 0.0495V = 49.5mV

When the variable resistor of 1kΩ is used the voltage can be

changed to maximum value of 0.151V or 151mV.

The variable voltage from the temperature sensor is delivered to

the IC through a resistor R4. The value of this resistor is

calculated by knowing the maximum possible voltage from the

sensor and the safe current needed to the IC. The Internal voltage

reference for the IC is from 1.2V to 12V. But here since the supply

voltage is 5V the value can not go beyond that. The Maximum

allowed current of the IC is 50μA. The resistor is calculated as;

R4 = V/I = 5/50μA = 100kΩ

3.3 DESIGN OF THE LM3915 UNIT

The circuit diagram of fig. 3.3 is the configuration Lm3915 unit.

40

The IC needs regulated DC voltage of 9 V through the VCC terminal

pin-3.

The reference is designed to be adjustable and develops a

nominal 1.25V between the REF OUT (pin 8) and REF ADJ (pin 7)

terminals.

The equations below are used calculate the reference voltage and

the current to the LED. (J. D and Ryder 1989)

VREF =1.25 V(1+VR4/R6) + VR4 ×80 µA

LED = 12.5V/R6 +VREF/2.2 kΩ

In this, R6 = 1 kΩ and VR4 = 4.7 kΩ. The VREF is calculated as

follows;

VREF = 1.25 x (1 + 4.7 k / 1 k) + 4.7k x 80 μA = 7.501V

The current to each of the LED is calculated as;

ILED = (12.5/1000) + (7.501/2200) = 0.0159 A ≈ 16 mA.

41

CHAPTER FOUR

CONSTRUCTION, CASING AND TESTING

4.1 INTRODUCTION

This chapter is going to give the basic procedure used in

constructing the circuit and casing the constructed work and

testing the constructed circuit to confirm its proper operation.

4.2 TEMPORARY CONSTRUCTION OF THE CIRCUIT ON

BREADBOARD

In the first place the circuit is constructed on a temporary

project board to confirm its ability to work according to the design

specifications. The components were inserted into the slots on the

board and connected accordingly using wires as jumpers. After

ensuring that all the wiring is done properly with all components

connected with right orientation, the circuit is connected to the

mains supply and the outputs at the different terminals gives

their respective voltages correctly. After this, the permanent

construction was made. Fig. 4.1 shows a typical breadboard.

42

4.3 PERMANENT CONSTRUCTION OF THE CIRCUIT ON

VEROBOARD

The circuit is finally constructed on a Vero board after assembling

the components on the board and using wires as jumpers to

connect some terminals. The whole board is

Soldered using soldering iron and lead. Fig 4.1 below gives typical

veroboard.

4.4 DESIGN OF THE CASE

The circuit is cased after the permanent construction discussed

above. The casing material used is a plastic because it is

43

availability everywhere. It is also easy to bend as well as suitable

for casing electronics project because it is an insulator.

CHAPTER FIVE

CONCLUSION, SUGGESTION FOR FURTHER WORK AND

REFERENCES

5.1 CONCLUSION

This project work of design and construction of Aquarium Probe

was carried out so as to provide a means of detecting the level

of temperature of an Aquarium at any time. At the end of this

work the actual design aim was achieved because the circuit

designed was working properly where it is used to detect level of

temperature of aquarium and produces an alarm when the

temperature is high.

This type of project is very significant since it provides a means of

solving some practical problems. In addition, it helps the students

to understand practical aspects of design and construction of

electronic circuits.

5.2 RECOMMENDATION FOR FURTHER WORK

44

This project has its own limitations in the area of its applications.

The basic limitation to this circuit is the fact that the circuit can

only detect the temperature but it can not regulate the

temperature when it rises.

The following recommendations are made in order to improve

the performance of the circuit.

• The sensitivity of the sensor should be improve to enable the

circuit detect all changes in temperature.

• A temperature control section should be included to monitor

the change in temperature.

• A digital read out should be included to show the actual

value of the temperature.

If the above recommendations are carried out, a better version of

the aquarium probe would be produced.

45

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Sensor” undergraduate Project. Electrical Engineering Dept.

Ahmadu Bello University Zaria.

D.F Stout (1976) Handbook Of Operational Amplifier Circuit

Design (McGraw-Hill, ISBN 007061799) pp 1-11

Edgar L. J (1925) "Method and apparatus for controlling electric

current" U.S. Patent 1,745,175

Gummel H. K. and R. C. Poon, (1970) "An integral charge control

model of bipolar transistors," Bell System Technology Journal, vol.

49, pp. 827--852

Hecht E. (2002). ‘’Optics’’ (4th Ed.). Addison Wesley. p. 591. ISBN

0195108183.

Horowitz, P. Hill .W (1989). ‘’The Art of Electronics’’ (2nd ed.).

Cambridge University Press. pp. 115. ISBN 0-521-37095-7.

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Ibrahim U. (2003) “Design And Construction Of A Fire Burglar

Alarm” Undergraduate Project. Electrical Engineering Dept.

Ahmadu Bello University Zaria.

Ivan Moreno, (2008). "Modeling the radiation pattern of LEDs".

Optics Express 16 (3): 1808. doi:10.1364/OE.16.001808. PMID

18542260.

Jonathan B. (2001) “Design And Construction Of A Comparator

Circuit” Undergraduate Project. Electrical Engineering Dept.

Ahmadu Bello University Zaria.

Jun J. L And Jians S. Y (1998) "Semiconductor Device Physics And

Simulation Springe"ISBN 0306457245

Malvino A.P,Electronics (1979) Principles (2nd Ed. ISBN 0-07-

039867-4) pp.476

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http://www.nytimes.com/2007/06/24/business/yourmoney/24nove

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AMACOM Div American Mgmt Assn. p. 42. ISBN 9780814471906.

http://books.google.com/books?

id=q7UzNoWdGAkC&pg=PA42&dq=transistor+inventions-of-the-

twentieth-century

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Ryder J. D (1989) “Electronics Fundamentals and application”,

Prentice– Hall of Indian books,

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Sciencenews.org. (2006) ‘’Light Impacts: Science News"..

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Texyt.com. (2007).’’Blue LEDs: A health hazard?".

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48