Revised Standard Grade Technological Studies
Applied Electronics
Contents
Introduction
Structure
Resources
Assessment
Detailed resource list
Component Electronic Systems
Electricity
Simple Circuits
Integrated Circuits: 555 timer
Modular Electronic Systems
Introduction: Electronics – a systematic approach
Modular boards
Analogue and digital signals
Problem solving in electronics
Switches
Truth tables
The comparator
NAND and NOR gate boards
Logic in Electronics
Switching logic
Binary numbers
Combinational logic
Pin-out diagrams
Practical tasks
Electronics Mini-Project
Introduction
Application of Technology
Example: Remote Controlled Buggy with Light and Sound
Computer simulation
Appendix 1: Infrared Remote Control
Remote controlled toy
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Standard Grade Technological Studies: Applied Electronics
Introduction
Electronics is a key influence in today’s society and is therefore a key area of study in
Technological Studies. This component unit should be studied early in the course after
pupils have some knowledge of systems.
Structure
This unit is split into three distinct sections:
• Component Electronic Systems
• Modular Electronic Systems
• Logic in Electronics.
These sections can be delivered in any order, with each approach having its own
advantages and disadvantages. Obviously there is some overlap between the units. For
example, logic appears in the Modular Systems section in the form of the inverter, the
OR, AND, NAND and NOR gates and as subsystem boards. Similarly, if the Modular
Systems section is delivered first then some areas of Component Systems are
mentioned.
The advantage of delivering the Modular Systems first is that it gives an easy
introduction to electronics; however, there is a danger that pupils will work through
this section and solve the problems without picking up a real understanding of
electronics.
Logic Systems naturally follows on from the Modular Systems section.
The advantage of delivering the Component Systems first is that the pupils gain a firm
understanding of electronics, components and circuits. This gives them a deeper
understanding, which helps in the subsequent sections. The disadvantage is that pupils
may find the theory contained in this section difficult early in the course, although as
they are General/Credit pupils the level should be well within their ability.
By delivering the Component Systems first, pupils will cover the use of multimeters
and prototype circuit boards that are used in the other sections.
If the Component Systems section is to be delivered last, then it will be necessary to
give the pupils some instruction in the use of multimeters, prototype circuit boards,
simple electronic theory and components.
The contents of this unit are set out comprehensively so that teachers do not require
the use of additional notes or textbooks. Pupils can move at their own pace in many
areas, but it must be stressed that these unit notes should not be used as an open
learning pack and it will be necessary to deliver many important lessons at crucial
times. These include an introduction to electrical theory (Ohm’s law, Kirchoff’s
second law, etc.), series circuits, parallel circuits, and an introduction to components
(recognition, use and characteristics).
Standard Grade Technological Studies: Applied Electronics
iii
Resources
The resources required to run this unit are the same as those being used in
Technological Studies at present. Some equipment may differ in type (for example
different meters, prototype boards and modular systems) and therefore the notes
provided will have to be interpreted differently.
The main resources are:
• a range of components
• prototype circuit boards (often referred to as breadboards)
• digital non-auto ranging multimeters
• a modular electronic system (for example E&L boards)
• circuit simulation software (for example, Crocodile Clips).
Circuit simulation software can be used for many of the activities but pupils must also
have experience of building physical circuits. This is necessary to experience the real
components as well as the problem-solving attached to building real circuits.
Teachers are encouraged to use other resources such as video and the interactive CDROMs that are available. Software that simulates Modular Systems is also available
and could be used to aid classroom management problems that arise from faulty
boards. A detailed list of resources is provided at the end of this introductory section.
Assessment
External
This unit of work and the exercises within will prepare the pupils for any electronics
questions that appear in the 90-minute exam at the end of the course. It will enable all
pupils to gain the knowledge and understanding required and give them suitable
practice in reasoning and numerical analysis.
Internal
The internal assessment of this unit requires pupils to carry out a structured
assignment. The assignment should take approximately three hours to complete but in
some circumstances this can be extended.
The pupils will be expected to:
• complete a specification from a given brief
• create appropriate diagrams to illustrate the problem parameters
• produce a graphical representation of a proposed solution
• perform a computer simulation of the proposed solution
• build and test the proposed solution
• evaluate the solution against the original specification.
It must be stressed that this is not like the existing main project report.
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Standard Grade Technological Studies: Applied Electronics
Example
The pump motor in an automatic heating system is designed to be on all the time, day
or night, unless it gets too hot in the daytime.
Design a system that drives the pump motor constantly except when it is hot during
the day.
For this assessment problem, pupils would be expected to produce:
• a limited specification from the information in the brief
• a system diagram illustrating the parameters
• diagram(s) showing a proposed solution using modular boards or a discrete
component circuit
• a parts/component list from classroom resources
• a computer simulation using Crocodile Clips or other suitable simulation software
• a physical solution to the problem
• a small written evaluation comparing the performance of the solution to the
specification.
Further information on the internal assessment can be found in Appendix 1 of the
Arrangement Documents: Guidelines for Internal Assessment.
Standard Grade Technological Studies: Applied Electronics
v
Detailed resource list
Relay
• Miniature 5 V DPDT (RS 376-981)
Component electronics systems
Low voltage power supply
Integrated circuit
• 555 timer IC
Prototype circuit boards
Digital multimeter (non-auto-ranging)
Resistors
• 100 R
• 220 R
• 270 R
• 390 R
• 1K
• Light-dependent (LDR) ORP 12
• Thermistor: ntc (RS 256-102)
Computer simulation software
• Crocodile Clips
Modular electronic systems
Modular circuit board systems
• E&L, or
• Alpha systems
Simulation software
• Control Studio
Potentiometers
• 10 K (RS 375-304)
• 100 K (RS 375-332)
Logic in electronics
Transistors
• BC 108
Modular circuit board system
• E&L, or
• Alpha systems
Diodes
• Light-emitting 5mm red
• 1N4001 (RS 261-148)
Computer simulation software
• Crocodile Clips
Capacitors
• 100 µF electrolytic
• 1 µF bead
Low-voltage power supply
Switches
• Miniature push (RS 331-758)
• Miniature slide (RS 339-673)
Prototype circuit board
Logic probe
Lamp
• Holder (RS 564-891)
• MES lamp 6 V (RS 586-172)
• MES lamp 12 V (RS 586-201)
Buzzer
• Piezo flying lead (RS 203-0233)
Motor
• 3−6 volt miniature
vi
Resistors
• 220 R
Diodes
• Light-emitting 5 mm red
Integrated circuits (TTL)
• 7400
• 7408
• 7432
• 7404
Standard Grade Technological Studies: Applied Electronics
Component Electronic Systems
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
1
Contents
Electricity
Simple Circuits
Integrated Circuits: 555 timer
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3
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Electricity
Introduction
Electricity is one of the most important forms of energy available to man. It affects
everyone’s lives in many ways. If you take time to think about your everyday life you
will realise that our lives are full of devices that depend upon electricity. These
devices depend on the electrical circuits inside them to work. The circuits often
change the electrical energy into other forms of energy such as heat, light and sound.
In this area of study you will learn how these circuits work and about the different
components within them.
Electric circuits
An electric circuit is a closed loop or network made up of electrical components such
as batteries, bulbs, switches and wires.
Switch
Battery
Lamp
Electric current
Electric current is the name given to the flow of negatively charged particles called
electrons.
electrons
Current is measured in amperes, usually referred to as ‘amps’ (A). Current is the rate
of flow of electrical charges (called electrons) through a circuit.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
3
Voltage
In most circuits a battery or voltage supply is used to drive the electrons through the
components. Voltage is measured in volts (V).
+
V
_
Current
Flow
R
Resistance
All materials conduct electricity. The materials that conduct electricity well are called
conductors and those that are poor conductors are called insulators. Metals are good
conductors while rubber and glass are good insulators.
A good conductor offers very little resistance to the flow of electrical current. In other
words, it lets currents flow with very little voltage being applied. Resistance is
therefore a measure of how much voltage is required to let a current flow. Resistance
is measured in ohms (Ω).
Electron flow − conventional current
Scientists in the early nineteenth century decided the direction of conventional current
flow. It seemed to them that current flowed from the positive side of power supplies
to the negative side. It was not until the twentieth century that electrons were
discovered and the true direction of current flow was proved.
As stated earlier, electric current is the flow of electrons but often it is more useful to
consider electric current to flow in the opposite direction. This is called conventional
current.
So although it is technically wrong, for convenience ‘conventional current’ will be
used in the circuits and calculations throughout this work.
+
V
_
Conventional
Current
R
Conventional current flows from positive to negative.
One of the main reasons for maintaining this convention is that symbols and other
data based on conventional current have become standard.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Batteries and voltage supplies
Batteries and voltage supplies are the source of power behind all electrical circuits.
Without a power source, electrical circuits will not work. In your work (as in most
electronic circuits) all power sources will be low-voltage − this normally means
everyday batteries or a low-voltage power supply.
The low-voltage supplies and batteries will normally supply between three and 12
volts. Electronic components normally work on much lower voltages and so the
circuits must be designed carefully.
The symbols for batteries and voltage supplies are as follows.
Single battery or c ell
Multiple batteries or cells
Voltage supply
6 volts
Note the positive and negative side of the battery:
-ve
+ ve
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
5
Direct current (d.c.)
The voltage supplied by batteries or low-voltage supplies is direct current (d.c.). This
is the normal type of supply to low-voltage circuits. Alternating current (a.c.) supplies
are high-voltage − usually 230 volts. This is the normal supply in homes and schools.
Many portable electric power tools work from 110 volts for safety.
Resistors
Resistors are basic components in electrical and electronic circuits. They limit the
amount of current flowing in circuits or parts of circuits. Resistors are roughly
cylindrical and have coloured stripes. They also have connection wires sticking out of
each end.
The stripes indicate the value of the resistors. The colours represent numerical values
according to a special code.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Resistor colour code
Resistors are marked with what is known as a resistor colour code. Each band that
surrounds the body of the resistor helps identify the value (in ohms) and the tolerance
(in per cent). In most resistors only four colour bands are used.
The colour code chart for resistors is shown below. The colours are used to represent
different numbers, and in this way we are able to tell the value for each digit.
First and
second
colour band
Digit
Black
0
x1
Brown
1
x 10
Red
2
x 100
Orange
3
x 1000 or 1 K
Yellow
4
x 10 000 or 10 K
Green
5
x 100 000 or 100 K
Blue
6
x 1 000 000 or 1 M
Violet
7
Silver means divide by
100
Grey
8
Gold means divide by 10
9
Tolerances:
• brown − 1%
• red − 2%
• gold − 5%
• silver − 10%
• none − 20%
White
Multiplier
Standard values
Resistors are supplied in a range of standard values: 1.0, 2.2, 3.3, 4.7, 5.6, 6.8, 7.5, 8.2
and 9.1. These standard values can then be multiplied by 10, 100, 1000, and so on.
Typical values of resistors are 220 R, 100 K, 680 R, etc. Some other popular sizes are
also available, such as 270 R and 390 R.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
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4-band resistor colour code layout
4 Band Resistor Colour Code Layout
1st band
1st digit
4th band
tolerance
2nd band
2nd digit
3rd band
multiplier
Example
If the colours on the above resistor are:
1st band − red
2nd band − violet
3rd band − brown
4th band − gold
then using the table on the previous page, the value of this resistor is 270 Ω and its
tolerance is 10 per cent. This is worked out as ‘2’ for the red first band, ‘7’ for the
violet second band and ‘times 10’ for the brown third band.
For most purposes you can ignore the tolerance. In the above example the
manufacturers guarantee that the resistor will not vary from the marked resistance by
more than 10 per cent.
Symbol for resistance
Although the symbol for ohms is ‘Ω’ it is often shown as a capital R; that is, 270
ohms can be expressed as either 270 Ω or 270 R.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Using the resistor colour code
Using the resistor colour code chart, record the resistance values of the following
resistors. Write your answers in your normal report notes/jotter.
1. 100 R ± 10%
blue – violet – brown – silver
2. 3 K9 ± 2%
orange – white – brown – gold
3. 100 K ± 10%
brown – black – red – gold
4. M2 ± 5%
brown – black – green – brown
Draw and note the colours of the resistors below. Use colour pencils to show the
correct colour bands.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
9
Exercises
1. Using the colour-code chart, determine the colours of the first three bands of the
following resistors.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Value
270 R
1 K5
33 K
1 M2
330 R
150 R
82 K
560 R
6 K8
750 R
390 R
2 M1
82 R
4700 R
9 K1
Colour
2. Using the colour-coding code, calculate the values of the following resistors.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
10
Value
First three colour bands
red
red
red
yellow
violet
black
grey
red
red
yellow
violet
orange
red
red
orange
orange
orange
orange
green
blue
brown
red
violet
black
grey
red
brown
brown
green
green
brown
grey
yellow
brown
black
yellow
green
blue
orange
brown
grey
black
brown
grey
green
blue
grey
orange
orange
orange
yellow
red
red
brown
grey
red
black
violet
brown
orange
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Diodes
Diodes are devices that allow current to flow in one direction only.
Current can pass this way only
Anode
Cathode
Symbol for Diode
Current will flow through the diode only when the anode (positive side) is connected
to the positive side of the circuit and the cathode (negative side) is connected to the
negative side of the circuit.
Light-emitting diodes
A light-emitting diode is a special diode that gives out light when current is flowing
though it. LEDs are used as indicators to tell when a circuit (or part or a circuit) is
working. You can tell the cathode of an LED as it is the short leg and there is a ‘flat’
on the plastic casing.
-ve
As with the normal diode, the current can only pass one way.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
11
Switches
Switches are useful input devices (or transducers) that have metal contacts inside
them to allow current to pass when then they are touching. There are several ways in
which the contacts in mechanical switches can be operated. The main types are −
push-button, toggle, key, slide, magnetic (reed) and tilt. These switches are ‘digital’
input devices as they can only be on or off.
Toggle
Slide
Key
Tilt
Roc ker
Reed
The switches shown above are all single pole with single or double throws. These are
known as SPST and SPDT switches. The symbols are shown below.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
13
Microswitches
Microswitches are small switches that are useful for detecting motion. They are
especially good as sensors and limit switches. Typical systems that use microswitches
are traffic barriers and lift systems.
The microswitch above has a roller fixed to a lever that detects movement and throws
the switch. It has three terminals: common, normally open (NO) and normally closed
(NC).
The microswitch below is commonly used in schools.
3 -- NO
1 -- C
2 -- NC
Like most microswitches, this one can be wired in three ways.
•
•
•
C and NO: this is a normal on/off switch.
C and NC: this allows current to flow when the switch is not operated.
C, NC and NO: when wired like this it acts as a changeover switch.
These microswitches are single-pole double-throw (SPDT) switches.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Simple Circuits
Series circuits
The diagram below shows a typical use for an LED circuit, where the LED indicates
that the car radio/cassette is on. The diagram also shows a simplified series circuit
layout for the LED indicator. The resistor is necessary to protect the LED from
drawing too much current and ‘blowing’.
The diagram below shows the above circuit using the component’s symbols. This is
called the circuit diagram.
Switch
I
6V
LED
The components in this circuit are connected in series. This means that they are
connected up in a line, one after the other (or end to end).
Series circuits are the simplest to deal with as the same current flows through all of
the components. The voltage, however, is divided up between the components – more
of this later.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
15
Series circuits and Kirchoff’s second law
When components are connected end to end (in series) to form a closed loop, the
same current flows through all components while the voltage is divided up amongst
them.
In other words, the sum of voltages dropped across each component (V1, V2 …) is
equal to the total voltage supply in the circuit. This is known as Kirchoff’s second
law:
VT = V1 + V2 + V3 + …
In the example below each bulb is rated at 6 volts and the supply voltage is 18 volts.
This shows Kirchoff’s second law in practice.
Prototype circuit boards
6V
6V
6V
18 V
Prototype circuit boards (often called breadboards) are used to build and test circuits.
They have the advantage that they are non-permanent: that is, the components can be
moved and used again. This makes it easy to make alterations or corrections to
circuits. Once a circuit has been proved on a prototype circuit board it is usually built
by a more permanent method on stripboard or printed circuit board (PCB).
METALLIC STRIP
CONNECTOR
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
15
The board above shows four mains sections of connection holes. The two centre areas,
separated by a gutter, are where most of the components are placed. The two outer
rows are used for the power connections.
The uncovered reverse side, seen below, shows how the connection holes are
interconnected.
The metallic strips connect the middle sections in columns of five, while the two sets
of outer rows are connected horizontally.
The diagram below shows how most common components can be inserted. Note that
the most complicated components are usually connected over the centre gutter. This is
especially true for transistors and integrated circuits (ICs).
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
LIGHT EMITTING DIODE
VARIABLE RESISTOR
SLIDE SWITCH
WIRE LINK
1
5
10
15
20
25
A
B
C
D
E
555
F
G
H
I
J
TRANSISTOR
1
5
10
15
20
LDR
25
ELECTROLYTIC
CAPACITOR
RESISTOR
Example 1
Build the LED series circuit for the car radio/cassette. It can be built on a prototype
circuit board or simulated on computer software such as Crocodile Clips.
Circuit diagram
Switch
I
6V
390R
or
390
LED
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
17
Prototype board circuit layout diagram
+ 6 volts
Push Switc h
390 ohm s
0 volts
LED
Remember to connect the LED ‘the right way round’; that is the short lead (cathode)
is connected to the zero volt line or negative battery terminal. The LED should light
when the switch is pressed.
Example 2
Build the lamp circuit below. It can be built on a prototype circuit board or simulated
or computer software such as Crocodile Clips.
Circuit diagram
Slide Switch
I
Lamp
6V
18
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Layout diagram
+ 6 volts
Slide Switc h
Diode
Lamp
0 volts
Remember to connect the diode ‘the right way round’; that is, the negative lead
(cathode) is connected to the zero volt line or negative battery terminal. The lamp
should light when the switch is moved to the right. Try connecting the diode ‘the
other way round’ to confirm its operation.
Computer simulation
The two series circuits can be built and simulated in a computer programme such as
Crocodile Clips.
LED circuit
As in the prototype circuit, when the switch is ‘pressed’ the LED should light.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
19
Lamp circuit
As in the prototype circuit, the lamp should light when the switch is pressed/moved
and it will not light when the diode is reversed. Note: Crocodile Clips uses a rocker
switch to represent the action of a slide switch.
Digital multimeters
The digital multimeter is used to measure voltage, current and resistance. It is very
simple to use and easy to read.
On/Off
Amps 20mA
a.c. d.c.
Volts
10A 50V
10V
1V
2 mA
100mV
200µA
200k
20k
200Ω
2k
Ohms
10A
mA
VΩ
COM
To measure d.c. voltage:
• connect the black lead to the ‘COM’ socket
• connect the red lead to the ‘VΩ’ socket
• make sure that ‘d.c.’ is selected
• move the dial into the voltage (volts) range
• select a suitable range (always slightly higher than the expected measurement)
• place the lead probes on the points where the voltage is to be measured.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
To measure direct current:
• connect the black lead to the ‘COM’ socket
• connect the red lead to the ‘mA’ socket
• make sure ‘d.c.’ is selected
• move the dial into the current (amps) range
• select a suitable range (always slightly higher than the expected measurement)
• connect the probes to the wire in which the current is to be measured.
To measure resistance:
connect the black lead to the ‘COM’ socket
connect the red lead to the ‘VΩ’ socket
make sure ‘d.c.’ is selected
move the dial into the resistance (ohms) range
select the range (always slightly higher than the expected measurement)
connect the probes to the ends of the component being measured.
Measuring d.c. voltage
Voltage is measured across components or parts of circuits as shown in the circuit
diagram below.
V
6v
This can be done in actual circuits or simulated with Crocodile Clips.
‘Across’ means in ‘parallel’ as opposed to ‘series’. Parallel circuits will be dealt with
later.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
21
Practical task
Using circuit simulation measure the voltage across all three components in the LED
circuit.
The voltage across each component is known as the voltage drop across the
component. This is the amount of voltage ‘used up’ or ‘dropped’ by each. The total
voltage dropped in the circuit should equal the total supply voltage as stated in
Kirchoff’s second law.
Record your results.
Measuring direct current
Current is measured through components or parts of circuits, as shown in the circuit
diagram below. Note that it is necessary to ‘break’ the circuit and connect the meter in
series with the components.
6V
A
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Practical tasks
1. Using circuit simulation, measure the current flowing through all three
components in the LED circuit.
In a series circuit the current flowing through all components is the same. Try placing
the meter at different parts of the circuit to prove this. In parallel circuits the same
current does not always flow through each component − you will find out about this
later.
Record the current flowing in this circuit.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
23
2. Using the prototype LED circuit, measure the voltages (potential difference, p.d.)
across each of the components.
Push Switc h
+ 6 volts
390 ohm s
volts
0 volts
LED
10A
mA
V
COM
Alter the position of the zero volt connection and measure the current flowing in the
circuit. Set the meter to ‘mA’.
Push Switc h
+ 6 volts
390 ohm s
mA
0 volts
LED
10A
mA
V
COM
Record all results.
Measuring resistance
When measuring resistance make sure that your circuit is disconnected from the
supply, otherwise this will affect the reading. Do not touch the meter probes or the
components when measuring, as your own electrical resistance will then be included.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Resistors connected in series
As resistors come in standard sizes, they are often connected in series to obtain a
specific size that is otherwise unavailable.
Practical tasks
1. Connect two resistors in series on a prototype circuit board and measure the
overall resistance.
R1
10A
mA
V
COM
R2
You should find that if Rtotal (or RT) is the total resistance measured across both
resistors then the equation for adding resistances in a series circuit is
Rtotal = R1 + R2
For three resistors in series
and so on.
Rtotal = R1 + R2 + R3
2. Using two unknown resistors, measure the resistance of each and calculate Rtotal.
Check your answer by measuring Rtotal as shown in the above diagram.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
25
Ohm’s law
You have already found that applying a voltage to a circuit results in a current flowing
through the circuit.
In the simple Crocodile Clips circuit below, double the voltage and you will see that
the current doubles as well. In other words if you double the voltage across a
component, the current flow through that component will also double.
Thus we can say that the current is proportional to the voltage drop across a resistor.
This rule is known as Ohm’s law. The rule applies to all metals, provided that their
temperature does not change.
R =
V
I
Current (A)
This relationship gives rise to the Ohm’s law formula:
Voltage (V)
which is more easily remembered as:
V=IxR
We can use the triangle trick to help transpose this formula. Cover up the quantity that
you are trying to find and the other two will be in the form that is needed.
V
I
26
R
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Ohm’s law in practice
For this exercise a simple series circuit is used.
I
=
V
R
6 volts
Lamp
Current 0.06 amps
The task is to calculate the resistance of the lamp using Ohm’s law.
V
I
6
R =
0.06
R =
∴ R = 100 Ω
Tasks
1. Calculate the total resistance (Rtotal = R1 + R2) and the current flowing through the
circuit. You can verify your answer by physical measurement or with Crocodile
Clips.
Switch
390 ohms
6v
LED
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
27
2. Using Crocodile Clips measure the current flowing in the LED circuit (you should
have done this earlier) and calculate the resistance of the LED.
+ 9 volts
220 ohms
Buzzer
240 ohms
0 volts
Check your answer by physically measuring the resistance of the resistor and LED
with a multimeter.
Ω
RESISTOR
10A
mA
VΩ
COM
The diagram above shows how to check the resistance of a resistor. A similar
technique is used to measure the resistance of the LED.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Worked example: series circuit
For the series circuit shown, calculate:
(a) the total resistance (RT)
(b) the circuit current (IC)
(c) the potential difference across both resistors (V1 and V2)
c
S
(a)
(b)
RT = R1 + R2
= 6 + 18
R T = 24Ω
VS = I C × R T
VS
IC =
RT
12
=
24
I C = 0.5 A
(c)
We can use Kirchoff’s second law to check the answers calculated for the potential
difference across the resistors:
V2 = I C × R 2
= 0.5 × 18
V2 = 9 V
VT = V1 + V2
= 3 + 9
VT = 12 V
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
29
Exercises: resistors in series
1. For the circuit shown below calculate:
(a) the total resistance of the circuit
(b) the circuit current.
12V
2. For the circuit shown below calculate:
(a) the total resistance
(b) the circuit current
(c) the voltage drop across each resistor.
(d) Use Kirchoff’s second law to verify your answers to (c).
6V
3. For the circuit shown below calculate:
(a) the total resistance of the circuit
(b) the circuit current.
24V
4. A circuit has three resistors 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 volts, determine the circuit current and
the supply voltage.
30
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Parallel circuits
Parallel circuits are circuits where there is more than one path for electricity to flow
along or that have more than one ‘branch’. Each branch receives the supply voltage,
which means that you can run a number of devices from one supply voltage. A good
example of a simple parallel circuit is a set of Christmas-tree lights where all the bulbs
require a 230 volt supply.
This arrangement ensures that if one or two bulbs ‘blow’ then the rest of them
continue to function and, importantly, you know which are faulty. In a series circuit if
one bulb blew then all the bulbs would go out and you would have to test them all to
see which one was faulty.
Parallel circuits can be arranged in many ways, but are normally set out so that you
can easily see the parallel ‘branches’. A simple parallel car-alarm circuit is shown
below with the switches wired up in parallel.
12 volts
The two switches in parallel represent the sensor switches connected to the doors.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
31
Resistors in parallel
As resistors come in standard sizes, they are often connected in parallel to obtain a
specific size that is unavailable. This practice of combining resistors has already been
seen in series circuits.
Practical tasks
1. Connect two resistors in parallel on a prototype circuit board and measure the
overall resistance.
R1
10A
mA
V
COM
You should find that if Rtotal (or RT) is the total resistance measured across both
resistors then the equation for adding resistances in a parallel circuit is
1
1
1
+
=
R2
R1
RT
For three or more resistors, the equation can be extended:
1
1
1
1
+ ...
=
+
+
RT
R1
R2
R3
2. Using two unknown resistors, measure the resistance of each and calculate Rtotal
when the resistors are connected in parallel. Check your answer by measuring the
total resistance as shown in the above diagram.
32
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
As stated earlier, each branch of a parallel circuit receives the supply voltage. Each
branch has its own current; that is, when the current reaches a junction it splits up,
with some current flowing into each branch. The total supply current is therefore the
sum of the currents flowing in the branches.
When resistors or resistive components are connected in parallel, the effect is to
reduce the resistance in the circuit.
I
I
1
I
T
I
T
2
There are two important points to remember about resistors in parallel.
(a) The voltage drop across each resistor is the same.
(b) The sum of the currents through each resistor is equal to the current flowing from
the voltage source.
Special case: two resistors in parallel
There is a special rule that can be applied when adding two resistors in parallel only:
total resistance (RT) = product/sum.
RT =
R1 x R 2
R1 + R 2
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
33
Worked examples: resistors in parallel
1. For the circuit below, calculate the total resistance of the parallel part of the circuit
and the total resistance in the circuit.
R1
R2
12 volts
The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R.
34
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
2. For the circuit below, calculate the total resistance of the resistors in parallel.
1
R
2
3
R
R
1
1
1
=
+
RT
R1
R2
1
1
1
=
+
RT
270
100
1
270 + 100
=
270 x 100
RT
370
1
=
27000
RT
∴ RT =
27000
370
For the series part
RT = R1 + R 2
R T = 73 + 240
∴ R T = 313Ω
∴ R T = 73Ω
The resistance values are R1 = 220 R, R2 = 100 R and R3 = 330 R.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
35
3. For the parallel circuit shown calculate:
1
1
1
1
=
+
+
RT
R1
R2
R3
1
1
1
1
=
+
+
RT
220
100
330
1
33000 + 72600 + 22000
=
RT
7260000
1
127600
=
RT
7260000
∴ RT =
7260000
127600
∴ R T = 57Ω
(a) the total resistance (RT)
(b) the circuit current (IC)
(c) the current in each resistor (I1 and I2).
c
s
(a)
(b)
RT =
8 × 12
8 + 12
96
=
20
= 4.8 Ω
=
RT
36
R1 × R 2
R1 + R 2
VS = I C × R T
IC =
VS
RT
12
4.8
= 2.5 A
=
IC
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
(c)
I1 =
VS
R1
12
8
I1 = 1. 5 A
=
VS
R1
12
=
12
I2 = 1 A
I2 =
We can use Kirchoff’s second law to check the
answers calculated for the current in each branch.
I C = I1 + I 2
= 1.5 + 1
I C = 2.5 A
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
37
Exercises: resistors in parallel
1. For the circuit shown below calculate:
(a) the total resistance of the circuit
(b) the circuit current.
9V
2. For the circuit shown below calculate:
(a) the total resistance of the circuit
(b) the circuit current
(c) the current flowing though R1 (10 R)
(d) the current flowing through R2 (24 R).
110V
3. For the circuit shown below calculate:
(a) the total resistance of the circuit
(b) the circuit current
(c) the current flowing through R1 (660 R)4
(d) the current flowing through R2 (470 R).
Use Kirchoff’s second law to verify your answers to parts (c) and (d).
240 V
4. A 6 R resistor and a 75 R resistor are connected in parallel across a voltage supply
of 12 V. Calculate the circuit current.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
5. A 440 R resistor is connected in parallel with a 330 R resistor. The current through
the 440 R resistor is 300 mA. Find the current through the 330 R resistor.
Combined series and parallel circuits
Until now we have been looking at series or parallel circuits individually. It 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 below.
You can see that R2 and R3 are connected in parallel and that R1 is connected in series
with the parallel combination.
Some points to remember when you are dealing with combined series and parallel
circuits are:
• the voltage drop across R2 is the same as the voltage drop across R3
• the current through R2 added to the current through R3 is the same as the current
through R1
• the voltage drop across R1 added to the voltage drop across R2 (which is the same
as across R3) would equal the supply voltage Vs.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
39
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).
48R
24R
10R
12V
(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
two resistors in parallel:
RP =
R2 × R3
R2 + 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:
R T = R1 + R P
R T = 24 + 8.28
R T = 32.28Ω
40
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
(b) It is now possible to calculate the circuit current:
VS = I C × R T
IC =
VS
RT
IC =
12
32.28
I C = 0.37A
(b) The voltage drop across R1 is found by using the resistance across and the current
through R1.
V = I× R
VR1 = I C × R 1
VR1 = 0.37 × 24
VR1 = 8.88V
(c) The current through R2 is found by using the resistance of R2 and the voltage drop
across R2. By using Kirchoff’s second law we know that the voltage drop across
the parallel arrangement must be:
VS = VR1 + VP
VP = VS − VR1
VP = 12 − 8.88
VP = 3.12V
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
41
By using Kirchoff’s first law we know that the circuit current IC will ‘split’ or divide
between the two resistors R2 and R3. In order to find the current through R2 we use:
V = I× R
VP = I 2 × R 2
I2 =
VP
R2
I2 =
3.12
10
I 2 = 0.312A
42
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Exercises: combined series and parallel circuits
1. For the circuit shown calculate:
(a) the resistance of the parallel combination
(b) the total circuit resistance.
7.5 V
2. For the circuit shown calculate:
(a) the total resistance
(b) the circuit current
(c) the voltage drop across each resistor.
24 V
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.
110 V
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
43
Power in electric circuits
Electrical power is measured in watts (W). Electrical power can be converted into
other forms of power using electric circuits. For example the power used in
overcoming electrical resistance can be converted into heat – this is the principle of an
electric fire.
The power in an electric circuit depends both on the amount of current (I) flowing and
the voltage (V) applied.
The formula for power in electric circuits is:
Power = Voltage x Current (watts)
P = V x I (W)
Worked example
An electric household lamp consumes 60 watts from a 230 volt supply. Calculate the
current drawn by the lamp and the resistance of the lamp.
P = V × I
∴I =
I =
P
V
60
240
I = 0. 25 A (or 250 mA)
V = I× R
∴R =
R =
V
I
240
0 .25
R = 960 Ω
44
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Exercises: power in electric circuits
1. In the following simplified circuit for a vacuum cleaner motor, calculate:
(a) the power consumption of the motor
(b) the voltage of the lamp
(c) the total power drawn from the power supply.
3 amps
240 volts
M
200 volts
2. The torch circuit below is supplied with two 4.5 volt batteries connected in series,
with the current being 20 mA. Determine:
(a) the resistance of the bulb
(b) the voltage across the bulb
(c) the total power drawn from the supply
(d) the power drawn by the bulb.
Switch
100 ohms
Bulb
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
45
For many of the following exercises it would be useful to draw the circuit diagram.
3. An electric iron rated at 800 W is connected to a 230 V supply. Calculate the
maximum current drawn by the iron. What is the power used by the iron at halfheat setting?
4. A kettle and a toaster use the same double socket. If the kettle draws a current of
10 A and the toaster 3 A, find the power used by each of the appliances. The two
sockets are wired in parallel to a 230 V supply.
5. An electric drill draws a current of 1.5 amps from a 110 volt supply. Calculate the
power rating of the drill.
6. An emergency power generator has to drive 80 lamps. Each lamp takes 60 W at
230 V. Calculate the current through each bulb if:
(a) they are connected in series
(b) they are connected in parallel.
7. How many 150 W lamps can be connected in parallel to a 250 V supply through a
5 A fuse?
8. In a stereo system each of the speakers is rated as 15 W per channel. If the supply
voltage is 230 V, find the current drawn by each speaker when the system is fully
on. Assume that the speakers are connected in parallel. If the system uses 2 W in
wasted heat energy in normal conditions at full power, what is the current drawn
from the supply and what fuse would you recommend for the mains plug?
9. The power ratings for the lamp bulbs on a car with a 12 V battery are as follows:
• headlamps – 60 W
• indicators – 24 W
• sidelamps – 6 W.
Find the current drawn by each lamp and the resistance of each bulb.
10. The rear screen heater in a car is connected to the 12 V system and draws a current
of 2 A. Find the resistance of the circuit. In reality the 12 V, 0.5 A interior light is
on the same circuit. State whether this is a parallel or series circuit and calculate
the power and current when both lamp and heater are on.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Voltage divider circuits
Input transducers
0V
Input transducers are devices that convert a change in physical conditions (for
example, temperature) into a change in resistance and/or voltage. This can then be
processed in an electrical network based on a voltage divider circuit.
If two or more resistors are connected in series (see diagram below), the voltage over
each resistor will depend on the supply voltage and the ratio of the resistances.
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.
The layouts of voltage divider circuits are conventionally represented as shown above.
A voltage divider circuit can be represented in a number of different ways. Some of
these are shown below.
0V
0V
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
47
Voltage divider circuit
If an input transducer changes its resistance as the physical conditions change, then
the resistance change has to be converted into a voltage change so that the signal can
be processed. This is normally done using a voltage divider circuit.
A typical voltage divider circuit is shown below.
VS
R1
V
R2
0 volts
As you can see, this circuit consists basically of two resistors connected in series. As
you already know, if you change the value of R1, the voltage across it will change, as
will the voltage across R2. In other words, the resistors divide the voltage up between
them.
Practical task
Use a prototype board to build the voltage divider circuit shown below. If the supply
voltage is 6 volts, R1 = 220 Ω and R2 = 330 Ω, what is the voltage across R2?
Volts
VS
R1
10A
mA
V
COM
R2
0V
The voltage across R2 is normally called the output or Vo. You should find that the
voltage is divided up according to the formula
VO =
48
R2
× VS
R1 + R 2
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Worked example: voltage divider circuit
Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 12 volts
R 1 = 80k
R 2 = 40k
V2
0 volts
Applying the voltage proportion formula:
R2
R1 + R 2
40
V2 = 12 ×
40 + 80
V2 = 4 volts
V2 = VS ×
The voltage over the 80 K resistor could be calculated in the same way, but this is
unnecessary for this circuit since we can use Kirchoff’s second law to confirm the
answer. The voltages over each of the components in a series circuit must add up to
the supply voltage, hence the voltage over the 80 K resistor is 12 V − 4 V = 8 V.
It is also possible to use Ohm’s law to solve these voltage divider problems.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
49
Exercises: Voltage divider circuits
1. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 12 volts
R 1 = 270R
R 2 = 810R
V2
0 volts
2. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 12 volts
R 1 = 390R
R 2 = 10K
V2
0 volts
50
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
3. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 6 volts
R 1 = 10K
R 2 = 47K
V2
0 volts
4. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 9 volts
R 1 = 10K
R 2 = 2.2K
V2
0 volts
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
51
Simple switches can be used in voltage divider circuits to give a digital signal (that is
definitely ON or OFF) to another part of a circuit.
In the example below a normally closed switch is used.
When the switch is pressed, the voltage divider comes into use and power is supplied
to the LED to give a definitely ON signal. Build the circuit in Crocodile Clips to test
this.
Digital switch types
Different types of switch were described earlier but they can be wired up to suit their
application. A switch with its contacts apart when it is not operating is called normally
open.
Double-pole switch symbols
Double-pole single-throw switch (DPST)
52
Double-pole double-throw switch (DPDT)
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Analogue input transducers
The two most common analogue input transducers are the thermistor and the lightdependent resistor (LDR).
Thermistor
A thermistor is a device whose resistance varies with temperature. It is a temperaturedependent resistor. There are two main types.
1. Negative temperature coefficient (−t or NTC) – where resistance decreases as
temperature increases.
2. Positive temperature coefficient (+t or PTC) – where resistance increases as
temperature increases.
The circuit symbols for and typical characteristics of the two types of resistor are
shown below.
NTC thermistors are the most commonly used.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
53
Graph of temperature versus resistance
The graph below shows accurately how the resistance varies with temperature for an
NTC thermistor.
Thermistor types
Strain gauges
Strain gauges 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 (beams, etc.) and as they are loaded, a reading on a voltmeter can be
obtained.
Strain gauge
Light-dependent resistor (LDR)
The LDR (sometimes called a photoresistor) is a component whose resistance depends
on the amount of light falling on it. Its resistance changes with light level. In bright
light its resistance is low (usually around 1 K). In darkness its resistance is high
(usually around 1 M).
54
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
The circuit symbol and typical characteristics of an LDR are shown above.
Graph of illumination versus resistance
The graph below shows accurately how the resistance varies as the amount of
illumination falling on an LDR varies.
Voltage divider circuits
One of the main purposes of the voltage divider circuits is to sense and process inputs
from analogue sensors. In this example a thermistor will be used. The resistor R2 of
the previous circuit has been replaced by an NTC thermistor.
VS = 9 volts
1
-t
VO
0 volts
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
55
Practical task 1
Build the above circuit and measure and record the output voltage (Vo) at room
temperature and at a higher temperature (use your fingers to warm up the thermistor).
Volts
VS
R1
10A
mA
V
COM
NTC Thermistor
0V
Measure the voltage drop and the resistance of the thermistor at both these
temperatures. Check your results by calculation and record them in a simple table.
NTC thermistor
Low temperature
High temperature
Voltage
Resistance
This voltage divider circuit uses a light-dependent resistor or LDR. The LDR replaces
R2 from the basic voltage divider circuit.
VS = 9 volts
10K
ORP12
VO
0 volts
56
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Volts
VS
R1
10A
mA
V
COM
LDR
0V
Practical task 2
Build the above circuit and measure and record the output voltage (Vo) at normal
room light conditions and when the LDR is covered (use your finger or a coin to do
this).
Measure the voltage drop and resistance of the LDR at both light states. Check your
results by calculation and record these in a simple table.
LDR
Light
Dark
Voltage
Resistance
Variable resistor (potentiometer)
A potentiometer configured as a variable resistor can be used in a circuit as a voltage
or current control device. They are often used in voltage divider circuits to adjust the
sensitivity of the input. In Standard Grade Technological Studies the majority of
applications will use a variable resistor or a potentiometer configured as a variable
resistor.
Potentiometers normally have three tags or terminals. The outer ones are connected to
the ends of the resistive material and the centre one is connected to the wiper.
The spindle of the potentiometer is connected to the wiper, which is able to traverse
the whole of the resistive material. As the spindle rotates, a sliding contact puts more
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
57
or less resistive material in series with the circuit. In this way the resistance in a
voltage divider circuit is varied.
Miniature potentiometers
Most modern circuits now use miniature potentiometers or variable resistors.
Examples of miniature potentiometers (not to scale)
Voltage divider circuits
The LDR voltage divider circuit can be set up to detect when it is light or when it is
dark.
VS = 9 volts
VS = 9 volts
ORP12
10K
ORP12
VO
10K
VO
0 volts
Detects when dark
Detects when light
The above circuits should be simulated on Crocodile Clips to confirm their operation.
58
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Sensitivity
With an analogue sensor it is normally desirable to adjust the sensitivity of the circuit.
Rather than using a fixed resistor we can replace it with a variable resistor (or
potentiometer).
VS = 9 volts
ORP12
47K
VO
0 volts
Practical task: voltage divider circuits
The picture below shows a typical situation where a light sensor circuit could be
useful.
To save money and inconvenience the residents want the outside light to come on
when it gets dark. They also want to be able to adjust the sensitivity from summer to
winter nights.
Build the following circuit using a prototype circuit board. The variable resistor is
rated at 10 kΩ.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
59
+ 9 volts
VR
LDR
0 volts
Adjust the sensitivity so that the output voltage (Vo) goes higher when your hand is
moved across the LDR at a distance of approximately 100 mm. You will have to
attach a multimeter to the circuit to see when this is happening.
Check this out using Crocodile Clips.
VS = 9 volts
10K
ORP12
V
0 volts
60
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Exercises: voltage divider circuits
1. Calculate the voltages that would appear across each of the resistors marked ‘X’ in
the circuits below.
5V
9V
0V
0V
2. In each of the following voltage divider circuits determine the unknown quantity.
12 V
0V
16 V
12 V
0V
0V
15 V
20 V
220R
0V
0V
0V
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
61
3. An NTC (negative temperature coefficient) thermistor is used in a voltage divider
circuit as shown below. 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°C
(b) 20°C.
4. What would happen to the voltage across the thermistor in the circuit shown above
as the temperature increased?
5. What would happen to the voltage across the resistor in the circuit shown above as
the temperature increased?
6. A thermistor (type 5) is used in a voltage divider circuit as shown below. The
characteristics of the thermistor are shown in the graph. If the voltage V2 is to be
4.5 V at 100 °C, determine a suitable value for R1. State whether V2 will increase
or decrease as the temperature drops. Explain your answer.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Transistors
The first major breakthrough in electronics came with the invention of the diode valve
at the beginning of the twentieth century. This was the first real electronic component
and was to lead to the modern diode and transistor.
A diode valve consisted of a heater inside a hollow rod that had been coated with a
substance which released electrons when heated. This was surrounded by a thin metal
cylinder, with all of this being contained in a bulb-like glass container. When the rod
was heated, electrons were released but, as in any diode, the electrons could only go
in one direction.
The diode was followed by the triode, which allowed the current flow to be
controlled. These valves could act as electronic switches or amplifiers. Radio and
television were developed using these amplifier valves. In the 1940s the first
computer was built using valves − it contained over 20,000 valves and filled a large
room.
In 1947 the transistor was invented. The transistor had many advantages over valves,
the main ones being size, efficiency, durability and cost. The next big advance in
electronics was the integrated circuit in 1958: two transistors were fitted on a silicon
chip. The developments since then have been rapid and chips now contain over a
million transistors.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
63
Transistors (bipolar)
The transistor is a semiconductor device. This means that it is sometimes a good
conductor of electricity and sometimes a poor one. A transistor is made up of three
layers of semiconductor materials that are either ‘n type’ or ‘p type’.
There are two types of bipolar transistor available: pnp or npn. We will deal only with
the npn type for convenience. (The only real difference is that the voltages and
currents should be reversed for a pnp transistor.)
C
N
This diagram represents an npn transistor. It
has three leads or legs.
C is the collector
B
P
B is the base
N
E is the emitter
E
When a positive voltage of about 0.6 volts is applied across the base and emitter, the
resistance between the collector and the emitter of the transistor drops from very high
to very low. In other words the transistor changes from being a very poor to a very
good conductor.
This means that to switch the transistor ‘on’ a small voltage of about 0.6 volts is
applied to the base. When the voltage reaches 0.7 volts the transistor is fully ‘switched
on’. In this condition the transistor is said to be fully ‘saturated’.
General-purpose transistor
The BC 108 is common general-purpose transistor. The diagram below shows the
position of the legs when viewed from underneath the case.
The transistor has to be connected into circuits correctly. The arrowhead on the
emitter indicates the direction of ‘conventional’ current flow − that is, opposite to the
electron flow.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
How does the transistor work?
Consider the circuit shown below.
9V
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. But, as no voltage is
being applied to the base of the transistor, it is acting as a barrier to electric current.
When switch S1 is closed, a very small voltage is applied to 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’.
Bipolar transistors amplify current. A small current flowing through the base of a
transistor causes a much larger current to flow from the collector to the emitter.
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65
The transistor as a switch
One of the main uses of a transistor is that of a very sensitive switch.
Assignment
Use Crocodile Clips or another circuit simulation package to set up the circuit below.
Begin with a value of 2200 K for the base resistor R and then reduce the resistance
using the values given in the table below. This can be carried out manually if a
suitable package is unavailable.
9V
Lamp
c
b
A
BC 108
1K
e
V
0V
Complete the following table during your investigation.
Base resistor
value (K)
2200
1000
470
220
100
47
33
22
10
1
Base/Emitter
Voltage (mV)
Base current (µ
µA)
Lamp
on/off
You should find that the circuit will ‘switch on’ the lamp when the base/emitter
voltage drop of the transistor is 0.7 volts. You will also have noted that as the
base/emitter voltage drop rises above 0.7 V the brightness of the lamp does not
increase. This is because once this level has been reached, the transistor is fully
‘switched on’, or saturated.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Practical tasks
1. Build the circuit below to demonstrate the operation of a transistor switch.
Lamp
A
+ 6 volts
IK
Flying
lead
c
b
e
0 volts
When the flying lead (a wire connected at one end only) is connected to hole ‘A’
the transistor should switch and the lamp should light.
Connect a multimeter (set at voltage) across the base and emitter of the transistor.
Volts
A
to lamp
10A
mA
V
COM
The multimeter should measure approximately 0.7 volts. As explained earlier, this
will switch the transistor ‘on’ and the lamp should light.
If this reading is incorrect or the lamp does not light when the flying lead is
connected to hole A, check all the connections and fix any faults until the circuit
works as expected.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
67
To make the circuit even more sensitive, a voltage divider with an LDR and variable
resistor can be used. This will enable small changes in the LDR resistance to switch
the transistor.
LAMP
LDR
2. Build the transistor switching circuit below.
Lamp
+ 6 volts
10K
c
b
1K
e
0 volts
LDR
Instructions
 Place all components as shown in diagram.
 Insert all connection wires.
 Make the 0 volt connection.
 Make the +6 volt connection.
 Set the variable resistor to mid-value.
 Cover the LDR and observe what happens.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Transistor circuits calculations
Ohm’s law, Kirchoff’s second law, series circuit and parallel circuit calculations are
just as important and appropriate in transistor circuits as they were in the previous
ones.
Example circuit
+ 6 volts
6V
60mA
Lamp
c
b
BC 108
1K
e
0V
The transistor circuit above is basically a parallel circuit. If the circuit is rearranged
slightly this becomes obvious.
Ic
c
6 volts
T
x
e
0V
Ie
b
Rb
Ib
The transistor (marked T) is at the junction of the parallel circuit. If we assume that no
voltage drops across the collector/emitter in the transistor then
Vxe = 6 volts (in the bulb branch)
As the two branches between x and e are in parallel, Vxe across the resistor branch
must also be 6 volts. Thus
VRb + Vbe = 6 volts
We know that Vbe must be 0.7 volts to switch the transistor on; therefore
VRb = 5.3 volts
It is now possible to calculate all other currents and resistance values.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
69
Relays
Although relays are often considered to be output devices, they are really output
switches from electric or electronic circuits. These output switches are used as inputs
for other circuits. In practice you can hear relays clicking on and off when a car’s
indicators are used.
How the relay works
When an electric current flows into the relay coil, the coil becomes an electromagnet.
This electromagnet attracts the armature and moves the contacts. This movement
provides the switching, just as the contacts in any other switch do.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
The relay is a very useful device because it is the vital link between microelectronics
and high-energy systems that require substantial amounts of current. The relay is
perhaps the most commonly used switch for driving devices that demand large
currents.
Relay symbol
in a circuit
Relays  connections
Miniature
relay
The connections for a typical miniature relay are shown below.
1 +
16 -




4
6
13
11
8
9
Connections 1 and 16 are those from the sensing or input circuit.
Connections 4 and 13 are the supply voltage to run the output.
Connections 6 and 11 are the normally closed output terminals.
Connections 8 and 9 are the normally open output terminals.
Relays  protective diodes
As seen earlier, relays have a coil that is energised and de-energised as the relay
switches on and off. During this process of switching, the coil can generate a large
reverse voltage (called a back e.m.f.). This reverse voltage can cause considerable
damage to components, especially transistors.
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71
The transistors and other sensitive components can be protected by the inclusion of a
diode that provides a path for the current caused by the reverse voltage to escape.
The circuit diagram is shown below.
A solenoid is another output transducer that has a coil inside. Circuits containing a
solenoid require a protective diode as well.
Coil
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Relay circuit
The circuit below shows a typical transistor circuit with a relay as an output.
+ 6 volts
-t
Relay
c
b
1K
BC 108
e
10K
0 volts
Practical tasks
1. Build the relay circuit below. When the temperature of the thermistor is increased
you should hear the relay switching and then switching once more as the
temperature decreases again. Note the diode, which is used to protect the transistor
(see later for more information).
+ 6 volts
NTC
b
diode
c
1K
Relay
e
0 volts
10K
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
73
2. This task requires you to connect a 12 volt lamp to the normally open output of
the relay so that when the temperature of the thermistor rises the light will switch
on.
Note the 12 volt supply for the bulb. Do not mix up the supplies or the 0 volt lines.
+ 12 volts
+ 6 volts
NTC
12V Lamp
b
diode
c
1K
Relay
e
0 volts
10K
0 volts
Alter the circuit so that the lamp comes on when it gets dark. Draw the circuit diagram
before you alter the prototype circuit above.
Relays can also be used to switch on (and off) solenoid-actuated pneumatics valves.
These normally run on a 12 volt supply. This is a method of controlling pneumatic
circuits and systems with microelectronics.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
3. As electric motors normally draw larger currents, relays are ideal devices for such
circuits. By using DTDP switching, relays can control the direction of rotation of
motors.
TO SENSOR
CIRCUIT
0V
Solenoid-actuated
3/2 pneumatic
valve
+V
The circuit below shows a motor control circuit. The motor will reverse direction
when the input switch is pressed.
+ 6 volts
b
diode
c
1K
e
0 volts
Change the circuit so that a change in temperature will automatically change the
direction of the motor. Draw the circuit diagram before making any alterations.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
75
4. The partial circuit below shows a transistor switch circuit with a relay as an output
and an LDR voltage divider circuit as an input.
Build and test a complete circuit showing the relay connected to a motor.
Instructions
Draw a full circuit diagram.
Investigate from earlier work the value of the potentiometer.
Make a layout diagram for building the circuit on a prototype circuit board.
After checking, build and test your circuit.
Note: Alternatively, this circuit could be built and tested using circuit simulation
software such as Crocodile Clips.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Integrated Circuits: 555 timer
An integrated circuit (or IC) is simply an electronic package that contains a number of
components on a silicon ‘chip’. The 555-timer IC that you are going to use is a very
versatile chip that has many applications.
As you can see, the 555 chip has eight pins. The pin functions are shown below.
1
8 +V (3 -15)
TRIGGER
2
7
OUTPUT
3
6
RESET
4
CONTROL
5 VOLTAGE
(LEAVE
UNCONNECTED)
NE555
0V
TIMING
PERIOD
CONTROL
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77
555 timer  capacitors
Capacitors are electronic components that store electricity for short periods of time
within electronic circuits or networks. They are made from two metal plates or films
separated by an insulator. In many capacitors, film is used so that the layers of metal
film and insulator can be wound into a cylinder. Capacitors are especially useful in
timer circuits with the 555-timer chip.
INSULATOR
METAL PLATE
OR FILM
There are two basic types of capacitor normally used in timer circuits: electrolytic and
polyester.
Electrolytic capacitors are polarity conscious. This means that they must be
connected ‘the right way round’. The negative lead must be connected to zero volts
with the positive terminal towards the higher voltage side of the circuit.
It is very dangerous to reverse connect capacitors.
ELECTROLYTIC
AXIAL
Axial
CAPACITATOR
capacitor
RADIAL
Radial
CAPACITATOR
capacitor
Polyester capacitors are for small-value uses and can be connected without regard to
polarity.
POLYESTER
Capacitance in measured in farads, but because this is a very large measurement most
capacitors are rated in F (microfarads) or in nF (nanofarads).
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
555 timer  practical tasks
1. This 555-timer circuit is used to switch an LED on for a specific time when the
chip is ‘triggered’. A typical application for this would be an egg timer.
Build the prototype circuit shown below.
+ 6 volts
1K
100K
555
1K
390R
+
0 volts
100uF
LED
Flying
lead
Instructions
 Briefly touch the bare end of the flying lead to 0 volts. The LED should light for a
fixed period.
 Adjust the variable resistor to obtain the longest fixed time for which the LED will
stay on.
 Change the capacitor to the values in the table below and record the maximum
time period for which the LED lights. Crocodile Clips or similar software could
be used for this task.
Capacitor value (F)
Maximum time
100
470
1000
2200
4700


Draw a graph to illustrate your answers.
Estimate what value of capacitor would give a time of approximately 60 seconds.
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79
1. The 555-timer circuit already built made an LED go on for a specific time when
the chip was triggered. The circuit diagram below shows the altered circuit with
the LED going off when the chip is triggered. Alter your circuit to show this
effect.
+ 6 volts
100K
1K
390R
1K
7
6
8
4
IC1
555
2
3
1
100F
0 volts
This circuit shows the 555 chip operating as a monostable device. This means that it is
stable in only one state, that is, it ‘jumps back’ to its initial state after a set time.
Note: As an alternative, build this new system using circuit simulation software.
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
2. This 555-timer circuit is used to make an LED flash on and off at a set frequency.
Build the prototype circuit shown below.
+ 6volts
1K
555
LDR 1K
390R
+
0 volts
100uF
1uF
LED
Instructions
 On powering up the circuit, the LED should flash on and off at a steady rate
(frequency).
 Cover the LDR to see what effect this has.
 Expose the LDR to bright light and observe the effect.
 Complete a table to show your findings.
Light conditions
Dark
Normal
Bright
Frequency
This circuit shows the 555 chip operating as an astable device. This means that it is
unstable in both states; that is, it flips constantly from one state to the other.
Frequency
Frequency is the regular rate at which a physical event repeats itself. In this circuit it
is the rate at which the LED flashes. In electronic circuits the common events are the
flashing of optical devices (LEDs and lamps) and the sounding of buzzers/speakers.
These outputs are driven by an electrical pulse from the electronic system or circuit.
Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
81
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Standard Grade Technological Studies: Applied Electronics – Component Electronic Systems
Modular Electronic Systems
Note
This section of the Applied Electronics unit has been constructed around the E&L
modular sub-systems boards. It is recognised that not all schools will use this system,
as others are also available. However, owing to various constraints, it has not been
possible to produce support materials in detail for all the systems currently available.
Schools using an alternative system will be able to use the generic diagrams to
provide meaningful input for the systems boards they do use.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
83
Contents
Introduction: Electronics – a systematic approach
Modular boards
Analogue and digital signals
Problem solving in electronics
Switches
AND gate logic
OR gate logic
Truth tables
The comparator
NAND and NOR gate boards
84
85
86
88
95
98
99
100
101
115
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Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Introduction: Electronics − A systematic approach
The systems represented in this section make use of E&L systems boards. Centres
using Alpha systems will be able to adapt the materials as appropriate. In all areas of
technology, including electronics it is useful to have a systems approach to problem
solving. This makes it easier to understand problems and enables you to solve these
electronic problems in a structured manner.
Before getting to the ‘nitty-gritty’ of any electronic system, it is necessary to have an
overview. This can be achieved by using a universal input−output system diagram.
Input
Proc ess
Output
All electronic circuits and systems have an input, a process and an output. Identifying
these basic parts is the first step in solving electronic circuit/system problems.
Input
Proc ess
Output
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85
Modular boards
Modular circuit boards provide a simple means of solving both simple and complex
electronics problems. It is easy to identify the input, process and output stages. The
boards are, however, bulky and expensive.
E&L modular system boards
E&L system boards make it easy to build working electronics systems to solve real
problems. These boards have input devices, process devices and output devices.
+V
+
+
S
0V
-
+
TP
S
0V
SB
0V
-
+
O/P
IND
SA
0V
-
-
+
+
+
S
0V
S
0V
S
0V
TP
-
-
-
OR GATE
0V
0V
THIS I S A
CHIP BU TTY
0v
E & L INSTRUMENTS Ltd
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E & L INSTRUMENTS Ltd
Using E&L boards
Before using the boards, there are a number of points that should be noted.
•
•
•
Collect all the boards that you will need before starting assembly.
Always connect the boards together on a flat surface.
Make sure that the four pins interlock properly.
•
Never connect the power supply until all the boards in the system have been
assembled.
•
•
When making alterations to a system, disconnect the power supply.
Always take care when using the boards. Never subject the components on the
boards to any force, as this will cause damage to the board.
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Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
All systems need two special boards. These are described below.
Power connection board
This board is necessary to supply power to the system. You will notice that there are
four connections.
+VE
SIGNAL
POS
SIG
0
0V
NEG
VOLTS
-VE
RANGE
+5V DC TO +8V DC
Transducer driver board
All systems require a transducer driver. This will normally be the second last board in
the system, that is the board before the output board.
+V
+
+
TP
S
0V
-
S
0V
-
0v
0v
E & L INSTRUMENTS Ltd
The transducer driver is a small transistor amplifier that provides the output devices
with enough power for them to operate. The signal (current and voltage) from the
input is otherwise too weak to power the output.
COLLECTOR
EMITTER
BASE
The terms in the above diagram are explained in the ‘Component Electronic Systems’
section.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
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Analogue and digital signals
All components in electrical and electronic circuits are either receiving or transmitting
electrical signals. These signals can be either analogue or digital.
Analogue devices
An analogue signal varies according to the physical surroundings. For example, the
E&L light-sensing unit will send out a voltage that is proportional to the amount of
light falling on the LDR.
TP
Light sensor
(ORP 12)
+
+
S
0V
S
0V
-
-
0v
E & L INSTRUMENTS Ltd
This E&L unit is called an input transducer because it converts the change in light to
a change in voltage.
Light
Intensity
LDR
Varying
Voltage
The graphs of analogue input transducers are typically a sloping line or a curve.
Typical analogue input transducers are:
• input voltage units
• light-sensing units
• temperate-sensing units
• moisture/rain sensor units
• sound-sensing units.
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Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Digital devices
A digital signal is one which has only two settings, on or off. In electronic terms it has
only two levels, high or low.
The push switch unit below is a typical simple digital transducer.
O/P
IND
+
+
S
0V
S
0V
TP
-
0v
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
This E&L unit is called an input transducer because it converts the change in physical
movement to a change in voltage.
Mec hanic al
Movement
Switc h
High or Low
Voltage
Voltage
The graph of typical digital input transducers is shown below.
Time
Typical digital input transducers are:
• switch units
• magnetic switch units
• pulse generator units.
Logic
In digital terms:
• on or high is Logic 1
• off or low is Logic 0.
This is why modern appliances have switches marked in this way.
1
0
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
89
Output transducers
Output transducers take an electrical signal and change it into a physical output. They
include the output boards in modular systems or output components in any electronic
system.
The main output transducers are shown below.
ACTIVE LOW
+
+
S
0V
S
0V
-
+
S
0V
-
+
+
-
S
0V
-
-
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
+
-
-
S
0V
-
E & L INSTRUMENTS Ltd
-
+
S
0V
S
0V
S
0V
E & L INSTRUMENTS Ltd
+
+
S
0V
+
S
0V
-
E & L INSTRUMENTS Ltd
System diagrams
The system diagram for a bulb unit is shown below.
Elec tric al
Energy
Bulb Unit
Light
Draw the system diagrams for the other output transducers.
How E&L boards work
Although there are four connections running through an E&L system, only the top
three are of importance at present.
+VE
SIGNAL
0 VOLTS
The top connection is the positive supply rail, the third is the 0-volt rail and the
middle one is the signal line.
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Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 1
Join the two boards shown below and connect up the power.
A
+
+
POS
SIG
S
0V
0V
-
1
0
K
TP
+
S
0V
-
NEG
C
RANGE
+5V D C TO +8V D C
VOLTS
INC.
E & L INSTRUMENTS Ltd
B
Set the potentiometer dial to position 1 and measure voltages A, B and C.
1
5
2
4
3
Using the connection pins and the test points (marked T.P.), complete the table below.
Position
1
2
3
4
5
Voltage A
Voltage B
Voltage C
You should find that VA + VB = VC
In other words, this circuit is a voltage divider circuit. The supply is split into two
paths.
Many of the circuits in the E&L modular boards are based on voltage divider circuits.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
91
Practical task 2
+V
TP
+
+
+
POS
SIG
S
0V
S
0V
S
0V
0V
-
+
+
+
S
0V
S
0V
S
0V
TP
-
-
NEG
-
-
-
0v
0v
0v
RANGE
+5V DC TO +8V DC
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
System diagram
Light
Intensity
Proc ess
Light
Instructions
• Draw a block diagram of the system shown.
• Connect the sub-systems as shown (individual boards are often referred to as subsystems).
• Make the power connection.
• Adjust the black dial (potentiometer) on the light-sensing unit to its mid-position.
Cover the LDR (light dependent resistor) with your hand/finger and note what
happens at the output.
• Try the same with the potentiometer turned fully clockwise and fully
anticlockwise.
• Write down what you think the purpose of the potentiometer on the light-sensing
unit is.
• Measure the signal voltages for the following conditions.
Potentiometer
Clockwise
Mid-point
Anticlockwise
LDR covered
LDR uncovered
Note: this system acts as a simple analogue to digital converter.
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Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 3
Power
Connection
Tem perature
Sensing Unit
Transducer
Driver
Bulb
Unit
The block diagram above shows a simple temperature-sensing system.
Instructions
• Copy the block diagram.
• Copy and complete the system diagram.
Proc ess
•
•
•
•
Connect up the system.
Connect to the power supply.
Heat the thermistor between your fingers and note what happens.
What is the effect of adjusting the potentiometer on the temperature sensing unit?
Practical task 4 − replacing the temperature sensing unit with a rain/moisture
sensing unit
Instructions
• Draw a block diagram for the new system.
• Copy and complete the system diagram.
Proc ess
•
•
•
Connect up the system.
How can you get the bulb to light?
Name the sensing component that is the input transducer.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
93
Practical task 5
+V
HIGH = ON
LOW = OFF
TP
+
+
+
POS
SIG
S
0V
S
0V
S
0V
0V
-
O/P
IND
TP
-
-
LATCH
NEG
+
+
S
0V
S
0V
-
TC4011BP
864BHB
-
+
+
S
0V
S
0V
-
-
-
0v
0v
0v
RANGE
+5V DC TO +8V DC
+
S
0V
TP
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E & L INSTRUMENTS Ltd
This system uses an inverter board.
Instructions
• Draw a block diagram for the new system.
• Copy and complete the system diagram.
Proc ess
•
•
•
•
•
•
Connect up the system.
Connect to the power supply.
Turn the potentiometer on the light-sensing unit to its mid-point. Cover the LDR
and note what happens.
State the function of the inverter.
How does this system differ from the previous one?
Write down a practical application of this system.
The inverter used in this system is often referred to as a NOT gate. This is because the
output from the inverter is NOT equal to its input.
If the input signal is high (or logic 1) then the output signal is low (or logic 0).
1
0
Inverter
If the input signal is low (or logic 0) then the output signal is high (or logic 1).
0
1
Inverter
From the diagrams above, you can work out that there are two input combinations.
These can be represented in a table called a truth table. Copy the symbol and table and
then complete the table.
A
0
1
Z
Symbol for NOT gate
Truth table
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Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Problem solving in electronics
It is best to have a structured systems approach to solving problems in any technology
sphere. This ‘systems approach’ is essential in electronics, as we cannot ‘see’ or
‘touch’ electricity.
Step 1
Identify the input(s) and output(s) of the system using a systems diagram.
Light
Intensity
Sound
(Buzzer)
Proc ess
Step 2
Try to identify the process needed to enable the system to convert the input to the
output. It will be helpful to look at previous work done and to do some research using
study notes and books.
Step 3
Investigate possible solutions using rough block diagrams.
Step 4
Draw up your final block diagram and discuss with your teacher before building.
Power
Connection
Light
Sensing Unit
Transducer
Driver
Buzzer
Step 5
Build and test your system. Make any adjustments or alterations that are necessary.
Step 6
Evaluate your solution.
Note: the modular circuit boards are excellent for simulating solutions to real-life
electronic problems. Once proved, a solution to a problem would be manufactured in
a more permanent construction. This would reduce the size and cost of the system.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
95
Problem 1
Government legislation states that food should be stored below −18 °C. Design an
electronic system that will warn the freezer user if the temperature rises above this
preset level.
Specification
• For the purposes of the simulation, the alarm should sound if the temperature rises
above normal room temperature.
• The alarm should alert the user even if they are not in the same room as the
freezer.
Solution
• Draw a system diagram to represent a suitable system.
• Draw a block diagram of a possible solution to the problem.
• Identify all sub-systems.
• Test your solution using modular boards.
• Explain how your system works.
Problem 2
It is often difficult for a cricket umpire to decide whether there is enough light to
continue play or not.
Specification
• For the purposes of the simulation, the alarm should operate if it gets darker than
the present room light-level.
• The alarm should alert the umpire.
Solution
• Draw a system diagram to represent a suitable system.
• Draw a block diagram of a possible solution to the problem.
• Identify all sub-systems.
• Test your solution using modular boards.
• Explain how your system works.
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Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 6
+V
O/P
IND
+
+
+
S
0V
S
0V
S
0V
+
+
+
S
0V
S
0V
S
0V
TP
POS
SIG
-
0V
TP
-
-
NEG
-
-
-
0v
0v
0v
RANGE
+5V DC TO +8V DC
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
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E & L INSTRUMENTS Ltd
Instructions
• Draw a system diagram for the above system.
• Draw a block diagram of the system shown.
• Connect the sub-systems as shown.
• Make the power connection.
• Test out the operation of the system by pressing the switch.
• Explain what would happen if an inverter was added.
• State where the inverter would be positioned.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
97
Switches
Switches are the most common types of input device. Most electronic systems use
switches. As shown earlier in this area of work, switches are digital devices.
Off (Logic 0)
On (Logic 1)
The switch used in task 6 above is a normally open switch. This means it is normally
off, that is at logic 0 state. You have to press the switch to turn it on and its spring will
return it to its normal position when the pressure is removed.
98
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 7
It is common for systems to have more than one input.
+
S
0V
+
O/P
IND
S
0V
TP
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
0v
+V
O/P
IND
0V
-
+
S
0V
TP
+
+
S
0V
SB
0V
-
+
POS
SIG
-
TP
O/P
IND
SA
0V
-
NEG
+
+
S
0V
S
0V
+
+
+
S
0V
S
0V
S
0V
TP
-
-
AND GATE
-
-
-
0v
0v
TH IS IS A
CHIP BUTTY
RANGE
+5V DC TO +8V DC
E & L INSTRUMENTS Ltd
0v
0v
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
This system uses an AND gate board.
Instructions
• Draw a system diagram for the system above.
• Draw a block diagram of the system shown.
• Connect the sub-systems as shown.
• Make the power connection.
• Press switch A only.
• Press switch B only.
• Press both switches at the same time.
• Explain how the AND gate works.
AND gate logic
0 or 1
AND Gate
0 or 1
0 or 1
It can be worked out that there are four input combinations. These can be represented
in a truth table. Copy the symbol and table and complete the table.
A
B
Z
Symbol for AND gate
A
0
0
1
1
B
0
1
0
1
Z
Truth table
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
99
Practical task 8
+
S
0V
+
O/P
IND
S
0V
TP
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
0v
+V
O/P
IND
0V
-
+
S
0V
TP
+
+
S
0V
SB
0V
-
+
POS
SIG
-
TP
O/P
IND
SA
0V
-
NEG
+
+
S
0V
S
0V
+
+
+
S
0V
S
0V
S
0V
TP
-
-
OR GATE
-
-
-
0v
0v
TH IS IS A
CHIP BUTTY
RANGE
+5V DC TO +8V DC
E & L INSTRUMENTS Ltd
0v
0v
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
This system uses an OR gate board.
Instructions
• Draw a system diagram for the system above.
• Draw a block diagram of the system shown.
• Connect the sub-systems as shown.
• Make the power connection.
• Press switch A only.
• Press switch B only.
• Press both switches at the same time.
• Explain how the OR gate works.
OR gate logic
It can be seen that there are four input combinations. These can be represented in a
truth table. Copy the symbol and table and complete the table.
A
Z
B
Symbol for OR gate
A
0
0
1
1
B
0
1
0
1
Z
Truth table
100
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Truth tables
Electronics is concerned with the processing of electrical signals.
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 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.
A
B
Z
Symbol for OR Gate
A
0
0
1
1
B
0
1
0
1
Z
0
1
1
1
Truth table
A and B are the inputs and Z is the output.
Results displayed in this way are known as truth tables.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
101
Problem 3
A washing machine manufacturer decides that to improve efficiency the washing
machine should not start until two conditions have been met.
Specification
The motor in the washing machine should only operate if:
• the water is at the correct temperature
• the water is at the correct level.
Solution
• Draw a system diagram to represent a suitable system.
• Draw a block diagram of a possible solution to the problem.
• Identify all sub-systems.
• Test your solution using modular boards.
• Explain how your system works.
Problem 4
Automatic doors should open from both inside and out. Design a simulation to solve
this problem.
Specification
For this simulation a combination of light sensors and switches can be used.
•
•
•
A light sensor should be used to open the doors from the other side.
A switch should be used to open the doors from the inside.
A solenoid unit should be used to simulate the door mechanism.
Solution
• Draw a system diagram to represent a suitable system.
• Draw a block diagram of a possible solution to the problem.
• Identify all sub-systems.
• Test your solution using modular boards.
• Explain how your system works.
102
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 9
+V
INC
FREQ
ACTIVE LOW
TP
+
+
+
S
0V
S
0V
+
+
+
S
0V
S
0V
S
0V
TP
POS
SIG
S
PULSE
0V
GEN.
-
0V
-
-
NEG
-
-
-
0v
+
0V
RANGE
+5V DC TO +8V DC
0v
+
EXT.C
PULSE
IND
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
This system uses a pulse generator input transducer.
Instructions
• Draw a system diagram for the system above.
• Draw a block diagram of the system shown.
• Connect the sub-systems as shown.
• Make the power connection.
• Turn the potentiometer dial on the pulse generator fully clockwise. Record the
output from the system.
• Turn the potentiometer dial on the pulse generator fully anticlockwise. Record the
output from the system.
• Explain in your own words what the pulse generator does.
• Is the pulse generator:
− an input transducer?
− an output transducer?
− a signal-processing device?
Practical task 10
Push
Switch
Power
Connec tion
Pulse
Generator
And Gate
Transducer
driver
Bulb
Unit
Instructions
• Draw the block diagram shown above.
• Press and hold the push switch and write down what happens.
• Slow down the frequency (rate of flashing) using the potentiometer on the pulse
generator. Observe the LEDs throughout the system.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
103
Practical task 11
INPUT
IND
OUTPUT
IND
+
TP
S
0V
+
S
0V
-
LATCH
-
TC4011BP
864BHB
EXT
RST
E & L INSTRUMENTS Ltd
This task uses a Latch unit.
+V
O/P
IND
INPUT
IND
HIGH = ON
LOW = OFF
OUTPUT
IND
ACTIVE LOW
+
POS
SIG
S
0V
0V
-
+
+
S
0V
TP
O/P
IND
TP
S
0V
-
-
NEG
+
+
S
0V
S
0V
LATCH
TP
-
-
TC4011BP
864BHB
+
+
+
+
S
0V
S
0V
S
0V
TP
S
0V
LATCH
+
-
TC4011BP
864BHB
S
0V
-
-
-
-
0v
0v
RANGE
+5V DC TO +8V DC
0v
ACTIVE
HIGH
EXT
RST
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
Instructions
• Draw a system diagram for the system above.
• Draw a block diagram of the system shown.
• Connect the sub-systems as shown.
• Make the power connection.
• Press the switch unit and record what happens.
• Now press the switch on the latch unit and record what happens in the system.
A latched system is one that remains on until it is reset. It is often called a memory
system as it ‘remembers’ that the switch has been pressed until the reset on the latch is
pressed.
Note: the inverter is needed because the latch unit only works as the voltage signal is
dropping – this is called negative-edge triggering. The inverter allows the system to
work when the voltage signal is rising, as happens with a normal switch unit. This
converts the overall system to positive-edge triggering.
104
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Problem 5
Your Technological Studies room has an infrared burglar alarm system fitted. The
trouble is that it sounds each time someone walks into the room. Redesign the system
so that it works in a more acceptable fashion.
Specification
• For the purpose of this simulation the alarm should sound if a light beam is
broken.
• There should be a master switch in the janitor’s office.
• The alarm should not sound when the system is not ‘set’.
• Once set, the alarm should sound if the light beam is broken.
• The alarm should continue to sound even if the burglar leaves the room.
• The janitor should be able to reset the system when the police arrive.
Solution
• Draw a systems diagram to represent a suitable system.
• Develop a block diagram of a possible solution to the problem.
• Test your solution using modular boards.
• The system is positive-edge triggered. Explain what this means.
• Evaluate your system.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
105
Practical task 12
+
+
S
0V
S
0V
-
-
E & L INSTRUMENTS Ltd
This task uses a relay unit.
+V
O/P
IND
+
+
+
POS
SIG
S
0V
S
0V
S
0V
0V
-
+
+
+
S
0V
S
0V
S
0V
TP
TP
-
-
NEG
-
-
-
0v
0v
0v
RANGE
+5V DC TO +8V DC
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
Instructions
• Draw a system diagram for the above system.
• Draw a block diagram of the system.
• Connect the sub-systems as shown.
• Make the power connection.
• Press the switch unit and record what happens at the output.
The ‘clicking’ sound at the output is caused by the contacts of a switch inside the
relay closing. The contacts are moved by the electromagnet energising.
SOFT IRON
ARMATURE
PIVOT
SOFT IRON
CORE
COIL
SPRING
TERMINALS
CONTACT
TERMINALS
COIL
TERMINALS
A point that must be understood is that when the relay contacts close, it provides no
power to the external circuit. To drive anything from a relay, a separate power supply
must be provided.
106
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 13
+
6V
-
+V
O/P
IND
+
+
+
S
0V
S
0V
S
0V
+
+
+
S
0V
S
0V
S
0V
TP
POS
SIG
-
0V
TP
-
-
NEG
-
-
-
0v
0v
0v
RANGE
+5V DC TO +8V DC
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
Instructions
• Draw a block diagram of the system.
• Draw a systems diagram for the E&L system.
• Draw a systems diagram for the motor.
• Draw a diagram to show how both sub-systems are linked.
• Connect the sub-systems as shown.
• Make the motor connections to the power supply and relay.
•
•
Make the power connection to the E&L system.
Press the switch unit and record what happens at the output.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
107
Problem 6
A car park barrier system has to be designed to operate as follows.
•
•
The barrier should open when a car breaks a light beam.
The barrier has to be powered by a more powerful motor operated by a relay.
Solution
• Draw a systems diagram to represent a suitable system.
• Develop a block diagram of a possible solution to the problem.
• Test your solution using modular boards.
• Evaluate your system.
Extension
Add a switch unit to represent a ticket machine: the barrier must not go up until a
ticket is taken and the light beam is broken.
108
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 14
+
O/P
IND
+
S
0V
TP
-
+
S
0V
-
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
-
S
0V
+
S
0V
TP
0v
0v
E & L INSTRUMENTS Ltd
+V
-
-
NEG
TP
+
+
S
0V
-
-
AND GATE
E & L INSTRUMENTS Ltd
TP
+
+
S
0V
S
0V
-
-
+
+
S
0V
S
0V
0v
E & L INSTRUMENTS Ltd
-
-
-
0v
THIS IS A
CHIP BUTTY
0v
E & L INSTRUMENTS Ltd
+
S
0V
TP
OR GATE
THIS IS A
CHIP BUTTY
0v
RAN GE
+5V D C TO +8V DC
O/P
IND
SA
0V
+
O/P
IND
SA
0V
SB
0V
-
+
-
0V
+
S
0V
+
S
0V
SB
0V
POS
SIG
-
TP
+
0v
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
Instructions
• Draw a systems diagram.
• Draw a block diagram of the system.
• Connect the sub-systems as shown.
• Make the power connection.
• Copy and complete the truth table to record the operation of the system. Remember that logic 1 means on and logic 0 means off.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
109
Truth table for practical task 14
Magnetic switch
0
0
0
0
1
1
1
1
110
Light sensor
0
0
1
1
0
0
1
1
Push switch
0
1
0
1
0
1
0
1
Bulb
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 15
O
U
T
+
0V
I
N
+
+
TP
TP
S
0V
S
0V
-
-
0v
E & L INSTRUMENTS Ltd
+V
+
O/P
IND
+
+
+
POS
SIG
S
0V
S
0V
S
0V
0V
-
+
+
S
0V
S
0V
TP
TP
-
-
I
N
0V
+
TP
-
NEG
O
U
T
TP
S
0V
-
-
0v
0v
0v
RANGE
+5V DC TO +8V DC
0v
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
+
O
U
T
I
N
TP
S
0V
0V
-
E & L INSTRUMENTS Ltd
+V
0V
+
POS
SIG
E & L INSTRUMENTS Ltd
+
+
S
0V
S
0V
+
+
+
S
0V
S
0V
S
0V
TP
TP
-
-
NEG
-
-
-
0v
0v
RANGE
+5V DC TO +8V DC
E & L INSTRUMENTS Ltd
0v
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
This task uses an input/output unit. This unit is used to connect other external
components and devices into the modular board systems.
Instructions
• Draw a systems diagram.
• Draw a block diagram of the system.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
111
•
•
•
•
Connect the sub-systems as shown.
Connect a normally open switch to the positive and ‘IN’ terminals on the I/O unit.
Make the power connection.
Test the system and describe what happens.
Practical task 16
Instructions
• Draw a systems diagram.
• Draw a block diagram of the system.
• Connect the sub-systems as shown.
• Connect a bulb to the positive and ‘OUT’ terminals on the I/O unit.
• Make the power connection.
• Test the system and describe what happens.
• Describe the advantage of using an I/O unit in electronic systems.
Practical task 17
Two outputs can be obtained using the I/O unit. To do this you must bridge both test
points (T.P.) on the I/O unit with a piece of single-core wire.
When the system sends a logic 1 signal to the I/O unit it gives out a logic 1 signal
through the positive and OUT connections and a logic 1 signal to the next board in
line.
Build up the system below to test this out.
+V
+
O/P
IND
+
+
+
+
+
S
0V
S
0V
TP
POS
SIG
S
0V
0V
-
S
0V
TP
S
0V
-
-
NEG
-
I
N
0V
+
TP
-
O
U
T
+
+
S
0V
S
0V
TP
S
0V
-
-
-
0v
0v
0v
RANGE
+5V DC TO +8V DC
E & L INSTRUMENTS Ltd
112
0v
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
Practical task 18
INC.
DELAY
+
TP
SA
0V
+
S
0V
-
-
DELAY
+
NEC556
0862
B
2
0v
+
E & L INSTRUMENTS Ltd
This task uses a delay unit. The delay unit reacts to an input signal by waiting a few seconds before it sends out a high (logic 1) signal to the next
sub-system.
+V
O/P
IND
INC.
DELAY
HIGH = ON
LOW = OFF
ACTIVE LOW
POS
SIG
+
+
+
S
0V
S
0V
S
0V
-
0V
O/P
IND
TP
-
-
NEG
TP
LATCH
+
+
S
0V
SA
0V
-
TP
-
+
+
S
0V
S
0V
-
DELAY
-
TC4011BP
864BHB
E & L INSTRUMENTS Ltd
+
S
0V
S
0V
-
-
-
+
NEC556B
08622
0v
ACTIVE
HIGH
+
0v
0v
RANGE
+5V DC TO +8V DC
+
S
0V
TP
0v
+
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
113
Instructions
• Draw a systems diagram.
• Draw a block diagram of the system.
• Connect the sub-systems as shown.
• Make the power connection.
• Adjust the potentiometer dial to the mid-position.
• Press the switch and hold. Note what happens.
• Try the same with the dial at different positions.
• Record the actual time delay for each position.
Position
Fully clockwise
Mid-position
Full anti-clockwise
Delay
Extended task
How could you adapt the system so that once the signal gets through to the buzzer, the buzzer stays on even after you let go of the switch unit?
Alter your system so that it operates in this way.
Note: the inverter is needed because the delay unit only works when the voltage signal is dropping (negative-edge triggered). The inverter allows
the system to work when the voltage signal is rising, as happens with a normal switch unit.
114
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
The comparator
The comparator board receives an input voltage from a sensor and compares it with
another voltage called the reference voltage.
•
•
If the input voltage is smaller than the reference voltage the comparator gives out
a ‘low’ signal (logic 0).
If the input voltage is greater than the reference voltage the comparator gives out a
‘high’ signal (logic 1).
The comparator may be used as an error detector in closed loop systems, where it
compares a feedback voltage to a reference voltage.
COMPARATOR
REQUIRED
TEMP
OUTPUT
DRIVE
OUYPUT
MOTOR
TEMP
SENSOR
FEEDBACK LOOP
The output signal from a comparator is digital, that is, it is logic 0 or logic 1. The
feedback signal that is fed into the comparator is usually analogue. Therefore the
comparator can be thought of as an analogue-to-digital converter.
VREF
INC
REF VOLTS
+
+
S
0V
S
0V
-
REF
TP
MAGIC
THINGY
0V
WITH
REF VOLTS
E & L INSTRUMENTS Ltd
The potentiometer on the comparator board sets the reference voltage.
V REF
+V
+
TP
POS
SIG
S
0V
0V
-
NEG
INC
REF VOLTS
+V
+
+
+
+
+
S
0V
S
0V
S
0V
S
0V
+
+
S
0V
S
0V
-
REF
-
-
I
N
0V
+
TP
-
O
U
T
TP
TP
-
-
S
0V
-
TP
0V
0v
0V
0v
RANGE
+5V DC TO +8V DC
0v
MAGIC
THINGY
0V
WITH
REF VOLTS
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
The temperature sensor sets the input voltage signal (feedback) to the comparator.
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
115
Practical task 19
Construct the comparator system shown below.
VREF
INC
REF VOLTS
+V
+
+
POS
SIG
S
0V
0V
-
1
0
K
TP
+
+
+
+
S
0V
S
0V
S
0V
+
+
+
S
0V
S
0V
TP
S
0V
-
-
-
REF
NEG
-
S
0V
-
-
-
TP
0v
0v
RANGE
+5V DC TO +8V DC
MAGIC
THINGY
0V
VOLTS
INC.
WITH
REF VOLTS
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
Instructions
• Set the input voltage unit to its maximum setting.
• Set the reference voltage to 2 volts (measure between the 0 volts and VREF
terminals on the comparator board with a multimeter).
• Set the multimeter probes to 0 volts and the T.P. terminals on the comparator.
• Gradually increase the input voltage until the bulb lights up.
• Note the input voltage reading the moment the bulb lights.
This task should prove the operation of the comparator as described earlier. The
comparator takes the analogue signal and gives a digital output.
COMPARATOR
116
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
NAND and NOR gate boards
There are two modular logic boards that are very useful in building systems. These are
the NAND gate and the NOR gate.
+
SB
0V
-
+
TP
O/P
IND
SA
0V
-
Inverter
+
S
0V
NAND GATE
And Gate
THIS IS A
CHIP BUTTY
0v
E & L INSTRUMENTS Ltd
NAND gate
This board is like a combination of AND gate and inverter (NOT gate) boards. This
may simplify some problems or give alternative solutions.
NOR gate
This board is like a combination of OR gate and inverter (NOT gate) boards. Again,
this may simplify some problems or give alternative solutions.
O/P
IND
SA
0V
-
Inverter
+
SB
0V
-
+
TP
+
S
0V
NOR GATE
Or Gate
THIS IS A
CHIP BUTTY
0v
NOR
E & L INSTRUMENTS Ltd
Additional problems
1. Design an electronic system that sounds a warning buzzer if somebody opens a
drawer and lets light in.
2. Design a system that will turn an electric pump on when the water in a storage
tank reaches a maximum level. The pump would be of a higher voltage than that
available in the control part of the system.
3. Design a system for a gardener that will automatically cool down a greenhouse if
the temperature gets too high.
4. Design an electronic system that tests people’s reactions by showing how quickly
they can press a button after they have seen a light flash on.
5. Design a control system for an electric kettle. (Use a bulb to stand in for the
heating element in the kettle.)
Standard Grade Technological Studies: Applied Electronics – Modular Electronic Systems
117
Specification: The kettle would be switched on with a push switch, should turn off
when the water is at the correct temperature and should not work if there is no
water in it.
6. Design a system that operates an automatic hand dryer, as found in many public
places.
Specification: The dryer should turn on automatically when the hands are put
under it and should switch off a few seconds after the hands have been removed.
7. Design part of the control system of a drinks machine.
Specification: When a coin is put into the machine, a cup is filled with liquid.
There must be a delay to make sure that the cup is in place before the valve opens
to allow the liquid to flow. The system must automatically close the valve when
the cup is full.
8. A builder has decided to include an automatic door for the garages in a housing
estate. The garage doors must be automatically opened without the driver leaving
the car. When the door is fully open, the drive motor (for the garage door) should
switch off. The weight of the garage door requires that the motor must be driven
by the mains supply. Design a system to satisfy this specification.
9. A chemical plant requires a storage area to be kept cool and dry for safety reasons.
If either of the conditions is not met, then a visible alarm should be activated to
warn personnel of a possible dangerous situation. It would be best if the alarm was
intermittent.
10. A local building society has asked for a new vault locking system to be installed.
It can only be opened when one of the two assistants and the manageress input
their security cards through the card readers of the security system.
11. The automatic flash in a camera will only operate when the following conditions
have been met.
• The ambient light conditions fall below a preset level.
• The lens cover is open.
• The shutter release button has been pressed.
12. In winter, it is important for a car driver to know if there is ice on the road ahead.
Design an ice alert system that will flash on a warning light when the temperature
is at or below freezing. For simulation purposes, room temperature can be
considered freezing point.
13. A model of a car windscreen wiper system is required. The system must give the
driver a variable-delay single wipe for occasions when the rain is not too heavy. A
motor unit can be used to simulate the wiper motor.
14. A railway crossing barrier and flashing light system has to be designed. It should
operate as follows.
• When the train is two miles from the crossing, the lights should start flashing.
• When the train is one mile from the crossing, the barrier should come down.
• Once the train has passed, the lights should stop flashing and the barrier
should go up.
Design a simulation of this system using modular circuit boards.
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Logic in Electronics
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
119
Contents
Switching logic
Making decisions
Logic gates
Integrated circuits
The NAND logic gates
The NOR logic gate
Boolean expressions
Logic in simple component circuits
NAND gate technology
Binary numbers
The decimal system
The binary system
Combinational logic
Truth tables for combinational logic systems
Dealing with NAND and NOR gates in combinational logic
Creating logic diagrams from truth tables
Creating logic systems from written specifications
Logic gate integrated circuits (ICs)
Logic ICs in prototype board circuits
Pin-out diagrams
Pin-out diagrams for common TTL logic Ics
Practical tasks
120
121
121
121
122
123
124
125
126
128
130
130
130
132
133
137
139
141
145
146
147
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Switching Logic
Making decisions
Although it may not always seem like it, electronics and electronic systems are very
logical in the way that they work. In the simplest form, if you want a light to come on,
then you press a switch. Of course, it gets more complicated than that. Most
technological systems involve making more complicated decisions: for example,
sorting out bottles into different sizes, deciding whether a room has a burglar in it or
not, or knowing when to turn a central heating system on or off.
Logic gates
Logic gates are very useful in dealing with and processing a combination of different
inputs. This switching logic can be applied to electrical switches and sensors,
pneumatic valves or hydraulic systems. Switching logic uses logic gates to perform
decisions. In previous work you have already seen NOT, AND and OR logic gates.
A
Z
NOT
A
B
Z
AND
A
B
Z
OR
It is worth remembering that logic gates are part of digital systems and, as such,
respond to either logic 1 or logic 0 signals only.
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
121
Integrated circuits
Although logic gates have electronic symbols, they are not discrete components: they
are contained in integrated circuits. A typical example is the TTL7400 IC shown
below. (The LS part gives more information about the chip for users.)
Integrated circuits (ICs) are silicon-based components containing complex circuits.
The simplest 14-pin IC that you will deal with is the TTL 7400 shown above. TTL
stands for transistor−transistor logic. The 7400 chip effectively contains four NAND
logic gates. Each NAND gate has the following transistor circuit.
+V
A
Z
B
From this circuit diagram, it is
easy to see why the term
transistor − transistor logic is
used.
0V
NAND logic gates are a combination of NOT and AND logic gates − see the
following page.
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The NAND logic gate
The NAND gate is effectively an inverted AND gate. In other words, the results
obtained from the output are the opposite to those of the AND gate. This gate is often
referred to as ‘NOT AND’.
When drawing up the truth table for the NAND gate it can be difficult to ‘picture’ or
imagine the results. The best way to do this is to pretend that it is an AND gate and
then invert (reverse) the results, thus giving you the outputs for the NAND gate.
A
Z
B
A
0
0
1
1
B
0
1
0
1
AND
Z
Symbol for NAND
Truth table
Practical task
Copy out the NAND gate symbol, the truth table and the block diagram.
Switc h
Unit
Power
Connec tion
Switc h
Unit
Nand Gate
Transduc er
driver
Bulb
Unit
Set up the E&L modular board electronic system and complete the table using the
system to confirm the outputs at Z.
Note: This task can be completed using a circuit simulation software package such as
Crocodile Clips.
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123
The NOR logic gate
The NOR gate is effectively an inverted OR gate. In other words, the results obtained
from the output are the opposite to that of the OR gate. This gate is often referred to as
‘NOT OR’.
As with the NAND gate, when drawing up the truth table for the NOR gate it can be
difficult to ‘picture’ or imagine the results. The best way to do this is to pretend that it
is an OR gate and then invert (reverse) the results, thus giving you the outputs for the
NOR gate.
A
Z
B
A
0
0
1
1
B
0
1
0
1
OR
Z
Symbol for NOR gate
Truth table
Practical task
Copy out the NOR gate symbol, the truth table and the block diagram.
Switch
Unit
Power
Connection
Switc h
Unit
Nor Gate
Transducer
driver
Bulb
Unit
Set up the E&L modular board electronic system and complete the table using the
system to confirm the outputs at Z.
Note: this task can be completed using a circuit simulation software package such as
Crocodile Clips.
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Boolean expressions
Each logic gate has a corresponding Boolean mathematical formula or expression.
The use of these expressions saves us having to draw symbol diagrams over and over
again.
The name Boolean is taken from an English mathematician, George Boole, who
founded symbolic logic in the nineteenth century.
Z=A
NOT
Z = A.B
AND
Z = A+B
OR
Z = A.B
NAND
Z = A+B
NOR
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Logic in simple component circuits
Boolean logic can also be seen in simple component circuits as well as in pneumatic,
hydraulic and other systems.
The circuits below show the five main types of logic.
NOT
AND
OR
NAND
NOR
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Logic gate exercises
For each of the following examples, state whether the output Z is at logic 0 or logic 1.
(b)
(a)
1
1
Z
Z
1
0
(c)
(d)
1
1
Z
Z
1
(f)
(e)
1
1
Z
0
Z
1
(g)
1
Z
1
(h)
1
Z
0
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NAND gate technology
NAND gate technology can be used to build other logic gates using NAND gates only.
(The same thing can be achieved using NOR gates, but NAND gate chips are more
common.)
NOT
AND
OR
NOR
XOR
Many manufacturers use only one type of gate (normally NAND) in the manufacture
of their products. This has several advantages.
•
•
•
128
You only have to stock one type of chip instead of a large range.
People only have to be familiar with the characteristics of this one chip.
Very often significant simplification of complex circuits is possible, thus reducing
the number of chips required.
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
NAND gate problems
Redraw the following logic systems replacing the logic gates with combinations of
NAND logic gates. Use the equivalents shown on the previous page.
C
A
Z
B
A
D
B
E
Z
C
A
D
B
A
Z
E
C
D
E
B
Z
C
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Binary numbers
The number system that we use is the decimal system; that is, we use a scale of ten.
This decimal system uses 10 different digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. You should
have noticed that truth tables use only two digits, 0 and 1. This is called a binary
system and it uses combinations of the digits 0 and 1 to form binary numbers.
The decimal system
The table below shows the decimal system of counting. The values of the columns (of
the left-hand table) working from right to left are 1, 10, 100 and 1000. These can be
written as powers of ten: 100, 101, 102, and 103. As indicated above, this type of
counting is called decimal and uses a base of 10.
Decimal
Binary
103
102
101
100
24
23
22
21
20
1000
100
10
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
16
8
4
2
1
1
1
1
0
0
0
0
1
1
1
1
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
The binary system
As stated above, the binary system uses only two digits, 1 and 0. This system is suited
to digital systems in electronics, where 1 represents ON or HIGH and 0 represents
OFF or LOW.
The right-hand table shows the binary equivalents of the decimal numbers in the table
on the left. The values of the columns working from right to left are 1, 2, 4, 8, and 16.
These can be written in powers of 2: 20, 21, 22, 23 and 24.
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Examples
The number 7 in decimal is 111 in binary: (one 4) + (one 2) + (one 1) = 7.
The number 13 in decimal is 1101 in binary: (one 8) + (one 4) + (no 2) + (one 1) = 13.
Binary number problems
1. Change the following decimal numbers to binary numbers.
Decimal
(a)
23
(b)
41
(c)
39
(d)
57
(e)
50
(f)
67
(g)
74
(h)
93
(i)
114
(j)
85
Binary
2. Change the following binary numbers to decimal.
Binary
(a)
11
(b)
1001
(c)
1100
(d)
1101
(e)
101
(f)
110
(g)
1111
(h)
10000
(i)
10001
(j)
11011
Decimal
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131
Combinational logic
So far in this unit of work we have only looked at simple logic systems on their own.
In reality, most logic systems use a combination of different types of logic gates in
one system. This type of logic control is known as combinational logic.
For example, if you study the simple block diagram of the electronic system below,
you will notice it has an AND gate, an OR gate and an inverter (NOT gate) in it.
Light
Sensor
Inverter
Power
Connection
Push
Switch
OR
Gate
Pressure
Pad
AND
Ga te
Transducer
Driver
Bulb
Unit
We can draw a logic diagram of this system, as shown below. There is more than one
logic gate in this diagram and so it is known as a combinational logic diagram.
LIGHT
SENSOR
PRESSURE
PAD
BUZZER
SWITCH
Questions
1. What is this system designed for?
2. What is the purpose of the AND gate?
3. Why is the inverter (NOT gate) included?
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Truth tables for combinational logic systems
Drawing up a truth table for a system with more than one logic gate is not too
difficult. As long as you know how each of the basic gates work, you can treat each
gate on its own and then work your way through the system.
Before going ahead to look at the outputs in the truth tables, it is worth reminding
ourselves of the number of combinations of inputs possible per number of actual
inputs.
One input
If there is only one input (A), then there are only two combinations (logic 0 or logic
1). So the incomplete truth table would be drawn up as below (ignoring the results in
the output column, Z).
A Z
0
1
Two inputs
If there are two inputs (A and B) they can be arranged in four different combinations:
• A and B both off
• A off and B on
• A on and B off
• A and B both on.
You cannot create any other combinations. The truth table would therefore be drawn
up as below (ignoring the results in the output column, Z).
A
0
0
1
1
B
0
1
0
1
Z
0
1
1
1
You should notice that the input columns are arranged in binary number order.
Three inputs
If there are three inputs (A, B and C) they can be arranged in eight different
combinations. The truth table for a 3-input system is shown below.
A
0
0
0
0
1
1
1
1
132
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
Z
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Summary
The pattern in the truth tables above is clear. Starting with one input giving two
combinations, you simply double the number of combinations each time an input is
added.
• 1 input: 2 combinations
• 2 inputs: 4 combinations
• 3 inputs: 8 combinations
• 4 inputs: 16 combinations
and so on.
You will never be asked to work with a system that has more than three inputs.
Worked example
The example below shows a logic diagram that has two logic gates. There are three
inputs, so this gives eight combinations in the truth table.
A
D
B
Z
C
Stage 1
Draw up the results for point D.(This is the output from the AND gate, being fed by
inputs A and B only.)
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
D
0
0
0
0
0
0
1
1
Z
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
133
Stage 2
Draw up the results for point Z. (This is the output from the OR gate, being fed by
output D and input C only.)
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
D
0
0
0
0
0
0
1
1
Z
0
1
0
1
0
1
1
1
By following this technique, logic system problems can be solved easily.
You could use a circuit simulation program to check your results.
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Exercises
Draw up a truth table for each of the following logic systems.
C
A
Z
B
A
D
B
E
Z
C
A
D
B
A
Z
E
C
D
E
B
Z
C
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135
Dealing with NAND and NOR gates in combinational logic
It can be confusing to have to remember the basic truth tables for NAND and NOR
gates. However, as it is easy to remember AND and OR truth tables, we can use this
to help. As we know the NAND gate is an inverted AND gate, we simply reverse
(invert) the answers in the AND gate truth table to get the results for a NAND gate.
We can use the same technique for NOR and OR.
Worked example
If we try and draw up the truth table for the system shown below, we must add some
extra columns for the ‘pretend’ results.
A
D
B
Z
C
A and B feed the NAND gate, but we treat it as an AND (see the extra column of the
truth table). Then, to obtain the results for D, we simply invert the results obtained in
our ‘pretend’ column. Now, C and D feed the next gate, which is a NOR. We
‘pretend’ it is an OR gate (see the extra column in the truth table) and then invert the
answers to obtain column Z.
Column for
Z as an
OR gate
Column for
D as an
AND gate
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
AND
0
0
0
0
0
0
1
1
D
1
1
1
1
1
1
0
0
OR
1
1
1
1
1
1
0
1
Z
0
0
0
0
0
0
1
0
You can check you results using circuit simulation software.
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Exercises
Draw up a truth table for each of the following logic systems.
A
B
Z
C
A
B
Z
C
A
B
Z
C
A
B
Z
C
A
B
Z
A
B
A
Z
B
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137
Creating logic diagrams from truth tables
When designing systems, it is normal to design a logic diagram from a prepared truth
table. This may seem difficult to start with, but if you concentrate on the combinations
which give a logic 1 condition in the output column, solutions can be found easily.
The truth table below shows two inputs, A and B, and one output, Z.
A
0
0
1
1
B
0
1
0
1
Z
0
0
1
0
Z = A. B
The output Z is at logic 1 in the third row down, and we can see that for this to happen
A must be at logic 1 and B must be at logic 0. In other words
Z = A AND NOT B
This means that we need a two-input AND gate, with B being fed through a NOT
gate. We can write the statement in shorthand Boolean as
Z = A. B
This means that the logic diagram is as shown below.
A
B
138
B
.
A B
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Worked example
In this problem we have three inputs, A, B and C, with one output, Z. From the truth
table we can see that there are two occasions when the output goes to logic 1.
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
Z
0
0
0
1
0
0
1
0
Z = A. B. C
Z = A. B. C
In other words, Z = 1 if (A is at logic 1 AND B is at logic 1 AND C is at logic 1) OR
if (A is at logic 1 AND B is at logic 1 AND C is at logic 0).
This means we need a two-input OR gate being fed from two three-input AND gates
as shown below.
A
A
..
A B C
B
..
..
(A B C) + (A B C)
C
..
A B C
C
The shorthand Boolean equation for this truth table is
Z = ( A . B . C) + ( A . B . C)
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
139
Exercise
Draw the logic diagrams for each of the following truth tables.
A
0
0
1
1
B
0
1
0
1
Z
0
1
0
0
A
0
0
1
1
B
0
1
0
1
(a)
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
Z
1
0
1
0
A
0
0
1
1
B
0
1
0
1
(c)
(b)
Z
0
0
0
0
0
1
0
1
(d)
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
Z
0
1
1
0
Z
0
1
0
0
0
1
0
0
(e)
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
Z
1
0
0
1
0
0
0
1
(f)
Creating logic systems from written specifications
Perhaps the most common application of switching logic is creating a logic system to
meet a given specification. Normally, by reading the specification carefully, the
system designer can almost ‘see’ the required logic system.
Worked example
A burglar alarm system is to sound if a master switch is on and either a light beam is
broken or a pressure pad is stood on.
Draw a logic diagram and a truth table for this system.
Read the specification carefully. You should notice that it has three inputs. These are:
• a master switch (M)
• a light sensor (L), and
• a pressure pad (P).
It has one output, an alarm bell (B).
The bell should go to logic 1 if the master switch is at 1 and either the light beam goes
to logic 0 or the pressure pad goes to logic 1. This can be written in Boolean as:
B = M . (L + P )
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Note: The alarm has to be triggered when the light beam is broken and so a NOT gate
is needed.
In other words, you need a two-input AND gate that is fed directly from M and also
from a two-input OR gate that is fed from L (through an inverter) and P. The logic
diagram is shown below.
M
B
P
L
L
The truth table for this system is shown below. Again, all you have to do is read the
specification carefully and then read across each row, one at a time, and decide
whether the bell should be ringing or not. There are some short cuts. For example, in
the first four rows the master switch is off; therefore the bell must be at logic 0 – even
if there is a burglar in the house.
M
0
0
0
0
1
1
1
1
L
0
0
1
1
0
0
1
1
P
0
1
0
1
0
1
0
1
B
0
0
0
0
1
1
0
1
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
141
Exercises
1. A house doorbell is to ring if a push button at the front door, a push button at the
back door or both buttons are operated. Draw a logic diagram and write a Boolean
equation.
2. A lift motor is to start only when, by closing, the door has actuated a switch and a
passenger has pressed a button. Prepare a truth table, a logic diagram and a
Boolean equation for this system.
3. The driver of a dustcart is to be able to operate the loading claw by pressing a
button, but only when the senior loader at the rear of the cart has pressed a button
to give the ‘all clear’. Draw a logic diagram and write a Boolean equation for this
system.
4. An automatic central heating system is to heat the radiators (R) if the mains switch
(M) is on, the timing control switch (T) is closed and the override button (O) is not
selected. Draw a logic diagram, truth table and Boolean statement for this system.
5. A drill is to operate if an isolator is closed, a guard is in place (closing a
microswitch), either ‘HI’ or ‘LOW’ speed is selected and a foot pedal is operated.
Draw a suitable logic diagram for this system. Draw up a truth table.
6. A large hall has three temperature sensors. A logic system is to operate the
radiator when any two of the temperature sensors fall below a preset level. Draw
up a truth table for this system and draw a logic diagram.
7. A burglar alarm will operate if the mains switch is on and either an electronic
beam is broken, a pressure pad is stood on or a window is opened. Draw a logic
diagram for this system.
8. At the start of a boxing match, a bell is to ring, provided:
• boxers A and B are present
• the referee and the time-keeper are present.
If either or both boxers fail to appear, the match is to start with the next pair of boxers,
C and D. Draw a logic diagram for this system.
9. A switching system for corridor lighting is shown below.
1
1
0
0
L
(a) Draw a truth table for this system.
(b) Write a Boolean equation for this system.
(c) Draw a logic diagram of an electronic system that could be used to achieve the
same control of the light.
10. A vending machine is to be controlled by the logic arrangement shown below.
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
BUTTON
DELIVERY
BLACK COFFEE
COFFEE
WHITE COFFEE
MILK
SWEET BLACK
COFFEE
SWEET WHITE
COFFEE
TEA
SUGAR
TEA
SWEET TEA
Explain what happens when a button is pressed. Do this for at least two selections.
Explain what will happen if two buttons are pressed at the same time.
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
143
Logic gate integrated circuits (ICs)
Integrated circuits consist of plastic cases filled with electronic circuitry. There are
many resistors, transistors and other components packed into the chips. There are
literally thousands of ICs on the market, all designed to do different jobs – logic gates,
amplifiers, timers, etc.
In this work we will be using the TTL (transistor−transistor logic) range of chips.
TTL chips require a stable 5V supply to work properly. (Great difficulties will be met
if any other voltage is used.) Any unconnected pins automatically go to logic 1. In
other words, if a wire connected to a pin is connected to the 0-volt rail (logic 0), it will
go to logic 0. If the wire is disconnected from the 0-volt rail it will go to logic 1.
However, it is good practice to connect pins to ‘high’ or ‘low’ as needed.
All TTL chips have a four-digit code number, which always starts with 74. For
example, a 7400 is a quad two-input NAND chip.
Although the chip contains complex circuitry, the internal wiring can be shown as
simple logic circuits with the inputs and outputs of each logic gate shown. This is
called a pin-out diagram.
+Vcc
14
13
12
11
10
9
8
Pin-out diagram for
7408 quad
two-input AND gate
1
2
3
4
5
6
7
Gnd
(0V)
Pin 14 is connected to the 5-volt stable supply and pin 7 to 0 volts.
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Logic ICs in prototype board circuits
Transferring and converting circuit diagrams to prototype layout diagrams can be
confusing at first, but once you grasp the technique you should find it quite
straightforward and enjoyable.
Here are some points worth noting before you start building logic circuits.
1. Although in theory we should wire up and use real mechanical switches to provide
logic 0 and logic 1 inputs to the chip, we can simply use wires to do this. When
logic 0 is required at a pin, the connecting wire is simply connected to the 0-volt
rail. Similarly, when logic 1 is required, the connecting wire is pushed into the 5volt rail.
5 volts
7400
220R
LED
0 volts
2. To show the output condition of any logic system, an LED will be used. This will
glow when the output is at logic 1 and be off at logic 0. Remember that LEDs are
polarity conscious; that is, they must be connected with the negative terminal
towards the 0-volt rail. The ‘flat’ or short leg on an LED is the cathode or negative
side.
-ve (Cathode)
3. LEDs must be protected from excess current. A protective resistor must be placed
in series with the LED to do this. As the current and voltage are already relatively
low, a 220 R resistor will suffice.
4. Remember that when connecting one component to another you must use parallel
vertical columns on the breadboard. If you connect two components into the same
column the prototype board will short circuit them.
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Pin-out diagrams
ICs are impossible to use without the manufacturer’s data sheets to show what
facilities are available on the chip and how the pins are to be connected. These data
sheets contain pin-out diagrams. A pin-out diagram is a graphical layout of the chip
and its contents.
Note: all chips have either a notch or a small dot (or both) above pin number 1 so that
the user can identify all the pins without them being numbered. The dot is always at
pin 1.
Pin-out diagrams for common TTL logic ICs
The description of each pin-out diagram gives details of the chip. For example, a ‘dual
four-input NOR’ means the chip has two (dual) NOR gates on it, each having four
inputs. A ‘quad two-input AND’ means the chip has four AND gates, each gate
having two inputs.
+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)
7404
+Vcc
7
Gnd
(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
Gnd
(0V)
7421
+Vcc
8
7
Gnd
(0V)
7420
+Vcc
14
13
12
11
10
9
1
2
3
4
5
6
7427
146
8
8
14
13
12
11
10
9
7
1
2
3
4
5
6
Gnd
(0V)
7432
8
7
Gnd
(0V)
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Pin-out and wiring diagrams − example
The following logic circuit could be constructed using ICs.
INPUT A
OUTPUT
INPUT B
Since the gates within an IC are identical, any one of them can be used. An example
of possible connections is shown in the IC circuit diagram below.
+Vcc
14
+Vcc
13
12
11
10
9
14
8
13
12
2
3
4
10
9
5
6
8
7432
7408
1
11
5
6
7
1
2
3
Gnd
(0V)
4
OUTPUT
INPUT A
7
Gnd
(0V)
INPUT B
Prototype circuit layout/wiring diagram
The two ICs are mounted on a prototype circuit board as shown below. Connections
between pins are made by 0.6 mm solid-core wire.
5 volts
7408
7432
220R
LED
0 volts
Input A
Input B
The circuit would now be tested against the truth table to check its operation. Inputs A
and B can be made by connecting to the 5-volt or 0-volt rails.
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Note: in order for a logic circuit to work, it must be powered up; that is, the correct
power supply must be connected to pin 14 and pin 7 must be connected to 0 volts.
Worked examples
Select the required IC and draw an IC circuit diagram for each logic system below.
INPUT A
OUTPUT
INPUT B
A
Z
B
A
Z
B
A
Z
B
Extension work
Draw up a truth table for each example and using circuit simulation check your
results.
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Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Practical tasks
Worked example
Here is the design for a logic system that is to be tested on a prototype circuit.
•
Identify the required pin-out diagrams and number each input and output being
used on the chips. In this case the chips are a 7404 and a 7408.
A
12
B
•
•
•
1
7404
13
7408
11
2
Insert the chips on to the prototype board and make the connections to the +V and
0-volt rails.
Make the other connections and insert the LED and resistor.
Create input wires as required.
Input A
5 volts
220R
7404
7408
LED
0 volts
Input B
Task 1
• Build the circuit shown above.
• Draw up a truth table for the system.
• Work your way through each row of the truth table and draw up the results in
output column Z.
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Task 2
Build the following logic system and establish a truth table showing all possible
combinations of the inputs.
1
A
2
7400
3
4
5
6
7400
Z
1
B
2
C
7408
3
5 volts
7400
7408
220R
LED
0 volts
Input A
150
Input B
Input C
Standard Grade Technological Studies: Applied Electronics – Logic in Electronics
Task 3
Three different logic systems are shown below. Develop a truth table for each one,
then build the system on a prototype circuit board. Confirm your results predicted in
the truth table.
Note: do not just test the logic-1 conditions; make sure that you test the outputs given
when logic 0 is applied.
A
B
Z
C
A
B
Z
C
A
Z
B
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Task 4
Set up the following logic system on a prototype circuit board and draw up a truth
table. Work your way through the truth table conditions and observe the output.
4
A
7400
6
5
1
12
3
7400
7400
2
11
13
9
7400
B
10
8
Name the type of logic gate obtained by this system.
PULSE
RUBBISH
RUBBISH
HI
LO
Test equipment
When trying to establish logic levels within a complex system or to monitor a logic
output without using an LED, we use a digital logic probe.
The logic probe is powered from the same supply as the logic circuit being tested and
the needle point is pushed against the various pins on the IC to test their logic level.
Normally the logic probe gives out a high-pitched sound and a red LED lights if the
pin being tested is at logic 1. If the point tested is at logic 0, a low-pitched sound is
emitted and a green LED lights.
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