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DET: TechnologicalStudies

Applied ElectronicsIntermediate 2

4597

Spring 1999

DET:Technological

StudiesApplied Electronics

Intermediate 2

Support Materials*+,-./

HIGHER STILL

Technological Studies Support Materials: Applied Electronics (Intermediate 2) 1

CONTENTS

Teacher’s guide

Students’ materials

Outcome 1

Outcome 2

Outcome 3

Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide

TECHNOLOGICAL STUDIES

INTERMEDIATE 2

APPLIED ELECTRONICS

TEACHER’S GUIDE

Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide1

TEACHERS GUIDE

Support Materials - Overview

The support materials for Technological Studies courses in Higher Still have beencreated to specifically address the outcomes and PC in each unit at the appropriatelevel. These materials contain a mixture of formal didactic teaching and practicalactivities.The support materials for each unit have been divided into outcomes. This willfacilitate assessment as well as promoting good teaching practice.

The materials are intended to be non-consumable, however it is at the discretion ofeach centre how to use these materials.

Each package of support materials follows a common format:

1. Statement of the outcome.2. Statement of what the student should be able to do on completion of the outcome.3. Learning and teaching activities.4. Sequence of structured activities and assignments.5. Formal Assessment

• NAB - assessing knowledge PC.• Computer simulation - assessing simulation PC.• Practical assignments - assessing practical PC.

It is important to note that the National Assessments have been designed to allowassessment either after each outcome has been completed or as an end of unitassessment when all outcomes have been completed depending on the needs of thecentre.

The use of SQA past external paper questions has been used throughout the materialsand the further use of these questions is encouraged.

Using past questions provides the opportunity for students to:

• Work at the appropriate level and rigour• Prepare for external assessment.• Consolidate teaching and learning.• Integrate across units.

Homework is a key factor in effective teaching and learning. The use of resourcessuch as P & N practice questions in Technological Studies is very useful forhomework activities and also in preparation for external assessment.

The use of integrated questions across units is essential in preparation ofstudents for External Assessment.


Support Materials - Content

Outcome 1 - Demonstrate knowledge and understanding of the relationshipbetween current and voltage in simple resistive d.c. networks.

The purpose of this unit of work is to investigate and analyse resistive d.c. networks.Student activities cover calculations on Ohm's Law, Kirchoff's 1st and 2nd Laws aswell as practical activities relating to d.c networks.

When students have completed this unit of work they should be able to:

• Determine the relationship between current and voltage in a d.c. network• Determine the relationship between supply current and branch currents in a

combined series-parallel resistive d.c. network• Determine the relationship between applied voltage and the series voltage drops in

a combined series-parallel resistive d.c. network• Perform calculations to determine equivalent resistance of a network.• Construct specified resistive networks• Test specified resistive networks.

Outcome 2 - Design and construct a simple electronic system to meet a givenspecification.

The purpose of this unit of work is to introduce student to simple electronic controlsystems based on voltage dividers and bi-polar transistors. The circuits should be ableto control output devices and component protection is introduced. Student activitiesinclude calculations relating to voltage dividers and transistor gain and construction ofsimple control systems.


• Use manufacturers data sheets to aid selection of appropriate input and outputtransducers for a given purpose

• Recognise that changes in the resistance of an input transducer can be convertedto changes in voltage using a voltage divider network

• Carry out calculations involving voltage divider networks• Perform calculations using transistor current gain equation• Recognise that the transistor can be used as a switch• Describe the operational characteristics of various electronic components• Perform calculations to verify the specified operation of a circuit• Construct specified control systems• Test specified control systems.


Outcome 3 - Design and construct a simple combinational logic system to meetgiven specifications.

The purpose of this unit of work is to introduce students to simple combinational logicsystems. Student activities include interpretation of data sheets and construction ofsimple combinational logic systems.


• Identify single logic gate symbols• Complete truth tables for single logic gates• Analyse combinational logic circuits• Complete truth tables for combinational logic circuits• Write Boolean expressions for simple combinational logic systems• Identify differences between the TTL and CMOS families of IC's• Identify types of logic gates, given pin layout diagrams or IC number (logic gate

IC's)• Use manufactures data sheets/CD-ROM to aid selection of appropriate integrated

circuits for a given purpose.• Correctly 'power up' an IC for use - on breadboard, circuit simulation and

diagrammatically


Resources

The resources listed below are items that the centre should provide for each student.It may be possible on some occasions for student to share resources, such asmultimeters during practical activities, however any activity that will be used tosatisfy an assessment requirement must be undertaken individually.

It is expected that centres already presenting Technological Studies at Standard Gradeor Higher Level will have the majority of these resources for the current courses.

Circuit Simulation Software ( Optional at Intermediate 2)

Crocodile Clips PC/MacOak Logic AcornElectronic workbench PC

Any circuit simulation software that will enable student to create and test d.c.networks, simple electronic systems and combinational logic systems would besatisfactory.

General equipment - required for all unitsPower supplyBreadboardsWire for links - 0.6mm solid core insulated wire.Wire strippersDigital multimeter

Outcome 1

Resistors - 120 R, 470 R, 1K

Outcome 2

Resistors - 220R, 1K, 10KVariable resistor - 20KLDR - ORP12Thermistors - Types 1, 2, 3, 4, 5Transistors (NPN various) - BC 108, BFY 51, 2N 3053Access to manufacturer data sheets/catalogue/CD - ROMLED's

Outcome 3

Various IC's (TTL or CMOS) containing common arrangements of two and threeinput logic gatesLED'sResistor values around 370 R.Logic probe.


Assessment

In most Higher Still courses there are two types of assessment, internal and external

Internal Assessment - this can be conducted in a number of ways:1. Knowledge based - tested through NAB2. Practical - tested in class under appropriate conditions.3. Software simulation (only used in some courses and units)

Internally assessment is subject to central moderation.

External Assessment - Assessed by means of an external examination

The external examination will provide the basis for grading attainment in courseawards and is marked externally.

To gain the award of the course, the student must pass all unit assessments as well asthe external assessment.

Recording and retention of evidenceAll evidence of performance should be retained by the centre for moderationpurposes.

NAB - Test

A record of the candidate's performance must be kept which shows:• The score achieved if a cut-off score is used• When a candidate has achieved an outcome

Practical assessment

A record of the candidate's performance must be kept which shows:• When circuit simulation is used - a brief description of the circuit being evaluated.• Whether the candidate has evaluated the circuit correctly.• Where a circuit is required to be constructed - a brief description of the circuit

being constructed.• Whether the candidate has constructed the circuit to the given specification.


Assessment Summary of each UnitThe following is a summary of the assessment requirements for each outcome.

Outcome 1 - Applied Electronics (Int 2)

1. National Assessment Bank item (Test) - Providing written and graphicalevidence for PC a and b.

2. Practical activity - providing performance evidence for PC c.The practical activities contained in the support materials will satisfy theassessment requirements for this aspect. Centres should ensure that whencandidates are carrying out the practical activity for assessment purposes,appropriate conditions are in place.


1. National Assessment Bank item (Test) - Providing written and graphicalevidence for PC a, b, c and d.

2. Practical activity - providing performance evidence for PC e and f. The practical activities contained in the support materials will satisfy theassessment requirements for this aspect. Centres should ensure that whencandidates are carrying out the practical activity for assessment purposes,appropriate conditions are in place. Assessment of the computer simulation aspect can be done using the assignmentsprovided in the support materials. Students should be able to evaluate the circuitseffectively to satisfy the assessment requirements; this can be done either inwriting or as a verbal report to the teacher/lecturer.


1. National Assessment Bank item (Test) - Providing written and graphicalevidence for PC a, b, c and d.

2. Practical activity - providing performance evidence for PC e and f.The practical activities contained in the support materials will satisfy theassessment requirements for this aspect. Centres should ensure that whencandidates are carrying out the practical activity for assessment purposes,appropriate conditions are in place.Assessment of the computer simulation aspect can be done using the assignmentsprovided in the support materials. Students should be able to evaluate the circuitseffectively to satisfy the assessment requirements; this can be done either inwriting or as a verbal report to the teacher.

Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1


INTERMEDIATE 2

APPLIED ELECTRONICS

SECTION 1

OUTCOME 1

Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 1

OUTCOME 1Demonstrate knowledge and understanding of the relationship between currentand voltage in simple resistive d.c. networks.

When you have completed this unit you should be able to:

• determine the relationship between current and voltage in a d.c. network• determine the relationship between supply current and branch currents in a

combined series-parallel resistive d.c. network• determine the relationship between applied voltage and the series voltage drops in

a combined series-parallel resistive d.c. network• perform calculations to determine equivalent resistance of a network• construct specified resistive networks• test specified resistive networks.

Before you start this unit you should have a basic understanding of:resistor colour codeselectrical circuit symbolsOhm’s lawuse of breadboardsuse of circuit test equipment: multimeter and/or logic probe.


ELECTRIC NETWORKS

An electric circuit is a closed loop made up from electrical components such asbatteries or voltage sources, bulbs, switches and wires.

AE.Int2. O1 fig 1

Voltage, Resistance and Current

Voltage

In most electric networks a battery or a voltage supply provides the energy source forthe circuit. Voltage is measured in volts (V).

Resistance

All materials conduct some electricity. Some are good and some are poor. Materialsthat are good at conducting are called conductors. Those which are poor, are calledinsulators. Examples of good conductors are silver and copper. Examples of goodinsulators are glass and rubber.

A good conductor is one that offers very little resistance to the flow of electriccurrent. In other words, it lets current flow with very little voltage being applied.Connecting wire used in the construction of electric circuits usually has a very lowresistance - it allows electricity to flow freely.

Resistors can come in the form of purpose made resistors that have fixed or variablevalues or in the form of any electrical component that offers resistance to the flow ofcurrent in the circuit.

Resistance is therefore a measure of how much voltage is required to let a currentflow.

Resistance is measured in ohms (Ω).


Current

Current (I) is the rate of flow of electricity in a circuit and is measured in Amperes(A)

Ohm's Law

If you apply a voltage to a resistor and make a current flow through it you should findthat doubling the voltage difference across the resistor doubles the current flowingthrough it. Thus we can say that current is proportional to the voltage differenceacross a resistor.The rule that current is proportional to the voltage difference is an important rule inelectronics and is known as Ohm’s Law.

The relationship between the voltage, resistance and current in a circuit gives rise toOhm’s Law formula:

Voltage = Current x Resistance

V = I x R

This triangle is designed to help you remember how to find V, R and I.

AE.Int2. O1 fig 2

To find R, cover R to give R = V/ITo find V, cover V to give V = I x RTo find I, cover I to give I = V/R


Assignments: Using Ohm’s Law

Use Ohm’s Law in the following examples.

1. A voltage of 6 V is applied across a 1K5 resistor. Find the current that will flowthrough the resistor.

2. An electric lamp has a resistance of 470 R and is connected to a supply voltage of

110 V. Calculate the current the lamp will draw from the supply. 3. Find the potential difference across a 47 K resistor that takes a current of 450 mA. 4. A current of 650 mA passes through a component that has a resistance of 2 K.

Find the potential difference across the component. 5. Calculate for the circuit shown: a) the value of the resistor

AE.Int2. O1 fig 3 6. Using the values from the circuit, determine: a) the reading on the ammeter. b) how much current would flow if the value of the resistor were halved

AE.Int2. O1 fig 4


7. Calculate the current flowing through this circuit.

AE.Int2. O1 fig 5


Series and Parallel Circuits

It is possible to connect resistive components in two ways, in series wherecomponents are connected end to end and in parallel where each component receivesthe supply voltage.

AE.Int2. O1 fig 5a

Series circuits

When components are connected end to end (in series) to form a closed loop thecurrent is common to all the components and the voltage is divided up amongst them.That is, the sum of the voltage drops across the circuit components must equal thetotal voltage input to the circuit.This is known as Kirchoff’s 2nd Law , which states: the sum of the emf’s (voltagesupplies) in a closed circuit is equal to the sum of the potential drops round thatcircuit.

VT = V1 + V2 + V3 .........

In a series circuit the Voltage Drop across each resistor is found in the following way:(remember, as there are no branches for the current to flow into, the supply currentmust flow through each resistor).

V1 = IT x R1 and V2 = IT x R2 and V3 = IT x R3 and so on for the number of resistors.

Study the example below which shows Kirchoff’s 2nd Law in practice.Each bulb is rated at 6 V, and the supply voltage is 18 V.

AE.Int2. O1 fig 6


Disadvantages of Series circuits are:• if one component fails, all components go off because the circuit is broken.• the supply voltage is shared out amongst the components, this means that a

component may not get the required voltage.

Resistors in Series

According to Kirchoff’s 2nd Law, when resistors are connected in series the supplyvoltage is shared out amongst them. When you add up the individual voltagesdropped over the resistors they should equal the supply voltage.

As there are no branches for the current to flow into, the supply current must flowthrough each resistor.

When resistors or resistive components are connected in series, the effect is to addmore resistance to that circuit. The total resistance can be found by simply adding upall the resistance values in the circuit.

There are two important points to remember about resistors in series:a) the same current flows through each resistor.b) the sum of the voltages across each resistor is equal to the voltage across the

combination.

The total resistance of the circuit is given by

RT = R1 + R2 + R3 .........

AE.Int2. O1 fig 7


Parallel Circuits

When components or resistive materials are connected in parallel, each componentreceives the supply voltage from the voltage source and the current is shared outamongst the components.

Study the example shown below:

AE.Int2. O1 fig 8

If you look at junction X, we can see that current I1 + I2 + I3 must be equal to the totalcurrent IT being supplied from the voltage source. This is known as Kirchoff’s 1stLaw, in other words, the sum of the current flowing towards a single junction in acircuit is equal to the sum of the currents leaving the junction.

I T = I1 + I2 + I3 .........

The current used in each branch of the circuit is found in the following way:(remember, in a parallel circuit the voltage drop across each resistor is the same and isthe value of the supply voltage).

I1 = VS/ R1 and I2 = VS/ R2 and I3 = VS/ R3 and so on for the number of branches.

An advantage of a parallel circuit over a series circuit is that if one of the componentsfail, it is only that one that is effected.

A disadvantage is that parallel circuits draw more current than series circuits.


Resistors in Parallel

A parallel circuit has a number of branches, each one having a load connected to it.Each branch receives the supply voltage, which is useful if you are trying to run 2 or 3devices from one supply voltage.

Each branch will have it’s own current, and the total supply current is found byadding up all the branch currents.

When resistors or resistive components are connected in parallel, the effect is toreduce the resistance in the circuit.

There are two important points to remember about resistors in parallel:a) the same voltage acts across each resistor.b) the sum of the currents through each resistor is equal to the current flowing from

the voltage source.

AE.Int2. O1 fig 9

The total resistance in a parallel circuit is given by:

1/RT = 1/R1 + 1/R2 + 1/R3 .........

Special Case: 2 Resistors in Parallel

There is a special rule that can apply for adding 2 resistors in parallel.

Total Resistance (RT) = Product/Sum

21

21

RR

RRRT +

×=

Note: This special formula only works for circuits with 2 resistors in parallel.


Worked examples: Series Circuit

1. For the series circuit shown calculate:a) The total resistance ( RT )b) The circuit current (IC )c) The voltage drop across both resistors (V1 ), (V2 ).

C

AE.Int2. O1 fig 10

a) RT = R1 + R2

= 6 + 18

RT = 24 Ω b) VS = IC x RT

IC = VS/ RT

= 12/24 IC = 0.5 A c) V1 = IC x R1

= 0.5 x 6

V1 = 3 V

V2 = IC x R2

= 0.5 x 18

V2 = 9 V


We can use Kirchoff’s 2nd Law to check the answers calculated for the voltage drops.

VT = V1 + V2

= 3 + 9

VT = 12V


Worked examples: Parallel Circuit

1. For the parallel circuit shown calculatea) The total resistance ( RT )b) The circuit current (IC )c) The current through each resistor (I1 ), (I2 ).

C

AE.Int2. O1 fig 11

a) 1/RT = 1/R1 + 1/R2

or it is possible to use the special case formula for 2 resistors in parallel RT = R1 x R2 / R1+ R2

= 8 x 12/ 8 + 12 = 96/20

RT = 4.8 Ω b) VS = IC x RT

IC = VS/ RT

= 12/4.8 IC = 2.5 A c) I1 = VS/ R1

= 12/ 8

I 1 = 1.5 A


I2 = VS/ R2

= 12/ 12

I 2 = 1 A

We can use Kirchoff’s 1st Law to check the answers calculated for the current in eachbranch.

IC = I1 + I2

= 1.5 + 1

IC = 2.5 A


Assignments: Series and Parallel circuits

1. For the circuit shown, calculatea) The total resistance of the circuitb) The circuit current

AE.Int2.O1 fig 11a

2. For the circuit shown, calculate:a) The total resistanceb) The circuit currentc) The voltage drop across each resistord) Use Kirchoff’s 2nd law to verify your answers in part c)

AE.Int2.O1 fig 11b


AE.Int2.O1 fig 11c


4. A circuit has three resistors connected in series. Their values are 15 R, 24 R and 60 R. Calculate the total resistance of the circuit.

5. Two resistors are connected in series. Their values are 25 R and 75 R. If the voltage drop across the 25 R resistor is 4 V, determine the circuit current and the supply voltage.


AE.Int2.O1 fig 11d

7. For the circuit shown, calculatea) The total resistance of the circuitb) The circuit currentc) The current flowing through R1 (10R)d) The current flowing through R2 (24R)

AE.Int2.O1 fig 11e


8. For the circuit shown, calculatea) The total resistance of the circuitb) The circuit currentc) The current flowing through R1 660R)d) The current flowing through R2 (470R)e) Use Kirchoff’s 1st law to verify your answers in parts c) and d)

AE.Int2.O1 fig 11f

9. A 66 R resistor and a 75 R resistor are connected in parallel across a voltage supply of 12 V. Calculate the circuit current.

10. A 440 R resistor is connected in parallel with a 330 R resistor. The current through the 440 R resistor is 300 mA. Find the current through the 330 R resistor.


Combined Series and Parallel Circuits

Until now we have been looking at Series or Parallel circuits individually. It ispossible and quite common to have series and parallel connections in the same circuit.Consider the combined series and parallel circuit shown in the figure.

AE.Int2.O1.fig 11g

You can see that R2 and R3 are connected in parallel and R1 is connected in series withthe parallel combination.

Some points to remember when you are dealing with combined series and parallelcircuits.

1. The voltage drop across R2 is the same as the voltage drop across R3. 2. The current through R2 added to the current through R3 is the same as the current

through R1. 3. The voltage drop across R1 added to the voltage drop across R2 (which is the same

as across R3) would equal the supply voltage Vs.


Worked Example: Combined Series and Parallel circuits

1. For the combined series and parallel circuit shown, calculate:a) The total circuit resistance (RT).b) The circuit current (IC).c) The voltage drop across resistor R1 (VR1).d) The current through resistor R2 (I2).

= 10 R

= 24 R

= 48 R

= 12 V

AE.Int2. O1.fig 11h

a) In the first instance you must calculate the equivalent resistance of the parallelarrangement (RP) of R2 and R3.

It is possible to use the special case formula for 2 resistors in parallel.

Ω=

=

+×=

+×

=

28.8

58

480

4810

4810

32

32

P

P

P

P

R

R

R

RR

RRR

The total circuit resistance (RT) is then found by adding RP to R1.

Ω=

+=

+=

28.32

28.824

1

T

T

PT

R

R

RRR


a) It is now possible to calculate the circuit current.

AI

I

R

VI

RIV

C

C

T

SC

TCS

37.0

28.32

12

=

=

=

×=

c) The voltage drop across R1 is found by using the resistance across R1 and thecurrent through R1.

VV

V

RIV

RIV

R

R

CR

88.8

2437.0

1

1

11

=

×=

×=

×=

d) The current through R2 is found by using the resistance of R2 and the voltage dropacross R2.

By using Kirchoff's 2nd Law we know that the voltage drop across the parallelarrangement must be:

VV

V

VVV

VVV

P

P

RSP

PRS

12.3

88.812

1

1

=

−=

−=

+=

By using Kirchoff's 1st Law we know that the circuit current IC will 'split' or dividebetween the two resistors R2 and R3.


In order to find the current through R2

AI

I

R

VI

RIV

RIV

P

P

312.0

10

12.3

2

2

22

22

=

=

=

×=×=


Assignments: Combined Series and Parallel Circuits

1. For the circuit shown, calculatea) The resistance of the parallel combinationb) the total circuit resistance

AE.Int2.O1 fig 11h

2. For the circuit shown, calculate:a) The total resistanceb) The circuit currentc) The voltage drop across each resistor

AE.Int2.O1 fig 11j

3. For the circuit shown, calculatea) The total resistance of the circuitb) The circuit currentc) The current through each resistord) The voltage drop across each resistor

AE.Int2.O1 fig 11k


Measuring current, voltage and resistance

The most commonly used device for measuring voltage, current and resistance is themultimeter, most of which are now digital.

Most digital multimeters have the facility to measure and display values of voltage,current and resistance. Other useful additional features are on some of the moreadvanced meters is the ability to measure capacitance, continuity and transistor gain.

There are some important points you need to know before attempting to measuredirect current and voltage.

Measuring Current

The ammeter setting is used to measure the flow of current in a circuit. In this modethe meter must be connected in series with the circuit components, that is, you mustbreak the circuit at a convenient point as shown in the figure below. In this way, thecurrent flowing in the circuit also flows through the meter and is recorded in Amps.

AE.Int2. O1 fig 12

A meter set in the ammeter mode has a very low resistance, so that it does not reducethe current that it is designed to measure.

Measuring Voltage

The voltmeter setting is used to measure the voltage across a component. In this modethe meter must be connected in parallel with the circuit components as shown in thefigure below.

AE.Int2. O1 fig 13


A meter set in the voltmeter mode has a very high resistance, so that when it isconnected across the component very little current is diverted from the component.

When measuring unknown voltages and currents, always set the meter to thehighest range then work down to increase the sensitivity of your measurements.


Assignments: Using a digital multimeter

Use the equipment provided to construct and investigate the following circuits.

Equipment:

BreadboardDigital multimeterPower supplyResistors - 120 R, 470 R, 1KWire for linksWire strippers

Connect the multimeter in the correct mode and measure the stated values in eachcircuit.For each circuit you should verify your measured results by calculation.

1. Measure and record the values of voltage and current using the positions shown.

AE.Int2. O1 fig 14 2. Measure and record the current flowing around this circuit.

AE.Int2. O1 fig 15


3. Measure and record the voltage across the resistor in this circuit.

AE.Int2. O1 fig 16 4. Measure and record the circuit current and the voltage across each resistor.

AE.Int2. O1 fig 17 5. Measure and record the circuit current, the current through each resistor and the

voltage across the resistors.

AE.Int2. O1 fig 18


6. Measure and record the circuit current, the current through each resistor and thevoltage across each resistor.

AE.Int2. O1 fig 19 7. Measure and record the circuit current, the current through each resistor and the

voltage across each resistor.

AE.Int2. O1 fig 20


Circuit Simulation Software

It is possible to use circuit simulation software such as ‘Crocodile Clips’ toinvestigate electric and electronic circuits. Circuit simulation is widely used inindustry as a means of investigating complex and costly circuits as well as basiccircuits.

Circuit simulators make the modelling and testing of complex circuits very simple.The simulators make use of libraries of standard components along with common testequipment such as voltmeters, ammeters and oscilloscopes.

Using Crocodile Clips or another similar software package construct and test thefollowing circuits.

1.

AE.Int2. O1 fig 21 2.

AE.Int2. O1 fig 22


3.



AE.Int2. O1 fig 25



INTERMEDIATE 2

APPLIED ELECTRONICS

SECTION 2

OUTCOME 2


OUTCOME 2Design and construct a simple electronic system to meet a given specification.

When you have completed this unit you should be able to:• Use manufacturers data sheets to aid selection of appropriate input and output

transducers for a given purpose• Recognise that changes in the resistance of an input transducer can be converted

to changes in voltage using a voltage divider network• Carry out calculations involving voltage divider networks• Perform calculations using transistor current gain equation• Recognise that the transistor can be used as a switch• Describe the operational characteristics of various electronic components• Perform calculations to verify the specified operation of a circuit• Construct specified control systems• Test specified control systems.

Before you start this unit you should have a basic understanding of:

Resistor colour codesElectrical circuit symbolsOhm’s lawKirchoff’s 1st and 2nd lawsUse of breadboardsUse of circuit test equipment: multimeter and/or logic probe.


TRANSDUCERS

Any electronic system can be broken down into three distinct parts

AE.Int2.O2.fig1

The input and output parts must ‘interface’ with the real world

A transducer is a device that converts one form of energy into another e.g. amicrophone is a transducer that changes Sound Energy into Electrical Energy.

Output TransducersOutput transducers in electronic systems are used to convert Electrical Energy intoanother form that can be detected by the user or used in some other way.

Common Output TransducersThe table gives some examples of common output transducers that you may have metbefore.

Output Transducer Output Energy

BulbLampLED

Light

BuzzerLoudspeaker

Earphone

Sound

MotorPump

Solenoid

Movement

Heating element Heat


Examples of Common Output Transducers

At the output of an electronic system the output transducer converts the electricalsignal in to some other useful form of energy such as heat, light, sound or mechanicalenergy.

Electric Motors

Electric motors convert the electrical signal into rotational kinetic energy. Before amotor is connected to a circuit it is necessary to know the characteristics of the motorin terms of working voltage and the maximum current to be drawn by it in order todetermine the correct choice of driver. The most common and likely choice to drive amotor from an electronic circuit would be the relay.

Solenoids

The solenoid consists of a magnetic core that is free to move position inside a coil.When current flows through the coil and it is energised, the magnetic core is pulledinto the centre of the coil (along the coil axis). This converts the electrical signal intolinear motion. A solenoid is used when in and out motion is required. Solenoidsrequire very large currents in order to produce meaningful force and they are usuallyswitched on and off by using relays.

AE.Int2.O2.fig1c - Circuit symbol for a solenoid


Input TransducersInput transducers convert a change in physical conditions (e.g. temperature) into achange in an electrical property (e.g. voltage) which can then be processedelectronically to produce either a direct measurement of the physical condition(Temperature in oC) or to allow something to happen at a predetermined level (e.g.switching on the central heating at 20 oC).

Common Input TransducersThe table gives some examples of common input transducers that you may have metbefore.

Physical condition to bemonitored

Input Transducer Electrical property thatchanges

TemperatureThermistor

ThermocouplePlatinum Film

ResistanceVoltage

Resistance

LightLDR

Selenium CellPhoto Diode

ResistanceVoltage

Resistance

DisplacementSlide Potentiometer

Variable TransformerVariable Capacitor

ResistanceInductanceCapacitance

Force Piezo electric crystal Voltage

Angle Rotary Potentiometer Resistance

It can be seen that electrical properties that change fall into three groups

1. Transducers that produce voltage.2. Transducers that change the value of resistance.3. Transducers that change either the value of inductance or capacitance.

Changes in the resistance of an input transducer are usually converted to changes involtage before the signal can be processed. This is normally done using a voltagedivider circuit.


Examples of common Input Transducers

The switch

We all make use of switches every day. We use them to turn on lights, personalstereos, hairdryers and numerous other devices. A switch in its simplest form is usedfor making and breaking an electrical circuit. It usually contains metal contacts whichwhen touching allow current to flow.

Switch types

There are several ways in which the contacts in mechanical switches can be operated.Some are push button, toggle, slide or magnetic (reed), tilt and electromagnetic relay.

Switches are wired up to suit their application. A switch with it’s contacts apart whenit is not operated is called a normally open switch.

The simplest type of switch is represented by the symbol shown below

AE.Int2.O2.fig2

Notice that the switch consists of two parts, a pole and a contact. This switch iscalled a single pole single throw switch (SPST). It is given this name because itssingle pole can be thrown into contact in one position only.


Three further commonly used switch layouts are given below.

AE.Int2.O2.fig3

AE.Int2.O2.fig4

AE.Int2.O2.fig5


Relay

The relay is not strictly speaking an output device but a switch that can be driven bythe output from an electric or electronic circuit. It is an electromechanical deviceconsisting of two main parts - the operating coil (which is essentially a solenoid) andthe contacts.

AE.Int2.O2.fig1a

AE.Int2.O2.fig1b - Circuit symbol for a relay

An electric current is sent through the coil that energises it. The coil becomesmagnetic and it attracts a spring-loaded armature that moves the contacts together(energised position). Switching the supply off to the coil causes the relay to re-set tothe normal (de-energised) position.

These contacts can then be used to switch on a very powerful circuit or a number ofcircuits.

The relay is a very useful device and is particularly useful for energising devicesthat require substantial amounts of current. It is perhaps the most commonly usedswitch for driving devices that demand large currents.


Variable Resistor (Potentiometer)

A potentiometer or variable resistor can be used in a circuit either as a voltage orcurrent control device.

AE.Int2.O2.fig5a

Potentiometers normally have three tags, the outer ones being connected to the endsof the resistive material and the centre one the wiper.The spindle of the potentiometer is connected to the wiper, which is able to traversefrom one end of the resistance to the other when the spindle is rotated. As the spindlerotates a sliding contact puts more or less resistive material in series with the circuit.


Light Dependent Resistor (LDR)

The LDR (sometimes called a photoresistor) is a component whose resistancedepends on the amount of light falling on it. It’s resistance changes with light level.In bright light its resistance is low (typically around 1K). In darkness its resistance ishigh (typically around 1M).

The circuit symbol and typical characteristics are shown below.

AE.Int2.O2.fig6

AE.Int2.O2.fig50 - Graph of Illumination / Resistance


Thermistors

A Thermistor is a device whose resistance varies with temperature. It is a temperaturedependent resistor. There are two main types:1. Positive temperature coefficient (+t) or (ptc) - where resistance increases as

temperature increases.2. Negative temperature coefficient (- t) or (ntc) - where resistance decreases as

temperature increases.

The circuit symbol and typical characteristics are shown below.

AE.Int2.O2.fig7

AE.Int2.O2.fig8


AE.Int2.O2.fig51 - Graph of Temperature / Resistance

Strain Gauges

These are really load sensors. They consist of a length of resistance wire and whenstretched their resistance changes. Strain gauges are attached to structural membersand as they are loaded you can obtain a reading on a voltmeter.

AE.Int2.O2.fig9


Criteria for selecting transducersOnce the physical condition to be monitored and the output requirements of thesystem have been identified, a choice of transducers has to be made. A number ofdifferent criteria may need to be taken into account, some of which are listed in thetable below.

Response Time Most transducers are required to respond to the change inconditions. If changes occur quickly a transducer with a fastresponse time may be required.

Linearity This is especially important when using transducers inmeasuring instruments

SensitivityIf changes in physical conditions produce only smallchanges in the electrical properties of an input transducer,then a differential amplifier (covered in later units and atHigher) may be required to amplify the small changes beforefurther processing can take place.

Physical Size This may be an important criterion dependent on the systemthe transducer is to be placed in. (e.g. a loudspeaker may beinappropriate as an output transducer for a personal stereo).

Robustness This may be dependent on the environment that thetransducer is exposed to ( or the users will be exposed to)

Accuracy Accuracy of the transducer could be of the utmostimportance in some situations.

Repeatability The ability of a transducer to consistently reproduce thereading for the same conditions.

Cost Given all the above, is the transducer cost effective for theapplication?

Full technical details of transducers and all electronic components are contained inmanufactures data sheets and increasingly in catalogues. (RS Components supply arange of data sheets plus the RS Catalogue on CD-ROM. The CD-ROM containsproduct technical details as well as data files in pdf. format that can be easily accessedand printed off if hard copies are required.)


Assignments: Use of manufacturer’s data sheets or CD-ROM.

Use manufacturers data sheets to answer the following questions

1. A kiln used in a brick making process has to heat the bricks to a temperature of600 0C. Which temperature input transducer would be most suitable formonitoring the kiln temperature and why?

2. A photographer wants to time how long his flash light bulb comes on for when he

takes a photograph. To do this, he connects a light sensor to a timer as shownbelow.

AE.Int2.O2.fig10

With reference to appropriate manufacturers data sheets, decide which of thefollowing light input transducers would be most suitable and why: - LDR,Selenium cell, and photo diode.

3. Most computers use LEDs as indicators to show various conditions. Why are

LEDs used in preference to normal filament bulbs?


INPUT SIGNALS

Voltage divider Circuits

If an input transducer changes it’s resistance as the physical conditions change, thenthe resistance change has to be first converted into a voltage change before thesignal can be processed. This is normally done by using a voltage divider circuit.

If two or more resistors are connected in series (see figure 11 below), the voltage overeach resistor will depend on the supply voltage and the ratio of the resistances.

In Outcome 1, you have already investigated circuits that have two or more resistorsin series in them and you will recall that changing the value of one of the resistors willhave the effect of changing the voltage dropped across that resistor.

In other words they were voltage divider circuits.

Voltage divider circuits work on the basic electrical principle that if two resistors areconnected in series across a supply, the voltage load across each of the resistors willbe proportional to the value of the resistors.

AE.Int2.O2.fig11

The layouts of voltage divider circuits are conventionally represented as shown abovein fig 11.

There are a number of different ways that a voltage divider circuit can be represented.Some of these are shown in fig 12.


AE.Int2.O2.fig12

These three diagrams each represent the same circuit, in slightly different form.This should not be a cause for concern since all that has occurred is that the diagramhas been rotated around on it’s side. The circuit diagram shown on the left is of thetype used in Outcome 1. The reason for the change to the style on the right is simplyso that inputs and outputs can easily be added to the circuit. As we progress throughthis section and onto more advanced circuits, it will become apparent to you whythese circuits are positioned as they are.

Consider fig 11

Increasing the value of one of the resistors will increase the voltage drop across it.(You can use Ohm’s Law to confirm this if you wish).

When monitoring physical conditions, one of the resistors in the circuit is an inputtransducer, the resistance of which will change depending on the physical conditions.

In general to calculate the voltage across any resistor in a series circuit, we can use theequation.


Voltage across resistor R = Supply Voltage x (size of resistor R/ Total resistance)

For fig 11

V VccR

R R2

2

1 2= ×

+

Worked Example

Calculate the voltage signal, V2 across the resistor R2, in the voltage divider circuitshown.

AE.Int2.O2.fig 13

Applying the voltage proportion formula

V VccR

R R2

2

1 2= ×

+

V2 1240

40 80= ×

+

V V2 4=

The voltage over the 80K resistor could be calculated in the same way, but this isunnecessary for this circuit since we can use Kirchoff’s 2nd Law to confirm theanswer. i.e. the voltages over each of the components in a series circuit must add up tothe supply voltage, hence the voltage over the 80K resistor is 12V - 4V = 8V.

It is also possible to continue to use Ohm’s Law to solve these voltage dividerquestions. You may choose whichever method you are most comfortable with.


Obtaining a signal voltage from a voltage divider circuit

If we were to replace one of the fixed resistors in a voltage divider circuit with ananalogue sensor, e.g. a Thermistor, we would now have a system which generates asignal voltage which is proportional to the change in the physical environment, in thiscase temperature. If you look at the E & L or Alpha analogue input boards you willfind that this is the method used to generate signal voltages.

AE.Int2.O2.fig 13a

Vsig changes in proportion with the resistance of the Thermistor Rth.

The Thermistor in fig 13a can be replaced by any analogue sensor, e.g. the LDR andwill generate a signal voltage proportional to the resistance of the sensor.


Assignments: Voltage divider equation

1. Using the formula described above, calculate the voltages that would appearacross each of the resistors marked “X” in the circuits below.

2. In each of the following voltage divider circuits determine the unknown quantity.

AE.Int2.O2.fig16


3. A ntc (negative temperature coefficient) Thermistor is used in a voltage dividercircuit as shown in fig 17. Using information from the graph shown, determinethe resistance of the Thermistor and hence calculate the voltage that would appearacross it when it is at a temperature of

a) 80 0C b) 20 0C

AE.Int2.O2.fig17 4. What would happen to the voltage across the Thermistor in the circuit shown in

fig 17 as the temperature is increased? 5. What would happen to the voltage across the resistor in the circuit shown in fig 17

as the temperature increases? 6. A Thermistor (type 5) is used in a voltage divider circuit as shown in fig 18. The

characteristics of the Thermistor are shown in the graph. If the voltage V2 is to be4.5V at 100 0C, determine a suitable value for R1.

State whether the V2 will increase or decrease as the temperature drops. Explain youranswer.

AE.Int2.O2.fig18


Sensing circuits

Light sensors

Obtain the relevant components and equipment then construct the light sensing circuitshown in fig 19.

AE.Int2.O2.fig 19

In normal light conditions, measure and record the voltage across the LDR and thefixed 10 K resistor.Cover the LDR, repeat the measurements and record them.

You should have found in this circuit configuration that the resistance of the LDRincreases as the light level decreases, so in this case the signal level (Vout) will rise asit gets dark.

Change the position of the LDR and the fixed resistor as shown in fig.20.

AE.Int2.O2.fig 20

Repeat the measurements taken on the first circuit record these.

You should have found that changing the position of the LDR and the fixed resistorallows the signal to change in the opposite direction i.e. the signal level will rise as itgets light.


Temperature sensors

This is a very similar arrangement to light sensing and like the light sensing circuits,temperature sensing circuits can be arranged to produce a signal to move in theopposite direction when the same temperature change is applied.Obtain the relevant components and equipment then construct the light sensing circuitshown in fig 21. Use a type 3 Thermistor (TH3).

AE.Int2.O2.fig 21

In normal room temperature conditions, measure and record the voltage across theThermistor and the fixed resistor.Apply heat to the Thermistor and repeat the measurements and record them.

Try reversing the positions of the Thermistor and the fixed resistor. Record whathappens.


Circuit Simulation Software

It is possible to use circuit simulation software such as ‘Crocodile Clips’ toinvestigate electric and electronic circuits. Circuit simulation is widely used inindustry as a means of investigating complex and costly circuits as well as basiccircuits.

Circuit simulators make the modelling and testing of complex circuits very simple.The simulators make use of libraries of standard components along with common testequipment such as voltmeters, ammeters and oscilloscopes.

Using Crocodile Clips or another similar software package construct and test thefollowing circuits.

1. Use the facilities of Crocodile Clips to adjust the circuit to the values given, thenuse the voltmeter facility to measure Vout (signal voltage). Record the value ofVout (signal voltage).

AE.Int2.O2.fig 22


2. Use Crocodile Clips to construct the following circuit.

AE.Int2.O2.fig 23

Adjust the variable resistor, alternately increasing and decreasing the resistance.Observe what happens to the reading on the voltmeter.Adjust the variable resistor to its midway value. Adjust the light level on the LDR.Observe what happen to the reading on the voltmeter.

The use of the variable resistor in sensing circuits is very important. It allowsthe level of the signal to be adjusted and acts as a means of controlling thesensitivity of the device.


Assignment: Sensing Circuits

1. Using the relevant information from manufacturers data sheets for the ORP 12LDR, determine the value of the cell resistance and hence calculate the values ofVout for the following circuits

AE.Int2.O2.fig 24 2. For the voltage divider circuits shown, calculate the maximum and minimum

values of Vout from each circuit.

AE.Int2.O2.fig25


3. Examine the circuit below in fig 26. State whether Vout will increase or decreaseas the light level falls.

AE.Int2.O2.fig26 4. The circuit for a temperature sensor is shown below in fig 27. Explain in a few

sentences how each component of this sensor sub-system contributes to its finaloperation.

AE.Int2.O2.fig27 5. One sub-system in a device for detecting changes in air pressure consists of a

voltage divider as shown in fig 27a. S is a pressure sensor and R is a fixed resistor.

AE.Int2.O2.fig 27a


a) A voltmeter (V1) is connected across the fixed resistor R. With the air pressure remaining steady, the voltmeter reading is 3 volts.

(i) If another voltmeter (V2) was connected across the sensor S, what would the reading on V2 be? (ii) What can you deduce about the resistances of the sensor and the resistor under these conditions? b) As the air pressure rises, the resistance of the sensor decreases. State how this affects the reading on the voltmeter across R. Explain your answer. c) The input to the voltage divider sub-system is “change in air pressure”. (i) What is the output from the sub-system? (ii) Using appropriate vocabulary, write a few sentences to suggest how

this output signal could be interpreted. 6. A digital device will process as logic ‘1’ an input signal greater than 2/3 Vcc and

as a logic ‘0’ an input signal less than 1/2 Vcc.

AE.Int2.O2.fig 28

Design and construct a light sensing circuit that will produce logic ‘1’ in daylight anda logic ‘0’ when covered.1. Measure and record the resistance of the LDR in daylight and when covered.2. Show by calculation, how a voltage divider would be constructed to provide the

required signal levels.3. Construct the circuit on breadboard and evaluate the voltage divider designed.4. Record the actual voltage signals produced in daylight and when covered.

Use a logic probe to check that the required logic levels have been achieved.


Semiconductor devices

Diodes

Diodes are made from two semiconducting materials usually silicon and germaniumjoined together. The junction between the material creates a one-way barrier toelectric current.The device allows current to flow in one direction only. The diode has a lowresistance when current is flowing forward and a very high resistance in the oppositedirection.The diode has two leads known as the ‘anode’ and ‘cathode’. On most common(glass) diodes, a coloured band indicates the cathode (the line in the circuit symbol).

Current will flow through the diode only when the anode is connected to thepositive side of a power supply, and the cathode to the negative side, thereforethe diode is said to be a 'polarity sensitive' device.

AE.Int2.O2.fig29

When a current can flow through the diode, it is called ‘ forward-biased’.When no current is allowed to flow through the diode it is called ‘reverse-biased’.Before any current can flow through a silicon diode, the potential difference must begreater than 0.6 V.

Obtain the relevant components and equipment then construct the diode circuitsshown in fig30. Use a type 1N 4001 diode that is commonly available.

AE.Int2.O2.fig30


Use of Diodes as a protective device in output transducers.

The ability of the diode to allow current to flow in only one direction can be made useof in circuit design and in particular with inductive output transducers such as relaysand solenoids.

The relay and solenoid are inductive devices and when the coil of a relay or a solenoidis energised and de-energised by the flow of current through it, it can generate a largereverse voltage (back e.m.f.). This reverse voltage can cause considerable damage tounprotected components like transistors in the drive circuit.The risk can be avoided by the inclusion of a protective diode that allows the energyto be dissipated by providing an alternative path for the current.

AE.Int2.O2.fig31

AE.Int2.O2.fig32


Light Emitting Diodes

A light emitting diode (LED) is a special diode and is made from a semiconductorjunction that gives out light when a forward voltage (forward biased) passes throughit. It is off when current flows in the opposite direction (reverse biased).

AE.Int2.O2.fig33

LEDs are used mainly for visual indicators that a circuit is working or that a signalhas been passed. Red is the most common colour of LED but increasingly green andyellow are being used.

LEDs normally operate when the voltage drop across them is between 1.4 - 2volts. The current through the LED should not exceed 20 mA.For the purposes of this course we will assume that all LEDs that we will use willrequire

LED voltage = 2 voltsLED current = 20 mA

The voltage drop across the LED of 2 volts is greater than that of a normal diode.This is because some energy is given out as light.

LED’s are polarised devices and must be connected the right way round in a circuit;they can be very easily damaged. Since the voltage to be applied across the LED isaround 2 volts they almost always have to have a resistor connected in series withthem.This resistor is called a current limiting resistor .

The value of the current limiting resistor can be calculated as follows.

AE.Int2.O2.fig34


Voltage Supply = 6 VVoltage across LED = 2 VVoltage across the resistor = 6 - 2 = 4 VMaximum current through LED = 20 mACurrent limiting resistor = V/I = 4/20 x 10-3 = 200 RNearest preferred value = 220 R


Assignments - Diodes

1. An LED is a polarity sensitive device. Explain the meaning of polarity sensitive. 2. What does the coloured band on a glass diode represent? 3. Explain what is meant when a diode is forward biased. 4. What happens if a diode is reverse biased? 5. Most circuits operate with a supply voltage that is too high for an LED and a

resistor is required in series with the LED.

(a) Explain the purpose of the series resistor.

(b) Calculate the value of the current limiting resistor in each of the followingcircuits. Using the calculated value, select the nearest preferred value. Thecurrent taken by the LED is 20 mA and the voltage across the LED is 2 V.

AE.Int2.O2.fig34a

6. For the following circuits, state whether the LED and lamp are on or off.

AE.Int2.O2.fig34b


7. A student is asked to design a circuit to test fuses. The specification for the circuitis as follows.

• The 1.5 V battery powering the circuit is tested before the fuse is inserted bypressing a switch. If the battery is in good condition, only the red LED lights.

• When an 'unblown' fuse is placed across the test points A and B, with the switchopen, only the green LED lights.

(a) Study the following circuits

AE.Int2.O2.fig34c

Place 3 ticks in the table below to show which of the three circuits shown abovesatisfy the specification.

Circuit 1 Circuit 2 Circuit 3 Circuit 4 Circuit 5 Circuit 6


8. Name two output devices that require a protective diode. 9. For the circuit below,

a) Which device is the diode protecting?b) Explain the need for the diode.

AE.Int2.O2.fig34d

8. Name two output devices that would not require a protective diode and explainwhy not.


Transistors (Bipolar)

The transistor is another semiconductor device. It is made of three layers ofsemiconducting material. The layers are made from either ‘n type’ material or ‘ptype’.There are two types of Bipolar transistor available, pnp or npn and this relates to theorder of the layers of semiconducting material. Both types of transistor operate in thesame way.For convenience only the npn type will be considered.(For pnp transistors, the currents and voltages should be reversed).

A npn transistor is made from two layers of ‘n type’ material and one layer of‘p type’. The layers are called the emitter, base and collector and each have a legattached to them, and thus a transistor is a three-legged device.

The BC 108 is a general-purpose transistor that is commonly used in schools. The BC108 comes in the T018 case. The diagram of the transistor shows the position of thelegs when viewed from underneath the case.There are different case types from the one shown in the figure. For other types referto manufactures data sheets or catalogues.

AE.Int2.O2.fig34e

AE.Int2.O2.fig35

The transistor has to be connected into circuits correctly. The arrowhead on theemitter indicates the direction of ‘ conventional’ current flow (positive to negative).


AE.Int2.O2.fig36Depletion layer prevents current from flowing.

The ‘p type’ layer, or depletion layer, acts as a barrier that prevents current flowingthrough the device from the collector to the emitter.

If a small current signal is introduced to the base, Ib, of the transistor then thedepletion layer is reduced and current flows throughout the transistor. As the basecurrent increases the depletion layer further reduces until a level is reached at whichthe transistor is fully ‘switched on’. In this condition the transistor is said to be fullysaturated.

AE.Int2.O2.fig37Current flow as depletion layer reduces

The current entering the base of the transistor is added to the collector current, Ic, toproduce the emitter current, Ie.

I e = Ic + Ib

Since in most applications Ib is about 1% of Ic, we can normally assume that:

I c = Ie


How does the transistor work?

Consider the circuit shown in fig38

AE.Int2.O2.fig38

When the switch S1 is open no current can flow in any part of the circuit. This mayseem strange since a 'complete' circuit appears to be made from the voltage source,through the bulb, the transistor and back to the voltage source.This can be explained by using the 'depletion layer' analogy shown and explained infigs 36 and 37.

When there is no current flowing to the base of the transistor, the depletion layer canbe considered as being large and acting as a barrier to current flow (a very highresistance) no current will flow through it, therefore the bulb will not light.

AE.Int2.O2.fig 39

When switch S1 is closed, a very small current flows through the base of thetransistor. When this happens the transistor allows current to flow through it and thebulb will light, the transistor is said to 'switch on'.

In terms of the depletion layer, when a small current is applied to the base through thebase resistor, the depletion layer reduces and current is allowed to flow throughout thetransistor via the load - in this case the bulb.


Bipolar Transistors amplify current. A small current flowing to the base of atransistor causes a much larger current to flow from collector to emitter.

AE.Int2.O2.fig40

It can be seen from fig 40 that:

Since Ib is much smaller than Ic, in normal situations we can assume Ic = Ie

IcIbIe +=


Switching effect of a transistor - Transistor as a switch

The transistor can be made to operate like a switch by controlling the current flowinginto the base of the transistor.

Assignment - Transistor as a switch

Use Crocodile Clips or another circuit simulation package to set up the followingcircuit. Begin with the Rb value 2200 K and reduce to the lowest value.(If you do not have access to a circuit simulation package then you should obtain thenecessary components from your teacher to construct this circuit.)

AE.Int2.O2.fig42

From your investigation of the circuit complete the following table.

Rb (K) V be (mV) I b (µA) I c (mA) I e (mA) Lampon/off

2200 µA µA

1000 µA µA

470220100473322101 mA

From completing the activity you should have found that the circuit would 'switch on'the lamp when the voltage signal across the base emitter junction (Vbe) of thetransistor was 0.7 volts.


You will also have noted that as Vbe rises above 0.7 V the brightness of the lamp doesnot increase. This is due to the fact that once this level has been reached the transistoris fully 'switched on', or Saturated.

A transistor is saturated when the maximum current flow across the collectoremitter junction is achieved.

In order to fully switch on a npn transistor, the voltage signal across the base emitterjunction must be between 0.6 V and 0.8 V. For most practical purposes a value of 0.7V is assumed.

Vbe = 0.7 V

Note: Any attempts to increase Vbe will simply increase the base current veryrapidly. This will damage the transistor. This can been seen for the last value ofRb, where Ib shows a significant increase, but Vbe does not.

This activity should also clearly show the link between Ib, Ic and Ie.


Current Gain of a Transistor

The result of introducing a small current to the base of a transistor is to 'switch on' alarger current across the collector emitter junction. In effect a small current signal isbeing used to produce a larger current signal. The factor by which the input signal isamplified is called Gain.

The Current Gain of a transistor is defined as follows:

The accepted symbol for transistor current gain in this mode is hFE.

Ib

Ich

Ib

IcGain

tBaseCurren

urrentCollectorCnCurrentGai

FE =

=

=


Assignments: Transistor Gain

Type (npn) Vce (max)

(V)I c (max) mA Pmax (mW) hFE Application

BC 107 45 100 300 110 - 450 Audio driverBC 108 20 100 300 110 - 800 General purposeBFY51 30 1000 800 40 min. General purpose2N3053 40 700 5000 50 - 250 General purpose

1. Calculate the gain of a transistor if the collector current is measured to be 10 mAwhen the base current is 0.25 mA.

2. Calculate the collector current through a transistor if the base current is 0.3 mA

and hFE for the transistor is 250. 3. What collector current would be measured in a BC107 transistor if the base

current is 0.2 mA? 4. What base current would be measured in a BFY50 transistor if the collector

current is 200 mA? 5. For the given specification, choose a suitable transistor from the table above. Material: npn Ic (max): 70 mA Vce (max): 20 V hFE (min): 500 Power: 200 mW Application: General purpose 6. This question refers to the table of values you obtained in the section on transistor

switching.Use values obtained for Ib and Ic to determine the gain of the transistor used inyour circuit. You may wish to try several of the values to confirm yourcalculations.


Biasing the transistor - Providing the correct voltage

The circuits shown if figs 38 and 39 show a resistor connected to the base of thetransistor. The purpose of this resistor is to control the flow of current to thetransistor. As we have stated before, the base current needs to be very smallotherwise the transistor could be damaged. In practice connecting a resistor to thebase of the transistor controls the base current. The two most common methods ofbiasing a transistor are shown below.

AE.Int2.O2.fig41

The voltage divider circuit, shown in the second method, should be familiar to you asa means of splitting up the input voltage into appropriate proportions.

Transistor control circuits

The principle advantage of electronic systems is the ability to process complexelectronic signals using very small amounts of energy. Electronic systems are able toachieve this because the current used by an electronic signal is extremely small.

Having processed an electronic signal, the problem that arises is that to operate eventhe simplest of output devices, such as a buzzer, motor or solenoid, a larger amount ofcurrent is required than the electronic system can provide.

To enable electronic systems to operate output devices, drive circuits are required toamplify the current from the electronic signal.

Amplifying devices are said to be active components, as opposed to non-amplifyingcomponents (resistors, capacitors etc.) that are known as passive components. Theextra energy required to operate the active component comes from an external powersource (battery, transformer etc.).


Transistor Circuit Construction

There are a number of factors to consider before you begin to construct transistordrive circuits.Some of the design issues you need to consider when using transistors are as follows:

1. What type of transistor will be needed. 2. Which leg is the emitter/base/collector. 3. How much current will the transistor be expected to take. 4. How will you bias the base of the transistor. 5. How will you work out the resistor values need.

Choosing an appropriate transistor

Base Resistor

When constructing transistor switching circuits a base resistor should always be used.If there was no resistor in series with the base of the transistor, the current flow intothe transistor could become too large and damage the transistor. For most commonapplications the base resistor value usually lies between 1 K and 10 K.

Collector Current

This is a most important aspect of transistor circuit design. It is important to knowwhat current will flow through the device into the collector. For example a 6 V MESbulb will typically have a maximum current of 60 mA. Therefore it will be necessaryto select a transistor that can cope with this value.

Transistor Gain

When you have decided what the values are for Ib and Ic, then you can calculate whatgain is required. Once you have calculated the gain required you would be able torefer to manufacturers data sheets to make your choice.


Circuit design with transistors

In practical circuit design it is usual to link a sensor (input) to the transistor (process)then on to a useful output device.

Consider the temperature sensing circuit show below.

A lamp is required to go on if the temperature of a certain room rises above a certainlevel.

AE.Int2.O2.fig43

The output device (in this case the lamp) will be on when it is hot.

It is possible to make the circuit more effective. This is achieved by replacing the 10K fixed resistor in the sensing circuit by a variable resistor. The inclusion of thevariable resistor allows the sensitivity of the circuit to be adjusted.


Assignment - Circuit sensitivity

Using Crocodile Clips or another circuit simulation package, construct the followingcircuit.The component values should be set to

Thermistor: 7 K at 25oCVariable resistor: 22 K

When you have set up this circuit you should begin by setting the variable resistor at1.1 K and the thermistor temperature at 0Oc. Gradually increase the temperature andobserve the readings on Vbe and the output of the lamp. Now set the variable resistorto 4.4 K and gradually increase the temperature until the lamp goes on.Try this for a few more setting on the variable resistor.

AE.Int2.O2.fig44

You should have found that you were able to adjust the circuit to switch on atdifferent temperature i.e. the circuit is responding to a smaller or larger physicalchange.


Assignment - Practical Circuit Construction

Light sensitive circuits

Gather the necessary components and construct the circuit shown in fig 45.

6V

0V

10K

1K

220R

AE.Int2.O2.fig45

1. Power up the circuit 2. In normal daylight conditions the LED should light. 3. Cover the LDR - you should find that the LED should not light.

Repeat the test with the circuit shown in fig 46

6V

0V

10K

1K

220R

AE.Int2.O2.fig46

1. Explain in your own words how the system operates.


Circuit sensitivity - Practical Circuits.

The switching effect of the transistor is controlled by the voltage that is supplied tothe base of the transistor via the voltage divider part of the circuit. Using a variableresistor in place of the 10 K fixed resistor means that you can adjust your circuit andchoose the light level at which your circuit switches on and off.

NB when a variable resistor is used in the voltage divider part of the circuit, it ispossible to adjust its value down to zero resistance and place a high voltage across thebase/emitter of the transistor. To protect the transistor from too large a current in thebase, a fixed resistor is inserted between the voltage divider part and the transistorbase (base resistor).

Assignment - Practical Circuit Construction

Light sensitive circuits with variable sensitivity

Gather the necessary components and construct the circuit shown in fig 47.

6V

0V

1K

220R

AE.Int2.O2.fig47

1. Power up the circuit 2. Set variable resistor to mid value 3. Cover the LDR and observe what happens.

4. Adjust the variable resistor between its minimum and maximum values and coverthe LDR

5. Evaluate the circuit for different values of light intensity and variable resistorvalues.

6. Explain in your own words how the system operates and is different to the lightsensing circuit with the fixed resistor in the voltage divider.



INTERMEDIATE 2

APPLIED ELECTRONICS

SECTION 3

OUTCOME 3


OUTCOME 3

Design and construct a simple combinational logic circuit to solve a givenproblem.

When you have completed this unit you should be able to:

• Identify single logic gate symbols• Complete truth tables for single logic gates• Analyse combinational logic circuits• Complete truth tables for combinational logic circuits• Write Boolean expressions for simple combinational logic systems• Identify differences between the TTL and CMOS families of IC's• Identify types of logic gates, given pin layout diagrams or IC number (logic gate

IC's)• Use manufactures data sheets/CD-ROM to aid selection of appropriate integrated

circuits for a given purpose.• Correctly 'power up' an IC for use - on breadboard, circuit simulation and

diagrammatically.Note: Use of Boolean expressions in this unit at Intermediate 2 level is intended as anintroduction to Boolean.

Students should be able to:Write Boolean expressions for basic logic gates.Write Boolean expressions for combinations of no more than three logic gates.Write Boolean expressions from truth tables.

Before you start this unit you should have a basic understanding of:

• Use of breadboards• Use of test equipment: multimeter and/or logic probe• Use of LED's.


Basic logic gates

There are seven different logic gates; these are the OR, AND, NOT, NOR, NAND,ExOR and the ExNOR. We will be concentrating on the five most common types. i.e.the first five listed.

When drawing circuits containing logic gates it is common to use logic symbols.Two sets of symbols might be used.

American Military Symbols (ANSI) Used in this and other countriesBritish Standard Symbols (BS3939) Used in this country

Throughout this course the more common American symbols will be used. These arethe symbols used by SQA.

NOT

AND

NAND

OR

NOR

AE.Int2.O3 fig1


Truth tables

Electronics is concerned with the processing of electrical signals.

AE.Int2.O3 fig1b

Input signals come from a variety of sources - a switch from a keyboard; a bar codereader; a temperature sensor; another part of a computer.

Output signals can have a variety of destinations - a monitor; a modem; an alarm;another part of a computer.

Digital signals can be at a HIGH voltage level or a LOW voltage level.In logic circuits a LOW signal is said to be at logic '0' a HIGH signal at logic ' 1'.The results can be recorded and used in a number of formats, the most common beingshown below.

The easiest way to represent how each gate behaves is to make use of truth tables. Atruth table shows all possible combinations of inputs and outputs to a logic gate.

OR gate

IN P U TS

O UT P U TA

B

IN P U T O UT P U TA B0 0 0

1 0 10 1 1

1 1 1

AE.Int2.O3 fig3

Results displayed in this way are known as truth tables.


Structure and layout of truth tables

Input conditions: Let us represent the inputs to a logic gate as two switches.

O UT P U TS W ITC H A

S W ITC H B

AE.Int2.O3 fig3

It is possible for each switch to be in one of two positions, either, on '1' or off '0'.These positions are known as input states.In order to determine the number of input conditions in a logic circuit, you must firstconsider how many inputs there are.

In the above diagram there are 2 inputs i.e. a 2 input problem.

Digital electronics is of course concerned with the Binary number system.Each of these inputs can therefore be in one of two STATES either '0' or '1'

Therefore the number of input conditions possible for a two input problem = 22

22

N UM B ER O F IN PU T STO T H E S Y ST E M

B IN A RY S TATE S

AE.Int2.O3 fig4

The number of input conditions = 22

= 4

i.e. there would be 4 lines in the truth table

For a 2 input gate the Truth table would be

INPUT OUTPUT

A B0 0

1 00 1

1 1

AE.Int2.O3 fig5


For a 3 input problem; either a single 3 input gate or a simple combination of gatese.g.

A

B

C

O U T P U T

O UT P U T

ABC

AE.Int2.O3 fig6

Number of inputs = 3

Input states = 2

Number of input conditions = 23

= 8

i.e. there will be 8 lines in the Truth table.

General case:

For n inputs

Number of input conditions = 2n


Truth tables for Individual logic gates

OR gate

IN P U T O UT P UTA B0 0

1 00 1

1 1

A

B

O UT P UT

IN P U TS

AE.Int2.O3 fig7

Assume both switches are initially at logic '0' or low

Copy and complete the truth table for the OR gate shown.

Answer:

When is the output from the OR gate 'high'?

Answer:

A possible application of this type of logic gate might be in a computer printer. Theprinter paper can be fed through either by pressing the button on the printer (line feed)OR by sending a signal from the computer.

COMPUTER SIGNAL

PRINTER BUTTON

PAPER FEED

AE.Int2.O3 fig8


AND gate

INPUTA B0 0

1 00 1

1 1

A

B

OUTPUT

OUTPUT

AE.Int2.O3 fig9

Copy and complete the truth table for the AND gate shown.

Answer:

When is the output from the AND gate 'high'?

Answer:

A possible application of this type of logic gate might be in a washing machine. Themotor in a washing machine should not operate until the water level is HIGH enoughAND a signal is sent from the control program.

W AT E R LE V EL S EN S O R

C O N T R O L PR O G R A M

M O TO R

AE.Int2.O3 fig10


NOT gate (Inverter)

INPUT OUTPUT

01

IN OUT

AE.Int2.O3 fig11

Copy and complete the truth table for the NOT gate shown.

Answer:

When is the output from the NOT gate 'high'?

Answer:

A possible application of this type of logic gate might be in a central heating system.The heating system should switch OFF when the temperature is too HIGH and ONwhen the temperature is too LOW.

T E M P E RAT UR E SE N S O R C EN T R AL H EAT IN G

AE.Int2.O3 fig12


NOR gate

INPUTA B0 0

1 00 1

1 1

A

B

OUTPUT

OUTPUT

AE.Int2.O3 fig13

Copy and complete the truth table for the NOR gate shown.

Answer:

When is the output from the NOR gate 'high'?

Answer:

A combination of the OR gate and NOT gate produce the NOR gate. The output ofthe OR gate is inverted by the NOT gate.

AE.Int2.O3 fig14

Try this for yourself by completing a Truth table for this combination.

A possible application of this type of logic gate might be in a car. To avoid accidentsat times of poor visibility, a warning indicator in the car should operate if the lightlevel is too low and the car headlamps are off.

H EA D LA M P S W IT CH

LIG H T LE V EL S E N SO R

W A RN ING IN D ICATO R

AE.Int2.O3 fig15


NAND gate

INPUT OUTPUTA B0 0

1 00 1

1 1

OUTPUTA

B

AE.Int2.O3 fig16

Copy and complete the truth table for the NAND gate shown.

Answer:

When is the output from the NAND gate 'high'?

Answer:

The NAND gate can be made by combining the AND gate and the NOT gate. Theoutput from the AND gate is inverted by the NOT gate.

AE.Int2.O3 fig17

Try this for yourself by completing a truth table for this combination.


A possible application of this type of logic gate might be in the maternity unit of ahospital. The temperature and pulse rate of premature babies has to be continuallymonitored. A warning alarm should sound if either the temperature or the pulse rateof the baby falls too LOW.

P ULS E RAT E SE N S O R

T E M P E RATU R E S E NS O R

W A RN ING A LA R M

AE.Int2.O3 fig18


Gate Networks (Combinational logic)

It is often necessary to use more than one gate to perform a decision process.For example, on some TV sets it is possible to change the channel either by pressingthe channel select button on the TV OR by pressing the channel select button on theremote control. The channel will only change however, if the TV set is switched on.i.e. The channel will change if the TV set button is pressed OR the remote button ispressed AND the TV set is on.

A logic diagram for this is shown

T V S E T B U TT O N

R EM O T E BU T TO N

C HA N NE LT V O N /O F F SW IT C H

AE.Int2.O3 fig19

Combinational logic circuits are that in which the output at any time is determinedentirely by the combination of input signals which is present at that time.

i.e. systems where the state of the output depends only on the state of the inputs.


Analysing a combinational logic system

In digital electronics we often encounter systems which will contain several logicgates that have been combined together.

Suppose that you need to know how a particular network will behave for eachpossible combination of inputs.[there are a variety of techniques possible]

Whilst there is no set or hard and fast method for analysing combinational logiccircuits, the following is a suggestion.

For a given logic diagram

A

B

B

C

D (O U T PU T )

AE.Int2.O3 fig20

STEP 1 Label all the different points on the circuit, including inputs andoutputs.

Notice that one input serves two gates, this is quite common in logic circuits and bothgates should be labelled accordingly.

STEP 2 Draw up a Truth table for the circuit, with a different column for eachletter.

NB. determine the number of input conditions for the truth table.

two input, therefore 22 = 4

i.e. 4 lines on the truth table.

INPUT OUTPUT

A B0 0

1 00 1

1 1

C0111

D

AE.Int2.O3 fig21


STEP 3 Determine what goes into column C. To do this, consider the OR gateon its own, with inputs A and B

A

B

C

AE.Int2.O3 fig22

enter findings into column C.

STEP 4 Determine what goes into column D i.e.. the output column. Considerthe AND gate on its own with inputs from C and B

C

B

D (OUTPUT)

AE.Int2.O3 fig23

enter findings into column D.

STEP 5 your final answer should look like this

INPUT OUTPUT

A B0 0

1 00 1

1 1

C0111

D0101

AE.Int2.O3 fig24

More complicated systems may need several stages before you finish, but if you worksystematically through the circuit in the way shown, considering one gate at a time,you should arrive at the correct answer.


Assignments: Truth tables

Complete a truth table for each of the combinations of gates shown below.It is possible to use circuit simulation software to verify your answers.

a)

d)

e)

f)

AE.Int2.O3 fig25


Boolean Expressions

Boolean algebra is a special form of algebra that has been developed for binarysystems. It was developed by George Boolean in 1854 and can be very useful forsimplifying and designing logic circuits.

VariablesThe most commonly used variables in logic circuit design are capital letters; such asA, B, C, Z and so on and are used to annotate inputs and outputs to systems.

In digital electronics we consider situations where the variables can only have one oftwo possible values, i.e. 'Logical 0' or 'Logical 1'. We are obviously dealing with abinary system.The statement A = 1 means that the variable A has the value of Logical 1. Similarly,if B = 0 it means that variable B has the value of logical 0.

Logical Operations: In Boolean algebra there are three logical operators, these arethe AND operation, the OR operation and the Inversion.

NB. It is essential to understand whilst undertaking analysis using Boolean that youmust not confuse the symbols used for the Logical operators with those used innormal algebra.

+ represents logical operator OR• represents Logical operator ANDA represents A bar i.e. NOT A ( the inverse of A)

AND Operator: The AND operation can be represented in Boolean notation by

A.B = Z

The dot between the A and the B is read as AND.

OR Operator: The OR operation can be represented in Boolean notation by

A+B = Z

The + between the A and the B is read as OR.

Inversion: The statement A=1 means that A is not equal to 1.The variable A is read as A bar and usually means NOT A . The bar over the top ofthe variable changes it's value , or inverts it . This is known as the NOT operation.


Basic logic gates and their Boolean representations

AND gate.

IN P U TZ =A .B

A B0 00

1 000 01

1 11

A

B

Z =A .B

OR gate.

IN P U TZ =A +B

A B0 00

1 100 11

1 11

A

B

Z =A +B

NOT gate.

AZ= A

INPUT

01

Z= A

01

Read as output Z is equal to NOT A


NAND gate.

IN P U TA B0 10

1 100 11

1 01

AZ = A .B

Z = A .B

B

This reads as output Z is equal to A AND B all NOT

The NAND gate is made up from a combination of an AND gate followed by aNOT gate. This arrangement demonstrates the Boolean notation quite clearly.

A

B

A .B A .B

NOR gate.

IN P U TA B0 10

1 000 01

1 01

A Z = A +B

Z = A +B

B

This reads as output Z is equal to A OR B all NOT

The NOR gate is made up from a combination of an OR gate followed by a NOT.

A A+B A+B

B


Assignment: Write down the Boolean expression for each of the following logicgates.

A

BZ

a )

A

BZ

d )

AB Z

C

g)

A

BZ

b )

A Z

e )

AB

ZCD

h )

A

BZ

c)

AB Z

C

f)

AB

ZCD

k)


Deriving the Boolean expression for a combinational logic circuit.

Consider the following circuit.

A

B

B

The Boolean expression for the circuit can be derived as follows:

1. Label the inputs on the left-hand side of the diagram (If one input serves more thanone gate ensure that the input is labelled accordingly).

2. Use the Boolean notation (operators) to give the output of the gate in terms of itsinput.

A

B

B

A .B

B

Write on the appropriate expression after each gate.

3. When outputs from other gates are inputs to a further gate, treat the expressions asyou would any other equation and make use of brackets (if necessary). Then write onthe appropriate expression after the next gate and so on until you reach the finaloutput.

A

B

B

A.B

B

A.B + B

The final Boolean expression is the output from the final gate A.B+B


Assignment: Derive the Boolean expression for each of the following combinationallogic circuits.

B )

c )

a )

d )


Deriving the Boolean expression from a Truth Table.

Obtaining a Boolean Expression from a Truth Table is a fairly straightforwardoperation.

Consider the Truth Table given.

Concentrate solely on the combinations of inputs that give a logic 1 condition in theoutput column. In the truth table given below there are two inputs A and B and oneoutput Z. The output Z is at logic1in the 1st, 3rd and 4th rows.

A B

0 10

1 100 01

1 11

C D Z

0 1

0 10 0

1 0

1. Note each combination that will give you a '1' at the output

A B

0 10

1 100 01

1 11

C D Z

0 1

0 10 0

1 0 A.B

A.B

A.B

write this at the side of the Truth Table, next to the line that it applies to.

2. Write down each of the combinations that have a '1' as their output. The equationsare joined by putting an OR sign between each.

A.B +A.B+A.B

The OR sign is used to join these and any other equations in Boolean which satisfythe output condition since

A.B OR A.B OR A.B

would give the correct output, therefore each of these equations must be considered inorder to obtain the final solution.


Assignment: Write down the Boolean expression for each of these truth tables.

a)A B Z

0 0 0

0 1 1

1 0 0

1 1 0

b)A B Z

0 0 1

0 1 0

1 0 1

1 1 0

c)

A B Z

0 0 0

0 1 1

1 0 1

1 1 0

d)

A B C Z

0 0 0 0

0 0 1 0

0 1 0 0

0 1 1 0

1 0 0 0

1 0 1 1

1 1 0 0

1 1 1 1


e)

A B C Z

0 0 0 0

0 0 1 1

0 1 0 0

0 1 1 0

1 0 0 0

1 0 1 1

1 1 0 0

1 1 1 0

f)

A B C Z

0 0 0 1

0 0 1 0

0 1 0 0

0 1 1 1

1 0 0 0

1 0 1 0

1 1 0 0

1 1 1 1

g)

A B C Z

0 0 0 1

0 0 1 0

0 1 0 0

0 1 1 1

1 0 0 1

1 0 1 1

1 1 0 0

1 1 1 1


Integrated circuits

In digital electronic circuits, logic gates are normally manufactured onto an integratedcircuit (IC). An IC can, and more often does, contain more than one logic gate.IC's can be found in a wide variety of packages. The most common is the plastic DIL(Dual In Line) pack, where the silicon chip is enclosed in the plastic case withconnecting pins to provide connections to the appropriate gates. Here the pins rundown either side of the plastic package as shown below. The number of pins can be a8, 14, 16, 18, 20, 24, 40 and even 60 pin.

14 13 12 11 10 9 8

1 2 3 4 5 6 7

+V cc

G nd(0V)

N O TC H - IN D IC ATE S TH E TO P

D O T- IN D IC AT ES P IN 1

P LA STIC C AS E

AE.Int2.O3 fig 26

Pin numbering starts with pin 1 that is always below the notch or identifying circle.The numbering then goes round in an anti-clockwise direction.

Advantages of Integrated Circuits:

Integrated circuits have the following advantages over discrete components.

1. They are smaller and lighter.

2. They are cheaper than discrete components due to manufacturing techniques andscale of integration (the number of components on each chip).

3. More reliable and easier to replace.


Types of Integrated Circuits available (Logic Families)

There are two main methods of producing electronic logic devices today, giving twofamilies of logic and these are MOS and TTL.

MOS Logic Devices

MOS stands for Metal Oxide Silicon. MOS devices use field effect transistors(FET's) to make up the gates. Various types of MOS are in use:

CMOS Complimentary Metal Oxide Silicon referring to the fact thatComplimentary p-type silicon and n-type silicon channels are used.

PMOS Where the channel is made from p-type silicon.

NMOS Where the channel is made from n-type silicon.

TTL Logic Devices

TTL stand for Transistor - Transistor Logic. These devices use bipolar transistorsto make up the gates

Most switching gate circuits use either Transistor - Transistor Logic (TTL) orComplimentary Metal Oxide Silicon (CMOS) gates.

TTL gates are available commercially in the 7400 series and CMOS IC's areavailable in the 4000 series.

Comparison of CMOS and TTL devices.

The table compares the two types of logic IC's.

Property CMOS TTL

Supply voltage 3 - 15 V (18 V max) 5 V (+ or - 0.25 V)

Current required 8µA 3 mA

Switching speed Slow Fast

Fan out 50 10

NB. A new hybrid chip is available on the market that has the advantages of bothtypes and is listed as the 74HCT00 series. The 74HCT series is a high speed CMOSsemiconductor and has all the advantages of high speed CMOS but with inputsconfigured for direct drop-in replacement of TTL. It operates in the voltage range 2-6V dc.


Summary of properties of CMOS and TTL devices.

CMOS - Advantages

1. The main advantage of CMOS devices is that they will operate on a any d.c.voltage in the range 3V-15V (18V max).

2. They have a low current drain, usually in the order of microamp.3. They have low power consumption.4. They have a high FAN OUT, usually in the order of 50 (the ability of the

output of a gate to drive a number of similar inputs to other gates).5. CMOS has very good noise immunity.

CMOS - Disadvantages

1. The major disadvantage of CMOS is it's slower switching speed, usually in theorder of 2 to 4 MHz. (4 * 106 switches per second)

2. CMOS devices can easily be destroyed by static electricity (in order to protectthem from static, when in use, all unused inputs should be connected to eitherthe zero volt line or the positive line).

Recent developments in MOS technology have seen major improvements in the areasof speed, with the latest MOS devices claiming speeds similar to TTL.

TTL - Advantages

1. The major advantage of TTL devices is their high switching speeds, usually inthe order of 50 MHz (50 * 106 switches per second)

2. No damage is done to TTL devices if inputs are left unconnected. (such inputswill set to +5V )

TTL - Disadvantages.

1. TTL devices are much less flexible in terms of their operating conditions thancorresponding CMOS devices.

2. Requires a stabilised voltage supply in the range +5V + or - 0.25V (usuallyexpensive).

3. The fan out is 10.4. Higher power consumption than CMOS devices.5. High current drain, usually in the order of milliamp.

The latest designs of TTL devices are combining higher and higher switching speedswith lower power dissipation.

In this course we will be using TTL logic gates for demonstrations and practicalwork, since they are more robust and we can leave the unused inputs floating.


Identifying Integrated Circuits

The pin diagram for a 7408 TTL logic gate is shown below

1 4 13 1 2 11 1 0 9 8

1 2 3 4 5 6 7

+ V c c

Gnd(0V)

AE.Int2.O3 fig 27

The 7408 IC is described as a quad two-input AND gate device.

The quad part indicates how many gates of the type are on the device and the two-input part refers to the fact that each AND gate has two inputs.

Other examples of TTL devices can be found in suppliers' catalogues such as RSComponents.


Examples of pin out diagrams

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

7404 7400

7421 7420

7427 7432

AE.Int2.O3 fig 28


Assignments: Integrated circuits

1. Use the data sheet on page 20 to answer these questions.

a) Which of the above ICs contain AND gates.b) Which IC contains 4 input NAND gates.c) Which IC contains NOT gates.d) What type of gate is contained in the 7400 IC.

2. Which ICs would be required to construct the following circuits.

a)

b )

c )

AE.Int2.O3 fig 29


Pin out and wiring diagrams

The following logic circuit could be constructed using ICs

IN PU T A

IN PU T BO U T PU T

AE.Int2.O3 fig 30

Since the gates within an IC are identical, any one of them can be used

The IC s are mounted on breadboard (care must be taken to ensure that the pins fromeach side of the IC are not connected). Connection between the pins is made byinserting 0.6mm solid core wire to the appropriate pin numbers.

Note: In order for your circuit to operate you must power up the IC using the correctpower source for the Logic family and the correct pins.

21 3 4 5 6 7

14 13 12 11 10 9 8

+Vcc

Gnd(0V)

21 3 4 5 6 7

14 13 12 11 10 9 8

+Vcc

Gnd(0V)

INPUT A

INPUT B

OUTPUT

AE.Int2.O3 fig 31


Assignments - Pin out diagrams

1. Draw connecting wires on the IC circuit diagram to show how the following circuitdiagrams could be constructed.

IN PU T A

IN PU T BO U T PU T

AE.Int2.O3 fig 32

IC circuit diagram

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

21 3 4 5 6 7

14 13 12 11 10 9 8

+ V cc

G nd(0V )

AE.Int2.O3 fig 33


2. The following circuit contains a 7427 and a 7404 IC.By referring to pin diagrams, draw the corresponding logic circuit.

21 3 4 5 6 7

14 13 12 11 10 9 8

21 3 4 5 6 7

14 13 12 11 10 9 8

7427 7404

INP U T A

INP U T B

INP U T C

O UT P U T

AE.Int2.O3 fig 34

3. Draw the corresponding logic diagram for the circuit shown below.

21 3 4 5 6 7

14 13 12 11 10 9 8

21 3 4 5 6 7

14 13 12 11 10 9 8

7432 7400

IN P UT A

IN P UT B

O U T PU T

AE.Int2.O3 fig 35


Practical IC circuits

The following section deals with the construction of circuits containing ICs. You willneed the following equipment.

1. Breadboard2. 0.6mm solid core insulated wire.3. Various IC's (TTL)only.4. One led.5. A resistor value around 370 R.6. Logic probe.7. 5V regulated supply.

Pin layout diagram

21 3 4 5 6 7

14 13 12 11 10 9 8

+Vcc

Gnd(0V)

AE.Int2.O3 fig 36

The pin layout diagram of a TTL IC is shown above, as previously mentioned insection 3.7.1 the orientation of the IC is determined from the location of the notch,which indicates the top and the dot that indicates pin 1.

In TTL logic gates, pin 7 is always the ground pin and pin 14 is always the +ve pin,i.e. pin 7 should always be connected directly to 0V or ground and pin 14 shouldalways be connected directly to +5V.

The logic levels of inputs and outputs can be determined by using the logic probe.The output condition can be monitored using the LED and resistor.


Assignments: Practical IC construction

1.Gather the necessary components and

a) Draw a truth table for the circuitb) Construct the circuitc) Test the circuitd) Check that the outputs from your circuit agree with the truth table

Remember that since the gates within an IC are identical, any one of them can beused.

Use the data sheet provided on page 20 for information.

21 3 4 5 6 7

14 13 12 11 10 9 8

21 3 4 5 6 7

14 13 12 11 10 9 8

7432 7400

INP U T A

INP U T B

INP U T C

O UTP UT

AE.Int2.O3 fig 37



21 3 4 5 6 7

14 13 12 11 10 9 8

21 3 4 5 6 7

14 13 12 11 10 9 8

7432 7404

INP U T A

INP U T B O UTP UT

AE.Int2.O3 fig38




AE.Int2 O3 fig 39



AE.Int2 O3 fig 40




AE.Int2 O3 fig41



AE.Int2 O3 fig42

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