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Transcript
DET: Technological
Studies
Applied Electronics
Intermediate 2
4597
Spring 1999
HIGHER STILL
DET:
Technological
Studies
Applied Electronics
Intermediate 2
Support Materials
*+,-./
CONTENTS
Teacher’s guide
Students’ materials
Outcome 1
Outcome 2
Outcome 3
Technological Studies Support Materials: Applied Electronics (Intermediate 2)
1
TECHNOLOGICAL STUDIES
INTERMEDIATE 2
APPLIED ELECTRONICS
TEACHER’S GUIDE
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide
TEACHERS GUIDE
Support Materials - Overview
The support materials for Technological Studies courses in Higher Still have been
created to specifically address the outcomes and PC in each unit at the appropriate
level. These materials contain a mixture of formal didactic teaching and practical
activities.
The support materials for each unit have been divided into outcomes. This will
facilitate assessment as well as promoting good teaching practice.
The materials are intended to be non-consumable, however it is at the discretion of
each centre how to use these materials.
Each package of support materials follows a common format:
1.
2.
3.
4.
5.
Statement of the outcome.
Statement of what the student should be able to do on completion of the outcome.
Learning and teaching activities.
Sequence of structured activities and assignments.
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 allow
assessment either after each outcome has been completed or as an end of unit
assessment when all outcomes have been completed depending on the needs of the
centre.
The use of SQA past external paper questions has been used throughout the materials
and 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 resources
such as P & N practice questions in Technological Studies is very useful for
homework activities and also in preparation for external assessment.
The use of integrated questions across units is essential in preparation of
students for External Assessment.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide
1
Support Materials - Content
Outcome 1 - Demonstrate knowledge and understanding of the relationship
between 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 as
well 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 given
specification.
The purpose of this unit of work is to introduce student to simple electronic control
systems based on voltage dividers and bi-polar transistors. The circuits should be able
to control output devices and component protection is introduced. Student activities
include calculations relating to voltage dividers and transistor gain and construction of
simple control systems.
When students have completed this unit of work they 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.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide
2
Outcome 3 - Design and construct a simple combinational logic system to meet
given specifications.
The purpose of this unit of work is to introduce students to simple combinational logic
systems. Student activities include interpretation of data sheets and construction of
simple combinational logic systems.
When students have completed this unit of work they 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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide
3
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 as
multimeters during practical activities, however any activity that will be used to
satisfy an assessment requirement must be undertaken individually.
It is expected that centres already presenting Technological Studies at Standard Grade
or Higher Level will have the majority of these resources for the current courses.
Circuit Simulation Software ( Optional at Intermediate 2)
Crocodile Clips
Oak Logic
Electronic workbench
PC/Mac
Acorn
PC
Any circuit simulation software that will enable student to create and test d.c.
networks, simple electronic systems and combinational logic systems would be
satisfactory.
General equipment - required for all units
Power supply
Breadboards
Wire for links - 0.6mm solid core insulated wire.
Wire strippers
Digital multimeter
Outcome 1
Resistors - 120 R, 470 R, 1K
Outcome 2
Resistors - 220R, 1K, 10K
Variable resistor - 20K
LDR - ORP12
Thermistors - Types 1, 2, 3, 4, 5
Transistors (NPN various) - BC 108, BFY 51, 2N 3053
Access to manufacturer data sheets/catalogue/CD - ROM
LED's
Outcome 3
Various IC's (TTL or CMOS) containing common arrangements of two and three
input logic gates
LED's
Resistor values around 370 R.
Logic probe.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide
4
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 NAB
2. 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 course
awards and is marked externally.
To gain the award of the course, the student must pass all unit assessments as well as
the external assessment.
Recording and retention of evidence
All evidence of performance should be retained by the centre for moderation
purposes.
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.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide
5
Assessment Summary of each Unit
The 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 graphical
evidence for PC a and b.
2.
Practical activity - providing performance evidence for PC c.
The practical activities contained in the support materials will satisfy the
assessment requirements for this aspect. Centres should ensure that when
candidates are carrying out the practical activity for assessment purposes,
appropriate conditions are in place.
Outcome 2 - Applied Electronics (Int 2)
1.
National Assessment Bank item (Test) - Providing written and graphical
evidence 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 the
assessment requirements for this aspect. Centres should ensure that when
candidates 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 assignments
provided in the support materials. Students should be able to evaluate the circuits
effectively to satisfy the assessment requirements; this can be done either in
writing or as a verbal report to the teacher/lecturer.
Outcome 3 - Applied Electronics (Int 2)
1.
National Assessment Bank item (Test) - Providing written and graphical
evidence 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 the
assessment requirements for this aspect. Centres should ensure that when
candidates 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 assignments
provided in the support materials. Students should be able to evaluate the circuits
effectively to satisfy the assessment requirements; this can be done either in
writing or as a verbal report to the teacher.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher’s Guide
6
TECHNOLOGICAL STUDIES
INTERMEDIATE 2
APPLIED ELECTRONICS
SECTION 1
OUTCOME 1
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
OUTCOME 1
Demonstrate knowledge and understanding of the relationship between current
and 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 codes
electrical circuit symbols
Ohm’s law
use of breadboards
use of circuit test equipment: multimeter and/or logic probe.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
1
ELECTRIC NETWORKS
An electric circuit is a closed loop made up from electrical components such as
batteries 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 for
the circuit. Voltage is measured in volts (V).
Resistance
All materials conduct some electricity. Some are good and some are poor. Materials
that are good at conducting are called conductors. Those which are poor, are called
insulators. Examples of good conductors are silver and copper. Examples of good
insulators are glass and rubber.
A good conductor is one that offers very little resistance to the flow of electric
current. 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 low
resistance - it allows electricity to flow freely.
Resistors can come in the form of purpose made resistors that have fixed or variable
values or in the form of any electrical component that offers resistance to the flow of
current in the circuit.
Resistance is therefore a measure of how much voltage is required to let a current
flow.
Resistance is measured in ohms (Ω).
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
2
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 find
that doubling the voltage difference across the resistor doubles the current flowing
through it. Thus we can say that current is proportional to the voltage difference
across a resistor.
The rule that current is proportional to the voltage difference is an important rule in
electronics and is known as Ohm’s Law.
The relationship between the voltage, resistance and current in a circuit gives rise to
Ohm’s Law formula:
Voltage = Current x Resistance
V=IxR
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/I
To find V, cover V to give V = I x R
To find I, cover I to give I = V/R
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
3
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 flow
through 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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
4
7. Calculate the current flowing through this circuit.
AE.Int2. O1 fig 5
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
5
Series and Parallel Circuits
It is possible to connect resistive components in two ways, in series where
components are connected end to end and in parallel where each component receives
the supply voltage.
AE.Int2. O1 fig 5a
Series circuits
When components are connected end to end (in series) to form a closed loop the
current 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 the
total voltage input to the circuit.
This is known as Kirchoff’s 2nd Law, which states: the sum of the emf’s (voltage
supplies) in a closed circuit is equal to the sum of the potential drops round that
circuit.
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 current
must 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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
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 supply
voltage is shared out amongst them. When you add up the individual voltages
dropped over the resistors they should equal the supply voltage.
As there are no branches for the current to flow into, the supply current must flow
through each resistor.
When resistors or resistive components are connected in series, the effect is to add
more resistance to that circuit. The total resistance can be found by simply adding up
all 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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
7
Parallel Circuits
When components or resistive materials are connected in parallel, each component
receives the supply voltage from the voltage source and the current is shared out
amongst 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 total
current IT being supplied from the voltage source. This is known as Kirchoff’s 1st
Law, in other words, the sum of the current flowing towards a single junction in a
circuit is equal to the sum of the currents leaving the junction.
IT = 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 is
the 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 components
fail, it is only that one that is effected.
A disadvantage is that parallel circuits draw more current than series circuits.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
8
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 3
devices from one supply voltage.
Each branch will have it’s own current, and the total supply current is found by
adding up all the branch currents.
When resistors or resistive components are connected in parallel, the effect is to
reduce 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
RT =
R1 × R2
R1 + R 2
Note: This special formula only works for circuits with 2 resistors in parallel.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
9
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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
10
We can use Kirchoff’s 2nd Law to check the answers calculated for the voltage drops.
VT
= V1 + V2
=3+9
VT
= 12V
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
11
Worked examples: Parallel Circuit
1. For the parallel circuit shown calculate
a)
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
I1 = 1.5 A
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
12
I2 = VS/ R2
= 12/ 12
I2 = 1 A
We can use Kirchoff’s 1st Law to check the answers calculated for the current in each
branch.
IC
= I1 + I2
= 1.5 + 1
IC
= 2.5 A
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
13
Assignments: Series and Parallel circuits
1.
For the circuit shown, calculate
a) The total resistance of the circuit
b) The circuit current
AE.Int2.O1 fig 11a
2.
For the circuit shown, calculate:
a) The total resistance
b) The circuit current
c) The voltage drop across each resistor
d) Use Kirchoff’s 2nd law to verify your answers in part c)
AE.Int2.O1 fig 11b
3.
For the circuit shown, calculate
a) The total resistance of the circuit
b) The circuit current
AE.Int2.O1 fig 11c
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
14
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.
6.
For the circuit shown, calculate
a) The total resistance of the circuit
b) The circuit current
AE.Int2.O1 fig 11d
7.
For the circuit shown, calculate
a) The total resistance of the circuit
b) The circuit current
c) The current flowing through R1 (10R)
d) The current flowing through R2 (24R)
AE.Int2.O1 fig 11e
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
15
8.
For the circuit shown, calculate
a) The total resistance of the circuit
b) The circuit current
c) 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.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
16
Combined Series and Parallel Circuits
Until now we have been looking at Series or Parallel circuits individually. It is
possible 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 with
the parallel combination.
Some points to remember when you are dealing with combined series and parallel
circuits.
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 R 2 (which is the same
as across R3) would equal the supply voltage Vs.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
17
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 parallel
arrangement (RP) of R2 and R3.
It is possible to use the special case formula for 2 resistors in parallel.
RP =
R 2 × R3
R 2 + R3
RP =
10 × 48
10 + 48
RP =
480
58
R P = 8.28Ω
The total circuit resistance (RT) is then found by adding RP to R1.
RT = R1 + R P
RT = 24 + 8.28
RT = 32.28Ω
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
18
a) It is now possible to calculate the circuit current.
VS = I C × RT
IC =
VS
RT
IC =
12
32.28
I C = 0.37 A
c) The voltage drop across R1 is found by using the resistance across R1 and the
current through R1.
V =I×R
V R1 = I C × R1
V R1 = 0.37 × 24
V R1 = 8.88V
d) The current through R2 is found by using the resistance of R2 and the voltage drop
across R2.
By using Kirchoff's 2nd Law we know that the voltage drop across the parallel
arrangement must be:
VS = V R1 + V P
V P = V S − V R1
V P = 12 − 8.88
V P = 3.12V
By using Kirchoff's 1st Law we know that the circuit current IC will 'split' or divide
between the two resistors R2 and R3.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
19
In order to find the current through R2
V =I×R
V P = I 2 × R2
I2 =
VP
R2
I2 =
3.12
10
I 2 = 0.312 A
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
20
Assignments: Combined Series and Parallel Circuits
1. For the circuit shown, calculate
a) The resistance of the parallel combination
b) the total circuit resistance
AE.Int2.O1 fig 11h
2. For the circuit shown, calculate:
a) The total resistance
b) The circuit current
c) The voltage drop across each resistor
AE.Int2.O1 fig 11j
3. For the circuit shown, calculate
a) The total resistance of the circuit
b) The circuit current
c) The current through each resistor
d) The voltage drop across each resistor
AE.Int2.O1 fig 11k
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
21
Measuring current, voltage and resistance
The most commonly used device for measuring voltage, current and resistance is the
multimeter, 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 more
advanced meters is the ability to measure capacitance, continuity and transistor gain.
There are some important points you need to know before attempting to measure
direct current and voltage.
Measuring Current
The ammeter setting is used to measure the flow of current in a circuit. In this mode
the meter must be connected in series with the circuit components, that is, you must
break the circuit at a convenient point as shown in the figure below. In this way, the
current 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 reduce
the current that it is designed to measure.
Measuring Voltage
The voltmeter setting is used to measure the voltage across a component. In this mode
the meter must be connected in parallel with the circuit components as shown in the
figure below.
AE.Int2. O1 fig 13
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
22
A meter set in the voltmeter mode has a very high resistance, so that when it is
connected across the component very little current is diverted from the component.
When measuring unknown voltages and currents, always set the meter to the
highest range then work down to increase the sensitivity of your measurements.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
23
Assignments: Using a digital multimeter
Use the equipment provided to construct and investigate the following circuits.
Equipment:
Breadboard
Digital multimeter
Power supply
Resistors - 120 R, 470 R, 1K
Wire for links
Wire strippers
Connect the multimeter in the correct mode and measure the stated values in each
circuit.
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
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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
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6. Measure and record the circuit current, the current through each resistor and the
voltage 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
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Circuit Simulation Software
It is possible to use circuit simulation software such as ‘Crocodile Clips’ to
investigate electric and electronic circuits. Circuit simulation is widely used in
industry as a means of investigating complex and costly circuits as well as basic
circuits.
Circuit simulators make the modelling and testing of complex circuits very simple.
The simulators make use of libraries of standard components along with common test
equipment such as voltmeters, ammeters and oscilloscopes.
Using Crocodile Clips or another similar software package construct and test the
following circuits.
1.
AE.Int2. O1 fig 21
2.
AE.Int2. O1 fig 22
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3.
AE.Int2. O1 fig 23
4.
AE.Int2. O1 fig 24
5.
AE.Int2. O1 fig 25
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TECHNOLOGICAL STUDIES
INTERMEDIATE 2
APPLIED ELECTRONICS
SECTION 2
OUTCOME 2
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
OUTCOME 2
Design 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 codes
Electrical circuit symbols
Ohm’s law
Kirchoff’s 1st and 2nd laws
Use of breadboards
Use of circuit test equipment: multimeter and/or logic probe.
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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. a
microphone is a transducer that changes Sound Energy into Electrical Energy.
Output Transducers
Output transducers in electronic systems are used to convert Electrical Energy into
another form that can be detected by the user or used in some other way.
Common Output Transducers
The table gives some examples of common output transducers that you may have met
before.
Output Transducer
Output Energy
Bulb
Lamp
LED
Light
Buzzer
Loudspeaker
Earphone
Sound
Motor
Pump
Solenoid
Movement
Heating element
Heat
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Examples of Common Output Transducers
At the output of an electronic system the output transducer converts the electrical
signal in to some other useful form of energy such as heat, light, sound or mechanical
energy.
Electric Motors
Electric motors convert the electrical signal into rotational kinetic energy. Before a
motor is connected to a circuit it is necessary to know the characteristics of the motor
in terms of working voltage and the maximum current to be drawn by it in order to
determine the correct choice of driver. The most common and likely choice to drive a
motor 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 pulled
into the centre of the coil (along the coil axis). This converts the electrical signal into
linear motion. A solenoid is used when in and out motion is required. Solenoids
require very large currents in order to produce meaningful force and they are usually
switched on and off by using relays.
AE.Int2.O2.fig1c - Circuit symbol for a solenoid
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Input Transducers
Input transducers convert a change in physical conditions (e.g. temperature) into a
change in an electrical property (e.g. voltage) which can then be processed
electronically 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 Transducers
The table gives some examples of common input transducers that you may have met
before.
Physical condition to be
monitored
Input Transducer
Electrical property that
changes
Temperature
Thermistor
Thermocouple
Platinum Film
Resistance
Voltage
Resistance
Light
LDR
Selenium Cell
Photo Diode
Resistance
Voltage
Resistance
Displacement
Slide Potentiometer
Variable Transformer
Variable Capacitor
Resistance
Inductance
Capacitance
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 in
voltage before the signal can be processed. This is normally done using a voltage
divider circuit.
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Examples of common Input Transducers
The switch
We all make use of switches every day. We use them to turn on lights, personal
stereos, hairdryers and numerous other devices. A switch in its simplest form is used
for making and breaking an electrical circuit. It usually contains metal contacts which
when 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 when
it 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 is
called a single pole single throw switch (SPST). It is given this name because its
single pole can be thrown into contact in one position only.
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Three further commonly used switch layouts are given below.
AE.Int2.O2.fig3
AE.Int2.O2.fig4
AE.Int2.O2.fig5
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Relay
The relay is not strictly speaking an output device but a switch that can be driven by
the output from an electric or electronic circuit. It is an electromechanical device
consisting of two main parts - the operating coil (which is essentially a solenoid) and
the 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 becomes
magnetic 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 to
the normal (de-energised) position.
These contacts can then be used to switch on a very powerful circuit or a number of
circuits.
The relay is a very useful device and is particularly useful for energising devices
that require substantial amounts of current. It is perhaps the most commonly used
switch for driving devices that demand large currents.
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Variable Resistor (Potentiometer)
A potentiometer or variable resistor can be used in a circuit either as a voltage or
current control device.
AE.Int2.O2.fig5a
Potentiometers normally have three tags, the outer ones being connected to the ends
of the resistive material and the centre one the wiper.
The spindle of the potentiometer is connected to the wiper, which is able to traverse
from one end of the resistance to the other when the spindle is rotated. As the spindle
rotates a sliding contact puts more or less resistive material in series with the circuit.
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Light Dependent Resistor (LDR)
The LDR (sometimes called a photoresistor) is a component whose resistance
depends 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 is
high (typically around 1M).
The circuit symbol and typical characteristics are shown below.
AE.Int2.O2.fig6
AE.Int2.O2.fig50 - Graph of Illumination / Resistance
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Thermistors
A Thermistor is a device whose resistance varies with temperature. It is a temperature
dependent 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
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AE.Int2.O2.fig51 - Graph of Temperature / Resistance
Strain Gauges
These are really load sensors. They consist of a length of resistance wire and when
stretched their resistance changes. Strain gauges are attached to structural members
and as they are loaded you can obtain a reading on a voltmeter.
AE.Int2.O2.fig9
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Criteria for selecting transducers
Once the physical condition to be monitored and the output requirements of the
system have been identified, a choice of transducers has to be made. A number of
different criteria may need to be taken into account, some of which are listed in the
table below.
Response Time
Most transducers are required to respond to the change in
conditions. If changes occur quickly a transducer with a fast
response time may be required.
Linearity
This is especially important when using transducers in
measuring instruments
Sensitivity
If changes in physical conditions produce only small
changes in the electrical properties of an input transducer,
then a differential amplifier (covered in later units and at
Higher) may be required to amplify the small changes before
further processing can take place.
Physical Size
This may be an important criterion dependent on the system
the transducer is to be placed in. (e.g. a loudspeaker may be
inappropriate as an output transducer for a personal stereo).
Robustness
This may be dependent on the environment that the
transducer is exposed to ( or the users will be exposed to)
Accuracy
Accuracy of the transducer could be of the utmost
importance in some situations.
Repeatability
The ability of a transducer to consistently reproduce the
reading for the same conditions.
Cost
Given all the above, is the transducer cost effective for the
application?
Full technical details of transducers and all electronic components are contained in
manufactures data sheets and increasingly in catalogues. (RS Components supply a
range of data sheets plus the RS Catalogue on CD-ROM. The CD-ROM contains
product technical details as well as data files in pdf. format that can be easily accessed
and printed off if hard copies are required.)
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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 of
600 0C. Which temperature input transducer would be most suitable for
monitoring 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 shown
below.
AE.Int2.O2.fig10
With reference to appropriate manufacturers data sheets, decide which of the
following 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?
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INPUT SIGNALS
Voltage divider Circuits
If an input transducer changes it’s resistance as the physical conditions change, then
the resistance change has to be first converted into a voltage change before the
signal 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 over
each 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 resistors
in series in them and you will recall that changing the value of one of the resistors will
have 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 are
connected in series across a supply, the voltage load across each of the resistors will
be proportional to the value of the resistors.
AE.Int2.O2.fig11
The layouts of voltage divider circuits are conventionally represented as shown above
in 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.
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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 diagram
has been rotated around on it’s side. The circuit diagram shown on the left is of the
type used in Outcome 1. The reason for the change to the style on the right is simply
so that inputs and outputs can easily be added to the circuit. As we progress through
this section and onto more advanced circuits, it will become apparent to you why
these 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 input
transducer, 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 the
equation.
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Voltage across resistor R = Supply Voltage x (size of resistor R/ Total resistance)
For fig 11
V 2 = Vcc ×
R2
R1 + R 2
Worked Example
Calculate the voltage signal, V2 across the resistor R2, in the voltage divider circuit
shown.
AE.Int2.O2.fig 13
Applying the voltage proportion formula
V 2 = Vcc ×
V 2 = 12 ×
R2
R1 + R 2
40
40 + 80
V 2 = 4V
The voltage over the 80K resistor could be calculated in the same way, but this is
unnecessary for this circuit since we can use Kirchoff’s 2nd Law to confirm the
answer. i.e. the voltages over each of the components in a series circuit must add up to
the 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 divider
questions. You may choose whichever method you are most comfortable with.
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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 an
analogue sensor, e.g. a Thermistor, we would now have a system which generates a
signal voltage which is proportional to the change in the physical environment, in this
case temperature. If you look at the E & L or Alpha analogue input boards you will
find 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 and
will generate a signal voltage proportional to the resistance of the sensor.
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Assignments: Voltage divider equation
1. Using the formula described above, calculate the voltages that would appear
across 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
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3. A ntc (negative temperature coefficient) Thermistor is used in a voltage divider
circuit as shown in fig 17. Using information from the graph shown, determine
the resistance of the Thermistor and hence calculate the voltage that would appear
across 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 be
4.5V at 100 0C, determine a suitable value for R1.
State whether the V2 will increase or decrease as the temperature drops. Explain your
answer.
AE.Int2.O2.fig18
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Sensing circuits
Light sensors
Obtain the relevant components and equipment then construct the light sensing circuit
shown in fig 19.
AE.Int2.O2.fig 19
In normal light conditions, measure and record the voltage across the LDR and the
fixed 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 LDR
increases as the light level decreases, so in this case the signal level (Vout) will rise as
it 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 resistor
allows the signal to change in the opposite direction i.e. the signal level will rise as it
gets light.
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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 the
opposite direction when the same temperature change is applied.
Obtain the relevant components and equipment then construct the light sensing circuit
shown 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 the
Thermistor 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 what
happens.
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Circuit Simulation Software
It is possible to use circuit simulation software such as ‘Crocodile Clips’ to
investigate electric and electronic circuits. Circuit simulation is widely used in
industry as a means of investigating complex and costly circuits as well as basic
circuits.
Circuit simulators make the modelling and testing of complex circuits very simple.
The simulators make use of libraries of standard components along with common test
equipment such as voltmeters, ammeters and oscilloscopes.
Using Crocodile Clips or another similar software package construct and test the
following circuits.
1. Use the facilities of Crocodile Clips to adjust the circuit to the values given, then
use the voltmeter facility to measure Vout (signal voltage). Record the value of
Vout (signal voltage).
AE.Int2.O2.fig 22
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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 allows
the level of the signal to be adjusted and acts as a means of controlling the
sensitivity of the device.
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Assignment: Sensing Circuits
1. Using the relevant information from manufacturers data sheets for the ORP 12
LDR, determine the value of the cell resistance and hence calculate the values of
Vout 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
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3. Examine the circuit below in fig 26. State whether Vout will increase or decrease
as 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 final
operation.
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
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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 and
a 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.
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Semiconductor devices
Diodes
Diodes are made from two semiconducting materials usually silicon and germanium
joined together. The junction between the material creates a one-way barrier to
electric current.
The device allows current to flow in one direction only. The diode has a low
resistance when current is flowing forward and a very high resistance in the opposite
direction.
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 the
positive side of a power supply, and the cathode to the negative side, therefore
the 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 be
greater than 0.6 V.
Obtain the relevant components and equipment then construct the diode circuits
shown in fig30. Use a type 1N 4001 diode that is commonly available.
AE.Int2.O2.fig30
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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 use
of in circuit design and in particular with inductive output transducers such as relays
and solenoids.
The relay and solenoid are inductive devices and when the coil of a relay or a solenoid
is energised and de-energised by the flow of current through it, it can generate a large
reverse voltage (back e.m.f.). This reverse voltage can cause considerable damage to
unprotected components like transistors in the drive circuit.
The risk can be avoided by the inclusion of a protective diode that allows the energy
to be dissipated by providing an alternative path for the current.
AE.Int2.O2.fig31
AE.Int2.O2.fig32
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Light Emitting Diodes
A light emitting diode (LED) is a special diode and is made from a semiconductor
junction that gives out light when a forward voltage (forward biased) passes through
it. 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 signal
has been passed. Red is the most common colour of LED but increasingly green and
yellow are being used.
LEDs normally operate when the voltage drop across them is between 1.4 - 2
volts. 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 will
require
LED voltage = 2 volts
LED 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 is
around 2 volts they almost always have to have a resistor connected in series with
them.
This resistor is called a current limiting resistor.
The value of the current limiting resistor can be calculated as follows.
AE.Int2.O2.fig34
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Voltage Supply
Voltage across LED
Voltage across the resistor
Maximum current through LED
Current limiting resistor
Nearest preferred value
=6V
=2V
=6-2=4V
= 20 mA
= V/I = 4/20 x 10-3 = 200 R
= 220 R
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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)
(b)
Explain the purpose of the series resistor.
Calculate the value of the current limiting resistor in each of the following
circuits. Using the calculated value, select the nearest preferred value. The
current 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
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7. A student is asked to design a circuit to test fuses. The specification for the circuit
is as follows.
• The 1.5 V battery powering the circuit is tested before the fuse is inserted by
pressing 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 switch
open, 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 above
satisfy the specification.
Circuit 1
Circuit 2
Circuit 3
Circuit 4
Circuit 5
Circuit 6
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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 explain
why not.
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Transistors (Bipolar)
The transistor is another semiconductor device. It is made of three layers of
semiconducting material. The layers are made from either ‘n type’ material or ‘p
type’.
There are two types of Bipolar transistor available, pnp or npn and this relates to the
order of the layers of semiconducting material. Both types of transistor operate in the
same 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 leg
attached 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 BC
108 comes in the T018 case. The diagram of the transistor shows the position of the
legs when viewed from underneath the case.
There are different case types from the one shown in the figure. For other types refer
to 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 the
emitter indicates the direction of ‘ conventional’ current flow (positive to negative).
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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AE.Int2.O2.fig36
Depletion layer prevents current from flowing.
The ‘p type’ layer, or depletion layer, acts as a barrier that prevents current flowing
through the device from the collector to the emitter.
If a small current signal is introduced to the base, Ib, of the transistor then the
depletion layer is reduced and current flows throughout the transistor. As the base
current increases the depletion layer further reduces until a level is reached at which
the transistor is fully ‘switched on’. In this condition the transistor is said to be fully
saturated.
AE.Int2.O2.fig37
Current flow as depletion layer reduces
The current entering the base of the transistor is added to the collector current, Ic, to
produce the emitter current, Ie.
Ie = Ic + Ib
Since in most applications Ib is about 1% of Ic, we can normally assume that:
Ic = Ie
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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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 may
seem 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 in
figs 36 and 37.
When there is no current flowing to the base of the transistor, the depletion layer can
be considered as being large and acting as a barrier to current flow (a very high
resistance) 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 the
transistor. When this happens the transistor allows current to flow through it and the
bulb 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 the
base resistor, the depletion layer reduces and current is allowed to flow throughout the
transistor via the load - in this case the bulb.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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Bipolar Transistors amplify current. A small current flowing to the base of a
transistor causes a much larger current to flow from collector to emitter.
AE.Int2.O2.fig40
It can be seen from fig 40 that:
Ie = Ib + Ic
Since Ib is much smaller than Ic, in normal situations we can assume Ic = Ie
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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Switching effect of a transistor - Transistor as a switch
The transistor can be made to operate like a switch by controlling the current flowing
into the base of the transistor.
Assignment - Transistor as a switch
Use Crocodile Clips or another circuit simulation package to set up the following
circuit. 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 the
necessary components from your teacher to construct this circuit.)
AE.Int2.O2.fig42
From your investigation of the circuit complete the following table.
Rb (K)
2200
1000
470
220
100
47
33
22
10
1
Vbe (mV)
Ib (µA)
Ic (mA)
Lamp
on/off
Ie (mA)
µA
µA
µA
µA
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 the
transistor was 0.7 volts.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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You will also have noted that as Vbe rises above 0.7 V the brightness of the lamp does
not increase. This is due to the fact that once this level has been reached the transistor
is fully 'switched on', or Saturated.
A transistor is saturated when the maximum current flow across the collector
emitter junction is achieved.
In order to fully switch on a npn transistor, the voltage signal across the base emitter
junction must be between 0.6 V and 0.8 V. For most practical purposes a value of 0.7
V is assumed.
Vbe = 0.7 V
Note: Any attempts to increase Vbe will simply increase the base current very
rapidly. This will damage the transistor. This can been seen for the last value of
Rb, where Ib shows a significant increase, but Vbe does not.
This activity should also clearly show the link between Ib, Ic and Ie.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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Current Gain of a Transistor
The result of introducing a small current to the base of a transistor is to 'switch on' a
larger current across the collector emitter junction. In effect a small current signal is
being used to produce a larger current signal. The factor by which the input signal is
amplified is called Gain.
The Current Gain of a transistor is defined as follows:
CurrentGain =
Gain =
hFE =
CollectorCurrent
BaseCurrent
Ic
Ib
Ic
Ib
The accepted symbol for transistor current gain in this mode is hFE.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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Assignments: Transistor Gain
Type (npn)
BC 107
BC 108
BFY51
2N3053
Vce (max)
(V)
45
20
30
40
Ic (max) mA
Pmax (mW)
hFE
Application
100
100
1000
700
300
300
800
5000
110 - 450
110 - 800
40 min.
50 - 250
Audio driver
General purpose
General purpose
General purpose
1. Calculate the gain of a transistor if the collector current is measured to be 10 mA
when 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:
Ic (max):
Vce (max):
hFE (min):
Power:
Application:
6.
npn
70 mA
20 V
500
200 mW
General purpose
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 in
your circuit. You may wish to try several of the values to confirm your
calculations.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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Biasing the transistor - Providing the correct voltage
The circuits shown if figs 38 and 39 show a resistor connected to the base of the
transistor. The purpose of this resistor is to control the flow of current to the
transistor. As we have stated before, the base current needs to be very small
otherwise the transistor could be damaged. In practice connecting a resistor to the
base of the transistor controls the base current. The two most common methods of
biasing a transistor are shown below.
AE.Int2.O2.fig41
The voltage divider circuit, shown in the second method, should be familiar to you as
a means of splitting up the input voltage into appropriate proportions.
Transistor control circuits
The principle advantage of electronic systems is the ability to process complex
electronic signals using very small amounts of energy. Electronic systems are able to
achieve 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 even
the simplest of output devices, such as a buzzer, motor or solenoid, a larger amount of
current is required than the electronic system can provide.
To enable electronic systems to operate output devices, drive circuits are required to
amplify the current from the electronic signal.
Amplifying devices are said to be active components, as opposed to non-amplifying
components (resistors, capacitors etc.) that are known as passive components. The
extra energy required to operate the active component comes from an external power
source (battery, transformer etc.).
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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Transistor Circuit Construction
There are a number of factors to consider before you begin to construct transistor
drive 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 into
the transistor could become too large and damage the transistor. For most common
applications 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 know
what current will flow through the device into the collector. For example a 6 V MES
bulb will typically have a maximum current of 60 mA. Therefore it will be necessary
to 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 what
gain is required. Once you have calculated the gain required you would be able to
refer to manufacturers data sheets to make your choice.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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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 certain
level.
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 10
K fixed resistor in the sensing circuit by a variable resistor. The inclusion of the
variable resistor allows the sensitivity of the circuit to be adjusted.
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Assignment - Circuit sensitivity
Using Crocodile Clips or another circuit simulation package, construct the following
circuit.
The component values should be set to
Thermistor:
Variable resistor:
7 K at 25oC
22 K
When you have set up this circuit you should begin by setting the variable resistor at
1.1 K and the thermistor temperature at 0Oc. Gradually increase the temperature and
observe the readings on Vbe and the output of the lamp. Now set the variable resistor
to 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 at
different temperature i.e. the circuit is responding to a smaller or larger physical
change.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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Assignment - Practical Circuit Construction
Light sensitive circuits
Gather the necessary components and construct the circuit shown in fig 45.
6V
220R
1K
10K
0V
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
220R
10K
1K
0V
AE.Int2.O2.fig46
1. Explain in your own words how the system operates.
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Circuit sensitivity - Practical Circuits.
The switching effect of the transistor is controlled by the voltage that is supplied to
the base of the transistor via the voltage divider part of the circuit. Using a variable
resistor in place of the 10 K fixed resistor means that you can adjust your circuit and
choose 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 is
possible to adjust its value down to zero resistance and place a high voltage across the
base/emitter of the transistor. To protect the transistor from too large a current in the
base, a fixed resistor is inserted between the voltage divider part and the transistor
base (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
220R
1K
0V
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 cover
the LDR
5. Evaluate the circuit for different values of light intensity and variable resistor
values.
6. Explain in your own words how the system operates and is different to the light
sensing circuit with the fixed resistor in the voltage divider.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2
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TECHNOLOGICAL STUDIES
INTERMEDIATE 2
APPLIED ELECTRONICS
SECTION 3
OUTCOME 3
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1
OUTCOME 3
Design and construct a simple combinational logic circuit to solve a given
problem.
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 an
introduction 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.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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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 countries
British Standard Symbols (BS3939) Used in this country
Throughout this course the more common American symbols will be used. These are
the symbols used by SQA.
OR
AND
NOT
NOR
NAND
AE.Int2.O3 fig1
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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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 code
reader; 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 being
shown below.
The easiest way to represent how each gate behaves is to make use of truth tables. A
truth table shows all possible combinations of inputs and outputs to a logic gate.
OR gate
IN P U TS
A
B
O UTP U T
IN P U T
A B
0 0
0 1
1 0
1 1
O UTP U T
0
1̀
1
1
AE.Int2.O3 fig3
Results displayed in this way are known as truth tables.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Structure and layout of truth tables
Input conditions: Let us represent the inputs to a logic gate as two switches.
S W ITC H A
O UTP U T
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 first
consider 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
N UM B ER O F IN PU TS
TO TH E S Y STE M
2
2
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 B
0 0
0 1
1 0
1 1
AE.Int2.O3 fig5
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
4
For a 3 input problem; either a single 3 input gate or a simple combination of gates
e.g.
A
A
B
C
O UTP U T
B
O UTP U T
C
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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Truth tables for Individual logic gates
OR gate
IN P U TS
A
O UTP UT
B
IN P U T
A B
0 0
0 1
1 0
1 1
O UTP UT
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. The
printer 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
PAPER FEED
PRINTER BUTTON
AE.Int2.O3 fig8
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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AND gate
A
OUTPUT
B
INPUT OUTPUT
A B
0 0
0 1
1 0
1 1
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. The
motor in a washing machine should not operate until the water level is HIGH enough
AND a signal is sent from the control program.
W ATE R LE V EL S EN S O R
M O TO R
C O N TRO L PR O G R A M
AE.Int2.O3 fig10
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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NOT gate (Inverter)
IN
OUT
INPUT OUTPUT
0
1
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 ON
when the temperature is too LOW.
TE M P E RATUR E SE N S O R
C EN TR AL H EATIN G
AE.Int2.O3 fig12
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
8
NOR gate
A
B
INPUT OUTPUT
OUTPUT A B
0 0
0 1
1 0
1 1
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 of
the 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 accidents
at times of poor visibility, a warning indicator in the car should operate if the light
level is too low and the car headlamps are off.
LIG H T LE V EL S E N SO R
W A RN ING IN DICATO R
H EA D LA M P S W ITCH
AE.Int2.O3 fig15
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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NAND gate
A
B
INPUT OUTPUT
OUTPUT A B
0 0
0 1
1 0
1 1
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. The
output 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.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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A possible application of this type of logic gate might be in the maternity unit of a
hospital. The temperature and pulse rate of premature babies has to be continually
monitored. A warning alarm should sound if either the temperature or the pulse rate
of the baby falls too LOW.
TE M P E RATU R E S E NS O R
W A RN ING A LA R M
P ULS E RATE SE N S O R
AE.Int2.O3 fig18
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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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 pressing
the channel select button on the TV OR by pressing the channel select button on the
remote 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 is
pressed AND the TV set is on.
A logic diagram for this is shown
R EM O TE BU TTO N
TV S E T B U TTO N
C HA N NE L
TV O N/O FF SW ITC H
AE.Int2.O3 fig19
Combinational logic circuits are that in which the output at any time is determined
entirely 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.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Analysing a combinational logic system
In digital electronics we often encounter systems which will contain several logic
gates that have been combined together.
Suppose that you need to know how a particular network will behave for each
possible combination of inputs.
[there are a variety of techniques possible]
Whilst there is no set or hard and fast method for analysing combinational logic
circuits, the following is a suggestion.
For a given logic diagram
A
C
B
D (O U TPU T)
B
AE.Int2.O3 fig20
STEP 1
Label all the different points on the circuit, including inputs and
outputs.
Notice that one input serves two gates, this is quite common in logic circuits and both
gates should be labelled accordingly.
STEP 2
Draw up a Truth table for the circuit, with a different column for each
letter.
NB. determine the number of input conditions for the truth table.
two input, therefore 22 = 4
i.e. 4 lines on the truth table.
OUTPUT
INPUT
A B
0 0
0 1
1 0
1 1
C
0
1
1
1
D
AE.Int2.O3 fig21
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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STEP 3
Determine what goes into column C. To do this, consider the OR gate
on its own, with inputs A and B
A
C
B
AE.Int2.O3 fig22
enter findings into column C.
STEP 4
Determine what goes into column D i.e.. the output column. Consider
the AND gate on its own with inputs from C and B
C
D (OUTPUT)
B
AE.Int2.O3 fig23
enter findings into column D.
STEP 5
your final answer should look like this
OUTPUT
INPUT
A B C
D
0 0 0
0
1
0 1 1
0
1 0 1
1 1 1
1
AE.Int2.O3 fig24
More complicated systems may need several stages before you finish, but if you work
systematically through the circuit in the way shown, considering one gate at a time,
you should arrive at the correct answer.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Boolean Expressions
Boolean algebra is a special form of algebra that has been developed for binary
systems. It was developed by George Boolean in 1854 and can be very useful for
simplifying and designing logic circuits.
Variables
The most commonly used variables in logic circuit design are capital letters; such as
A, 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 of
two possible values, i.e. 'Logical 0' or 'Logical 1'. We are obviously dealing with a
binary 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 are
the AND operation, the OR operation and the Inversion.
NB. It is essential to understand whilst undertaking analysis using Boolean that you
must not confuse the symbols used for the Logical operators with those used in
normal algebra.
+ represents logical operator OR
• represents Logical operator AND
A 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 of
the variable changes it's value , or inverts it. This is known as the NOT operation.
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Basic logic gates and their Boolean representations
AND gate.
A
Z=A .B
B
IN P U T
A B
0 0
0 1
1 0
1 1
Z=A .B
0
0
0
1
OR gate.
A
Z=A +B
B
IN P U T
A B
0 0
0 1
1 0
1 1
Z=A +B
0
1
1
1
NOT gate.
A
Z= A
INPUT
0
1
Z= A
1
0
Read as output Z is equal to NOT A
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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NAND gate.
A
INP U T
A B
0 0
0 1
1 0
1 1
Z= A .B
B
Z= A .B
1
1
1
0
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 a
NOT gate. This arrangement demonstrates the Boolean notation quite clearly.
A
A .B
A .B
B
NOR gate.
A
INP U T
A B
0 0
0 1
1 0
1 1
Z= A +B
B
Z= A +B
1
0
0
0
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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Assignment: Write down the Boolean expression for each of the following logic
gates.
a)
b)
A
B
c)
A
Z
d)
B
A
Z
e)
A
Z
B
g)
h)
A
B
C
Z
Z
f)
A
Z
B
A
B
C
Z
A
B
C
D
Z
k)
A
B
C
D
Z
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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 than
one gate ensure that the input is labelled accordingly).
2. Use the Boolean notation (operators) to give the output of the gate in terms of its
input.
A
A .B
B
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 as
you would any other equation and make use of brackets (if necessary). Then write on
the appropriate expression after the next gate and so on until you reach the final
output.
A
A.B
A.B + B
B
B
B
The final Boolean expression is the output from the final gate A.B+B
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Assignment: Derive the Boolean expression for each of the following combinational
logic circuits.
a)
B)
c)
d)
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Deriving the Boolean expression from a Truth Table.
Obtaining a Boolean Expression from a Truth Table is a fairly straightforward
operation.
Consider the Truth Table given.
Concentrate solely on the combinations of inputs that give a logic 1 condition in the
output column. In the truth table given below there are two inputs A and B and one
output Z. The output Z is at logic1in the 1st, 3rd and 4th rows.
A
B
C
0
0
1
1
0
1
0
1
0 1 1
0 0 0
0 1 1
1 0 1
D
Z
1. Note each combination that will give you a '1' at the output
A
B
C
0
0
1
1
0
1
0
1
0 1 1 A.B
0 0 0
0 1 1 A.B
1 0 1 A.B
D
Z
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 equations
are 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 satisfy
the output condition since
A.B OR A.B OR A.B
would give the correct output, therefore each of these equations must be considered in
order to obtain the final solution.
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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
A
B
Z
0
0
1
0
1
0
1
0
1
1
1
0
A
B
Z
0
0
0
0
1
1
1
0
1
1
1
0
b)
c)
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
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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
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
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
f)
g)
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Integrated circuits
In digital electronic circuits, logic gates are normally manufactured onto an integrated
circuit (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 with
connecting pins to provide connections to the appropriate gates. Here the pins run
down either side of the plastic package as shown below. The number of pins can be a
8, 14, 16, 18, 20, 24, 40 and even 60 pin.
+V cc
14
13
12
11
10
9
8
N O TC H - IN D IC ATE S TH E TO P
P LA STIC C AS E
D O T- IN D IC AT ES P IN 1
1
2
3
4
5
6
7
G nd
(0V)
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 and
scale of integration (the number of components on each chip).
3. More reliable and easier to replace.
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Types of Integrated Circuits available (Logic Families)
There are two main methods of producing electronic logic devices today, giving two
families 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 that
Complimentary 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 transistors
to make up the gates
Most switching gate circuits use either Transistor - Transistor Logic (TTL) or
Complimentary Metal Oxide Silicon (CMOS) gates.
TTL gates are available commercially in the 7400 series and CMOS IC's are
available 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
3 mA
Switching speed
8µA
Slow
Fan out
50
10
Fast
NB. A new hybrid chip is available on the market that has the advantages of both
types and is listed as the 74HCT00 series. The 74HCT series is a high speed CMOS
semiconductor and has all the advantages of high speed CMOS but with inputs
configured for direct drop-in replacement of TTL. It operates in the voltage range 26V dc.
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Summary of properties of CMOS and TTL devices.
CMOS - Advantages
1.
2.
3.
4.
5.
The main advantage of CMOS devices is that they will operate on a any d.c.
voltage in the range 3V-15V (18V max).
They have a low current drain, usually in the order of microamp.
They have low power consumption.
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).
CMOS has very good noise immunity.
CMOS - Disadvantages
1.
2.
The major disadvantage of CMOS is it's slower switching speed, usually in the
order of 2 to 4 MHz. (4 * 106 switches per second)
CMOS devices can easily be destroyed by static electricity (in order to protect
them from static, when in use, all unused inputs should be connected to either
the zero volt line or the positive line).
Recent developments in MOS technology have seen major improvements in the areas
of speed, with the latest MOS devices claiming speeds similar to TTL.
TTL - Advantages
1.
2.
The major advantage of TTL devices is their high switching speeds, usually in
the order of 50 MHz (50 * 106 switches per second)
No damage is done to TTL devices if inputs are left unconnected. (such inputs
will set to +5V )
TTL - Disadvantages.
1.
2.
3.
4.
5.
TTL devices are much less flexible in terms of their operating conditions than
corresponding CMOS devices.
Requires a stabilised voltage supply in the range +5V + or - 0.25V (usually
expensive).
The fan out is 10.
Higher power consumption than CMOS devices.
High current drain, usually in the order of milliamp.
The latest designs of TTL devices are combining higher and higher switching speeds
with lower power dissipation.
In this course we will be using TTL logic gates for demonstrations and practical
work, since they are more robust and we can leave the unused inputs floating.
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Identifying Integrated Circuits
The pin diagram for a 7408 TTL logic gate is shown below
+ Vc c
14
13
12
11
10
9
8
1
2
3
4
5
6
7
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 twoinput 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 RS
Components.
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Examples of pin out diagrams
+Vcc
+Vcc
14
13
12
11
10
9
1
2
3
4
5
6
8
14
13
12
11
10
9
7
1
2
3
4
5
6
G nd
(0V)
7404
+Vcc
8
7
G nd
(0V)
7400
+Vcc
14
13
12
11
10
9
1
2
3
4
5
6
8
14
13
12
11
10
9
7
1
2
3
4
5
6
G nd
(0V)
7421
+Vcc
8
7
G nd
(0V)
7420
+Vcc
14
13
12
11
10
9
8
14
13
12
11
10
9
8
1
2
3
4
5
6
7
1
2
3
4
5
6
7
7427
G nd
(0V)
7432
G nd
(0V)
AE.Int2.O3 fig 28
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Assignments: Integrated circuits
1. Use the data sheet on page 20 to answer these questions.
a)
b)
c)
d)
Which of the above ICs contain AND gates.
Which IC contains 4 input NAND gates.
Which IC contains NOT gates.
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
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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Pin out and wiring diagrams
The following logic circuit could be constructed using ICs
IN PU T A
O UT PU T
IN PU T B
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 from
each side of the IC are not connected). Connection between the pins is made by
inserting 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 correct
power source for the Logic family and the correct pins.
+Vcc
+Vcc
14
13
12
11
10
9
1
2
3
4
5
6
8
14
13
12
11
10
9
7
1
2
3
4
5
6
Gnd
(0V)
INPUT A
OUTPUT
8
7
Gnd
(0V)
INPUT B
AE.Int2.O3 fig 31
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Assignments - Pin out diagrams
1. Draw connecting wires on the IC circuit diagram to show how the following circuit
diagrams could be constructed.
IN PU T A
O UT PU T
IN PU T B
AE.Int2.O3 fig 32
IC circuit diagram
+Vcc
+Vcc
14
13
12
11
10
9
8
14
13
12
11
10
9
8
1
2
3
4
5
6
7
1
2
3
4
5
6
7
G nd
(0V)
G nd
(0V)
AE.Int2.O3 fig 33
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2. The following circuit contains a 7427 and a 7404 IC.
By referring to pin diagrams, draw the corresponding logic circuit.
INP U T A
O UTP U T
14
13
12
11
10
9
8
14
13
12
7427
1
2
3
10
9
8
5
6
7
10
9
8
5
6
7
7404
5
4
11
6
7
1
2
3
4
INP U T B
INP U T C
AE.Int2.O3 fig 34
3. Draw the corresponding logic diagram for the circuit shown below.
14
13
12
11
10
9
8
14
13
12
7432
1
2
3
4
11
7400
5
6
7
1
2
3
4
IN P UT A
O U TPU T
IN P UT B
AE.Int2.O3 fig 35
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Practical IC circuits
The following section deals with the construction of circuits containing ICs. You will
need the following equipment.
1.
2.
3.
4.
5.
6.
7.
Breadboard
0.6mm solid core insulated wire.
Various IC's (TTL)only.
One led.
A resistor value around 370 R.
Logic probe.
5V regulated supply.
Pin layout diagram
+Vcc
14
13
12
11
10
9
1
2
3
4
5
6
8
7
Gnd
(0V)
AE.Int2.O3 fig 36
The pin layout diagram of a TTL IC is shown above, as previously mentioned in
section 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 should
always 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.
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Assignments: Practical IC construction
1.
Gather the necessary components and
a)
b)
c)
d)
Draw a truth table for the circuit
Construct the circuit
Test the circuit
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 be
used.
Use the data sheet provided on page 20 for information.
INP U T C
INP U T A
O UTP UT
INP U T B
14
13
12
11
10
9
8
14
13
12
7432
1
2
3
4
11
10
9
8
5
6
7
7400
5
6
7
1
2
3
4
AE.Int2.O3 fig 37
2.
Gather the necessary components and
a)
b)
c)
d)
Draw a truth table for the circuit
Construct the circuit
Test the circuit
Check that the outputs from your circuit agree with the truth table
INP U T A
O UTP UT
INP U T B
14
13
12
11
10
9
8
14
13
12
7432
1
2
3
4
11
10
9
8
5
6
7
7404
5
6
7
1
2
3
4
AE.Int2.O3 fig38
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3.
Gather the necessary components and
a)
b)
c)
d)
Draw a truth table for the circuit
Construct the circuit
Test the circuit
Check that the outputs from your circuit agree with the truth table
AE.Int2 O3 fig 39
4.
Gather the necessary components and
a)
b)
c)
d)
Draw a truth table for the circuit
Construct the circuit
Test the circuit
Check that the outputs from your circuit agree with the truth table
AE.Int2 O3 fig 40
Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 3
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5.
Gather the necessary components and
a)
b)
c)
d)
Draw a truth table for the circuit
Construct the circuit
Test the circuit
Check that the outputs from your circuit agree with the truth table
AE.Int2 O3 fig41
6.
Gather the necessary components and
a)
b)
c)
d)
Draw a truth table for the circuit
Construct the circuit
Test the circuit
Check that the outputs from your circuit agree with the truth table
AE.Int2 O3 fig42
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