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Technological Studies
Electronic Systems
Electronic Systems
1 The advantages and limitations of electronic systems .................................. 2
2 Electronic system .............................................................................................. 3
(a) Input sub-system .............................................................................................................. 3
(i)
(ii)
(iii)
(iv)
Switches ............................................................................................................................................... 3
Light sensor .......................................................................................................................................... 4
Temperature sensor .............................................................................................................................. 4
Pulse generators ................................................................................................................................... 4
(b) Process sub-system .......................................................................................................... 5
(i) Transistor switch .................................................................................................................................. 5
(ii) Logic gate ............................................................................................................................................ 7
(iii) Binary adder ......................................................................................................................................... 9
(iv) Flip-flop ............................................................................................................................................. 11
(v) Amplifier ............................................................................................................................................ 11
(vi) Comparator ........................................................................................................................................ 12
(vii) Analogue to digital converter ............................................................................................................. 12
(viii) Integrated circuits............................................................................................................................... 14
(c) Output sub-system ......................................................................................................... 15
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
Light bulb ........................................................................................................................................... 15
Light emitting diode ........................................................................................................................... 15
Solenoid ............................................................................................................................................. 16
Buzzer ................................................................................................................................................ 16
Loudspeaker ....................................................................................................................................... 16
Motor ................................................................................................................................................. 17
Relay .................................................................................................................................................. 17
3 Electronic control ........................................................................................... 18
(a) General principle............................................................................................................ 18
(b) Electronic circuit symbols ............................................................................................. 19
(c) Electronic control circuit design .................................................................................... 19
(i) A practical example ............................................................................................................................ 19
(ii) Safety Measures ................................................................................................................................. 22
Interactive information……………………………………………………….23
Exercise………………………………………………………………………...24
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Electronic Systems
1 The advantages and limitations of electronic
systems
Electronic systems have many advantages, such as:
(a) Small in size and light in weight, e.g. mobile phones and electronic notebooks.
(b) Information can be transmitted to distant destinations easily and quickly using electric
wires, e.g. fax machines.
(c) Electronic signals can be converted into radio waves easily and transmitted to the
surroundings, e.g. radios.
(d) Fast operation and high accuracy, e.g. electronic calculators, electronic game machines,
hard disk drives, etc (Fig. 1b).
(e) Lower energy consumption allows batteries to be used for a longer time.
Fig. 1 (a) Fax machine
(b) hard disk drive
However, electronic systems have various limitations, such as:
(a) The transmission of electronic signals is subject to interference from electric fields and
magnetic fields.
(b) The system may not work properly under unfavourable conditions (for example, extremely
cold, hot or humid environment).
(c) The cost of development is very high. Much investment is needed for production lines to
go into operation.
(d) Electronic parts and circuit boards are relatively fragile and may be broken easily upon
impact.
(e) The efficiency of electronic systems is low unless they are used together with mechanical
systems.
(f) It is often difficult to repair electronic components and circuit boards. Therefore, they
must be replaced when they are broken.
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2 Electronic system
Electronic systems can be divided into the input sub-system, the process sub-system and the
output sub-system
(a) Input sub-system
Input sub-systems include various input components and electronic circuits, such as switches,
light sensors, temperature sensors, pulse generators, etc.
(i) Switches
The main function of switches is to enable or interrupt the flow of electric current. They are
used in electric circuits. Switches come in many forms, such as button, control rod, slide, rotary
and sensor (Fig. 2). When designing an electronic system, one should always look at the situation
and choose the most suitable switch. Fig. 3 shows the circuit symbols of some common switches.
(a) Button switch
(b) Slide switch
(c) Rotary switch
(d) Sensor switch
Fig. 2 Different kinds of switches
(a) General
(b) Button switch
Fig. 3 Circuit symbols of switches
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(ii) Light sensor
A light sensor is in fact a variable resistor. Its resistance varies with the intensity of the light it
receives (Fig. 4). The resistance will increase when the sensor is exposed to weak light, and
decrease when exposed to strong light. Therefore, a light sensor can convert differences in light
intensity (light signals) into differences in current and voltage (electric signals). Light sensors are
mainly used in automatic control, infrared detection, etc.
Fig. 4
(a) Light sensor
(b) Circuit symbol of a light sensor
(iii) Temperature sensor
The resistance of a temperature sensor varies with temperature. They are commonly used in
switching circuits. The resistance of a common temperature sensor increases under low temperature
and decreases under high temperature. Temperature sensors are responsible for temperature
detection in fire alarm systems, and body temperature measurement in electronic thermometers (Fig.
5).
Fig. 5 (a) Electronic thermometer (b) A thermistor
(c) Circuit symbol of a thermistor
(iv) Pulse generators
The term pulse is used to describe a sharp difference in electric voltage or current that takes
place within a very short time. If we express the change of pulse voltage (vertical axis) with time
(horizontal axis) in the form of waves, we can see the variation of the pulse voltage. Examples of
pulse waveforms include square waves, triangular waves, saw-tooth waves, drum waves, etc (Fig.
7).
Fig. 6 Pulse generator
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(a) Triangular waves
Electronic Systems
(b) Saw-tooth waves
(c) Drum waves
Fig. 7
Square wave is the most common form of pulse waves, because its shape can show clearly the
difference between high and low voltage, which allows the representation of the two digital values 1
and 0 (Fig. 8a). Therefore, square waves are commonly used as the input and output signals of digital
systems. Ideal square waves are supposed to have only either high voltage or low voltage and no
other voltages in between, but in reality such ideal square waves are very difficult to produce.
Usually square waves look like those in Fig. 8b.
(a) Ideal square waves
(b) Real square waves
Fig. 8
A pulse generator can be applied to a high voltage pulse circuit and used to change the strength
of pulses. Pulse generators are often used in camera flashlights, because they can supply high
voltages within a very short time.
(a) Process sub-system
The process system within an electronic system changes the input electronic signals according
to preset instructions. The processed electronic signals form the output of the system. There are
numerous components and systems that can process electronic signals, examples include transistor
switches, logic gates, binary adders, flip-flops, amplifiers, comparators, integrated circuits, etc.
(i) Transistor switch
A transistor is an electronic component that can be used to switch on/off electronic circuits, or
to amplify current or voltage. A transistor is formed by three p-type and n-type semi-conductors,
arranged in p-n-p or n-p-n form (Fig. 9). A transistor has three poles: base (B), collector (C) and
emitter (E). The two types of transistors are represented by different symbols. The symbol of a
PNP transistor contains an arrow pointing at its base, while that for an NPN transistor contains an
arrow pointing away from its base. As NPN transistors can be mass-produced more easily, we will
use it for the following discussions.
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Fig. 9 (a) Transistor
Electronic Systems
(b) PNP-type transistor
(c) NPN-type transistor
A transistor circuit can be connected in many different ways. As shown in Fig. 10, if we
connect the emitter of an NPN-type transistor to both its base and collector, the resulting circuit is
called a common emitter circuit. In a common emitter circuit, the base-emitter voltage forms the
input voltage Vin, while the collector-emitter voltage forms the output voltage Vout.. For the
transistor to work properly, Vin has to be larger than the cut-off voltage VBE(ON), which is about
0.7V. Otherwise, the transistor will be in a cut-off state and will cease to be conducting.
The currents flowing in the base, collector and emitter are called base current IB, collector
current IC and emitter current IE respectively. As the current flowing into and out of the transistor
must be the same, it can be deduced that IE = IB + IC.
Fig. 10 Common emitter circuit
Fig. 11 Input / Output voltage properties curve
The curve in Fig. 11 shows the different height of two plateaus of the output from the transistor.
They can be applied to switches and the components of logical calculation.
Heat sensitive switch
Fig. 12 shows a simple heat sensitive switch circuit that involves the use of a transistor. As the
thermistor RT has a higher resistance under low temperature, the potential difference Vin across
variable resistor R is smaller, and if Vin is smaller than 0.7V, the transistor will remain in a cut-off
state, the output voltage Vout would be 6V, and IC would be zero. Therefore, the indication light L
will not glow.
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Fig. 12 (a) Heat sensitive switch circuit
(b) A pictorial diagram of the heat
sensitive switch circuit
When the temperature rises, RT will decrease, causing the potential difference across R Vin to
increase and the output voltage Vout to decrease close to zero. IC will increase to a maximum
(saturation), causing the indication light L to glow. This circuit can be used in a fire alarm system.
R can be used to set the temperature that will set off the alarm.
(ii) Logic gate
A transistor circuit can be used to make one or more logic gates. A logic gate can process only
binary signals, such as high/low voltage, true/false, conducting/non-conducting, etc. All these
signals can be represented by the two logic values 0 and 1. The relations between the input and
output of a logic gate can be expressed in a truth table. Examples of logic gates include NOT gate,
AND gate, OR gate, NAND gate, NOR gate, etc.
NOT Gate
If you look at the transistor input/output voltage properties curve in Fig. 11, you can see that
the values of Vin and Vout are exactly by opposite. Therefore, the transistor is called a NOT gate,
which is designed to produce an output opposite to the input. If we input the logic value A, the
NOT gate will generate a result that is “not A”, or A . Table 1 shows the truth table of a NOT gate.
Fig. 13 shows the circuit symbol of a NOT gate.
Input A
Output F  A
0
1
1
0
Table 1 Truth table of a NOT gate
Fig. 13 Circuit symbol of a NOT gate
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AND Gate
An AND gate is a logic gate that generates the output 1 only if its inputs are also 1. Its output
is called “A and B”, represented by A  B. Table 2 shows the truth table of an AND gate. Fig. 14
shows the circuit symbol of an AND gate.
Input A Output B
Output
F  A B
0
0
0
0
1
0
1
0
0
1
1
1
Table 2 Truth table of an AND gate
Fig. 14 Circuit symbol of an AND gate
NAND Gate
The output of a NAND gate is the exact opposite of that of an AND gate. Table 3 shows the
truth table of a NAND gate. Fig. 15 shows the circuit symbol of a NAND gate. The output of a
NAND gate is called “not (A and B)”, represented by A  B .
Input A Output
B
Output
F  A B
0
0
1
0
1
1
1
0
1
1
1
0
Table 3 Truth table of a NAND gate
Fig. 15 Circuit symbol of a NAND gate
OR Gate
In an OR gate, whenever one of the two inputs A and B equals 1, the output would be 1. The
output of an OR gate is called “A or B”, represented by A + B. Table 4 shows the truth table of an
OR gate. Fig. 16 shows the circuit symbol of an OR gate.
Input A
Output B
Output
F  A B
0
0
0
0
1
1
1
0
1
1
1
1
Table 4 Truth table of an OR gate
Fig. 16 Circuit symbol of an OR gate
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NOR Gate
The output of a NOR gate is the exact opposite of that of an OR gate. Table 5 shows the truth
table of a NOR gate. The output of a NOR gate is called “not (A or B)”, represented by A  B . Fig.
17 shows the circuit symbol of a NOR gate.
Input A
Output B
Output
F  A B
0
0
1
0
1
0
1
0
0
1
1
0
Table 5 Truth table of a NOR gate
Fig. 17 Circuit symbol of a NOR gate
(iii) Binary adder
Decimal number
0
1
2
3
4
5
6
7
Binary number
0
1
10
11
100
101
110
111
Table 6 Decimal and binary numbers
Many electronic systems need calculation to finish their jobs. One example would be the
circuit of an electronic calculator. Calculation in electronic systems are mainly done in binary
numbers, that is, numbers formed by the two digits 0 and 1. Table 6 shows the corresponding
decimal and binary forms of a set of numbers. Fig. 18 shows how binary numbers can be converted
into decimal numbers.
Value
23
22
21
20
Binary
1
0
1
1
10112 = 123 + 022 +121 + 120 = 8 + 0 + 2 + 1 = 1110
Fig. 18
When an electronic system wants to add two numbers together, it has to use a binary adder,
which is formed by a half adder and a full adder. Both half adders and full adders can be made from
integrated circuits.
Half adder
A half adder adds two binary digits together and outputs the result. Fig. 19 shows the four
possibilities when adding two binary numbers A and B together. After they are added together, the
adder will return the sum S and the carry C.
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Fig. 19
Fig. 20
Fig. 20 shows the circuit symbol of a binary half adder. The adder has two inputs A and B, and
two outputs C and S.
Full Adder
A half adder can only process the summation result of two one-digit binary numbers, while a
full adder can process the summation result of two multiple-digit binary numbers.
Fig. 21
(a)
(b)
A full adder can be made from two half adders and an OR gate. Fig. 22 shows its electronic
circuit diagram. To perform four digit summation, we need a combination of three full adders and
one half adder, as shown in Fig. 23. Such arrangement forms the basic mode of a binary adder in an
electronic system.
Fig. 22
Fig. 23
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(iv) Flip-flop
Electronic systems often have to store numbers for further use in calculations. The component
responsible for this task is called a memory. A flip-flop is a simple kind of memory. Flip-flops
come in many models, such as S-R flip-flop, S-R NOR gate flip-flop, D type flip-flop, J-K flip-flop,
etc. Fig. 24 shows the circuit symbols of different flip-flops.
(a) S-R flip-flop
(b) J-K flip-flop
(c) A flip-flop device
Fig. 24
A flip-flop can be formed by logic gates. The S-R flip-flop in Fig. 24a is formed by two
transistors. Although the operation of a flip-flop is rather complicated, it still plays an important
role in an electronic system because it is needed to make memories, which are very useful.
Examples of memory include the RAM found in computers.
(v) Amplifier
The function of an amplifier is to strengthen weak signals, one example would be the amplifiers
used by radios to strengthen the weak signals received by its antenna. Fig. 25a shows the circuit of a
simple amplifier that employs transistors to amplify the input voltage. When the input voltage Vin
falls within the linear amplification area, a linear relation will form between Vin and the output voltage
Vout. When used with a suitable base resistor RB and load resistor RL, a transistor can amplify the
input voltage of alternate current. The result is shown in Fig. 25b.
Fig. 25
(a)
(b)
An amplifier can amplify not only the voltage, but also the power. The extra energy is
supplied by a second electric energy source in the output circuit. Fig. 26a shows two common
symbols of amplifiers. Fig. 26b shows the circuit of an amplifier.
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Fig. 26
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(a) Symbols of amplifiers
(b) Circuit of an amplifier
(vi) Comparator
A comparator (Fig. 27) is commonly used to compare the input voltage with a preset value. If
the input voltage is smaller than the preset value, the comparator will output a lower voltage. On
the contrary, when the input voltage is larger than or equal to
the
preset value, the comparator will output a higher voltage.
The
output allows other systems to respond correspondingly.
For example, in a fire alarm system, a comparator is
to compare the preset temperature with the signal it receives
the temperature sensor. When there is a fire, the voltage
generated by the temperature sensor will become larger than
preset voltage signal, the comparator will then activate the
used
from
signal
the
alarm.
Fig. 27 A comparator
(vii) Analogue to digital converter
Electronic signals in an electronic system can be transmitted in two forms: analogue signals
and digital signals. An analogue signal, such as the sinusoidal waves in Fig. 28a, carries with it a
continuous stream of varying signals. A digital signal, such as the square waves in Fig. 28b, can
transmit only two values.
Fig. 28 (a) Example of analogue signals
(b) Example of digital signals
Most electronic sensors receive analogue signals. Due to the resistance in the circuit, these
signals may be weakened greatly after transmission and repeated calculation, resulting in errors. As
digital signals can carry with them only one of the two values, the values stored in them cannot be
changed easily. Therefore, digital signals are more suitable for use in repeated calculation and
transmission.
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Fig. 29 Circuit of a square wave generator
Analogue signals can be converted into digital signals. Fig. 29 shows a transistor circuit that
can convert the sinusoidal wave voltage Vin generated by the low voltage power source into square
wave voltage Vout (Fig. 30).
Fig. 30 Sinusoidal wave is changed to square wave
An analogue to digital converter (ADC) can convert the analogue signals it receives from the
electronic sensor into digital signals, so as to allow further processing and transmission. Its
principle of operation is shown in Fig. 31. Similarly, a digital to analogue converter (DAC) can
convert digital signals into analogue signals.
Fig. 31 The principle of operation of an analogue to digital converter
Fig. 32 shows an example of an analogue to digital converter. The electronic scale seen in the
picture contains a microprocessor that can execute certain orders repeatedly. Analogue signals,
which are the weight of the food, are sent to the strain gauge, where an analogue to digital converter
converts them into ‘1’ and ‘0’ digits, which are in turn converted into decimal numbers for display
on the screen.
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Fig. 32 Operational flow diagram of an electronic scale
(viii) Integrated circuits
As technology advances, electronic systems also become more and more sophisticated. The
numbers and brands of electronic components also increase sharply. Examples include transistors,
diodes, resistors, capacitors, connecting wires, etc. However, when a large number of electronic
components are put together, the size of the circuit will become very large. Much energy will be
wasted, and a large amount of heat will be produced.
Integrated circuits (IC) are formed by a large number of electronic components combined
together on a tiny microchip (usually silicon made)
with
the help of advanced technology (e.g. photo
chemical machining). The components are then
sealed
in a plastic shell, exposing only a few legs for
connection.
Integrated circuits reduce not only the size of
circuits, but also their energy consumption.
Integrated circuits are often used to produce
microprocessors, read-only memories (ROM) and
random access memories (RAM) (Fig. 33)
Fig. 33 Example of integrated circuits
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(c) Output sub-system
The output sub-system of an electronic system is responsible for the display and execution of
the output. Examples include light emitting diodes, solenoids, buzzers, loudspeakers, motors,
relays, etc.
(i) Light bulb
A light bulb usually consists of a hollow glass ball that contains metal filaments. The metal
filaments, made of tungsten, will glow when heated. To prevent the filaments from oxidizing too
quickly, light bulbs are usually filled with inert gas such as nitrogen and argon (Fig. 34a).
Halogen bulbs contain gases like iodine, which allows the filament to be heated to a higher
temperature so the bulb will become brighter. The temperature of the filament is very high. In a
100 W light bulb, the temperature of the filament can be as high as 3300 oC.
Fig. 34 (a) Structure of a light bulb
(b) Light bulbs are used as decorations on a bridge
A light bulb can be used as the output device of an electronic system. For example, the
colourful light bulbs found on buildings are often controlled by electronic systems (Fi g. 34b).
Another example would be the light bulbs found in traffic lights.
(ii) Light emitting diode
A light emitting diode (LED) is an output component that emits light and allows current to
pass through in only one direction (Fig. 35). If we connect the longer leg of an LED to the positive
terminal of the power source, the LED will allow current to pass through and glow. On the contrary,
if we connect the shorter leg to the positive terminal, the LED will not allow current to pass through
and so will not glow. LED are very small in size, so they are often used as indication lights in
electrical appliances. The other advantages of LED include low energy consumption, low heat
emission and they are safe to use.
Fig. 35
(a) Light emitting diode (LED)
(b) Circuit symbol of an LED
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(iii) Solenoid
A solenoid consists of a series of metal coils arranged together. When current passes through
the solenoid, it will generate a magnetic field like that of a magnet (Fig. 36). If we add a soft metal
core to the solenoid, the solenoid will be strengthened to form an electromagnet. As the magnetic
field generated is very strong, the solenoid will attract nearby metal objects. However, if the current
is cut off, the solenoid will cease being magnetic. Electromagnets are often used in cars, motors,
lifters, etc.
Fig. 36 (a) A solenoid
(b) When the current passes through the solenoid,
a magnetic field is generated
(iv) Buzzer
A buzzer is an output component that emits a buzzing sound when current passes through it
(Fig. 37). As buzzers can only emit one kind of sound, they are mainly used in alarms and signal
lights.
Fig. 37 (a) Buzzer
(b) Circuit symbol of a buzzer
(v) Loudspeaker
A loudspeaker converts electric energy into sound energy. A loudspeaker is formed by a paper
or plastic cone and a movable coil. At the end of the coil lies a permanent magnet (Fig. 38). When
an electronic signal induced by the input sound passes through the coil, the current will react with
the magnetic field of the permanent magnet, producing a force that moves the cone. As the cone
vibrates and vibration is transmitted to nearby air, sound will be emitted. In this manner, the cone
has amplified the vibration and thus copied and boosted the input sound.
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Fig. 38 (a) Loudspeaker
Electronic Systems
(b) Structure
(c) Circuit symbol of a loudspeaker
(vi) Motor
A motor converts electrical energy into magnetic energy and kinetic energy. Examples include
electric fans, air conditioners, washing machines, electric dryers, etc. Motors are formed by a metal
coil suspended in a magnetic field generated by a permanent magnet or electromagnet.
A magnetic field will be produced when current passes through the metal coil. Due to the
attraction and repulsion of the two magnetic fields, the coil rotates and moves the axis. A rotating
coil is called an electric main (Fig. 39).
Fig. 39 (a) Electric motor
(b) Principle of motor
(c) Circuit symbol of a motor
Electric motors can be divided into two classes: direct current motors and alternate current
motors. In both models, a force is generated to drive the machines when current passes through the
metal coil. Direct current motors are driven by direct current, while alternate current motors are
driven by alternate current.
(vii) Relay
A relay is a device that uses a relatively small current to connect or cut off a larger current. As
high-power motors usually employ large current, it would be dangerous for users to switch on
directly. Thus a relay can allow the user to start or stop the machine safely. A relay is a combination
of components such as electromagnet, armature, spring, contact, etc (Fig. 40). When the switch is
closed, current passes through the solenoid. The magnetic core is magnetised, attracting the
armature and closing the contact, resulting in a closed circuit. When the switch is open, the current
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in the coil is cut and the magnetic core is demagnetised. The armature is restored to its original
position by the spring, cutting off the contact. The result is an open circuit. In this manner, a relay
can be used to control the switches of motors, electrical appliances, etc.
Common symbol
Fig. 40
(a) Relay
A.C. relay
(b) Circuit symbol of a relay
3 Electronic control
(a) General principle
Electronic Control means the process of using electronic devices to perform designated tasks.
Examples include thermostat control, complex aviation control, automatic door control, etc. Fig. 41
shows the block diagram of a typical electronic control circuit:
Fig. 41 Block diagram of a typical electronic control circuit
External signals provide operational orders for electronic control devices. These signals can be
in the form of light, heat, electric current, pressure, speed, sound, etc. A signal receiver collects
these signals and sends them to the signal converter.
A signal converter is a device that converts physical signals into electronic signals. It is an
important part of electronic control systems. All electronic devices work with electronic signals
(for example, electric current, voltage, resistance, etc) and not physical signals.
The electronic signals generated by the signal converter are usually very weak. Therefore, an
electronic amplifier is required to amplify them so they can be used to drive other components.
Typical electronic amplifiers are usually formed by components such as resistors, transistors and
other semi-conductors.
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(b) Electronic circuit symbols
In order to simplify the representation of electronic circuits, engineers employ different
symbols to represent different circuit components. With the help of these symbols, one can easily
draw an electronic circuit diagram and describe the relations between each electronic component.
Fig. 42 shows some common circuit symbols.
Fig. 42 Common circuit symbols
(c) Electronic control circuit design
Before designing an electronic control circuit, one should first understand the properties of
each electronic control component. After that, one can follow certain basic steps and combine the
components to form the correct circuit.
(i) A practical example
Scenario
We shall use a simple streetlight system
to show the basic steps in designing an
electronic control circuit. The system uses
time to control the streetlight, so it can turn
on at night and off in the daytime. One
drawback of this system is that the light will
remain off on cloudy days or in the winter,
when the streets are quite dark.
Fig. 43 The lighting decoration in the street
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General concept
In order to avoid the problem mentioned above, we should improve the system so that its
operation depends on the brightness of the streets.
Design details
Fig. 44a shows the block diagram of the original streetlight system. Fig. 44b shows the block
diagram of the improved system. The improved system uses a light sensor (such as a light sensitive
resistor) to detect the brightness of the streets. The signals from the sensor are fed back to the
system to control the streetlight.
(a)
(b)
Fig. 44 Block diagram of the original streetlight system
Data collection
As this electronic control system involves the use of light sensitive resistors and switches of
high voltage streetlight, refer to related texts on light sensitive resistors, transistor switches, relays
and control circuits.
Solution
Fig. 45 shows a light sensitive switch circuit that uses a transistor as the switch. The system
uses a relay to operate the streetlight. The variable resistor is set at 200 k.
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(a) A pictorial diagram of a light sensitive switch circuit
Fig. 45
(b) A light sensitive switch circuit
Situation 1
When the light sensitive resistor is exposed to light, its resistance will be reduced to
approximately 9k. The potential difference VLDR across the light sensitive resistor is (refer to the
Fig. 45):
VLDR ＝
9
 6 ＝ 0.258 V
9  200
As a result, the potential difference across it will be smaller than the cut-off voltage of the
transistor at about 1 V, causing the transistor to go into a cut-off state. As no current will pass
through the base of the transistor, the output voltage Vout will be 6V. The circuit is an open circuit.
The streetlight circuit will not be activated.
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Fig. 46 The variable resistor and the light sensitive resistor serve as a potential divider in the circuit
Situation 2
When the light sensitive resistor is put in a relatively dark place, its resistance will rise to about
600k. The potential difference across it will exceed the cut-off voltage of the transistor, causing a
current to flow through the transistor (refer to Fig. 46):
VLDR ＝
600
 6 ＝ 4.5 V
600  200
The output voltage Vout will fall close to zero. The relay will then switch on the streetlight
circuit as well as the streetlight itself. A diode is connected to the relay to prevent the transistor
from damage by the eddy current generated when the relay is turned on or off. By adjusting the
variable resistor R, the system can be set to activate at a lower or higher level of brightness.
Realisation, Testing and Evaluation
After finding the solution, we can test the above system by connecting a light bulb to it.
We can also learn to make fine adjustments to the system, such as adjustments to the resistor R.
Next, we can evaluate the performance of the system and find ways to improve it. But
remember, when connecting the circuit, one must pay attention to safety.
(ii) Safety Measures
For safety reasons, one should pay attention to the following:
1. When assembling circuits, do not deviate from the design plans.
2. To ensure safety, choose the appropriate voltage.
3. Positive poles should be distinguished from negative ones.
4. Handle electronic components with care so as to avoid percussion and damage to the
components.
5. Use a relay to control high power / voltage electronic components.
6. Understand the functions and output signals of electronic components so as to avoid faulty
connection, which may cause overheating and electric leakage.
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Technological Studies
Electronic Systems
Exercise
1. Draw the circuit symbols of the following electronic components:
(a) Thermistor
(b) Light sensor
(c) Light emitting diode
(d) PNP type transistor
(e) NPN type transistor
(f) Relay
2. State the working principles of a transistor and show how to distinguish different types of the
transistors.
3. Draw the circuit symbols of the following logic gates, and write down the truth tables of AND
gate and NOT gate.
(a) AND gate
(b) OR gate
(c) NOT gate
(d) NAND gate
(e) NOR gate
4. What is a logic circuit?
5. In what mode of signal does an input exist in a logic circuit?
6. What is an amplifier?
7. What are analogue and digital converters?
8. Give three examples of using analogue and digital converters in daily life.
9. Design an electronic control circuit to fit the following processing requirements:
Patients are sent to the Intensive Care Unit (ICU) after operations in a hospital. They need
medical staff to take special care of them. Some kind of help devices are needed to be installed
beneath the beds, so that the patients can press the alarm buttons to ask for help in the day time
(when light is available). At night, the alarm system may cause a nuisance to other patients.
Thus the system will be switched off and replaced by other helping systems.
(a)
(i) What types of input devices should be used?
(ii) Which logic gates should be used for the system?
logic gate.
(iii) What types of output devices should be used?
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Write down the truth table of that
Technological Studies
(b)
Electronic Systems
By using the following decision module, show how to connect the circuit to establish the help
system for the patients.
10. Design an electronic control circuit to fit the following processing requirements:
Some careless parents always forget to lock the brake of a baby cart. When they do not hold
the cart tight, the baby cart will slide down the slope. Accidents may happen and the baby will
get hurt. Design an alarm device for the baby cart. The alarm will show a siren when the
parents forget to lock the brake and their hands do not grasp the handle of the cart. (Note: When
the sliding switch is ON, the braking system of the baby cart will be locked.)
(a)
(i) Which two types of input devices should be used?
(ii) Which logic gates should be used for the system?
(iii) What types of output devices should be used?
(b)
By using the following decision module, show how to connect the circuit to establish the
electronic safety system for the baby cart.
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