Design & Technology
Electronics
Electronics components are made up of three types of materials:
conductors, insulators and semi-conductors. Components can
be either separate devices linked together in a circuit, or
integrated circuits incorporating large numbers of semiconductor components etched onto a chip of silicon inside tiny a
plastic case.
Materials
Modern electronic systems are controlled by microprocessors and
microcontrolers - computer-like components which are programmable
[programmable: able to take instructions in the form of a sequence of
commands ] and can therefore be used in different ways in different
products.
Electronic components work together in circuits, and these can be
represented in circuit diagrams using standard symbols for the
components. You need to know some common inputinput: Inputs are
everything that goes into the system. There are three main inputs: 1.
Physical inputs - in primary industries these are sun, soil, water; in
secondary industries these are raw materials (such as cotton for
textiles or metal for cars); 2. Labour - either skilled or unskilled
workers; 3. Capital - money invested in the business to pay for raw
materials, staff, machinery and buildings. and output [output:
Whatever comes out of a system. In electronics, output components
deliver a circuit's end result, while the output of a component is the
size of electrical signal at its output terminal ] components, and be
familiar with two common types of integrated circuit - the 555 timer
and Op-Amp circuit.
There are three types of materials used in electronic components:
An electrical insulator
1. Electrical conductors are materials that allow electricity to
flow through them easily.
2. Electrical insulators are materials that prevent electrical flow.
In the diagram to the right, the insulating material (plastic)
surrounds the conducting material (copper wires).
3. Semi-conducting materials exhibit both conducting and
insulating properties. The way in which the material is
connected to a power supply determines whether it will conduct
an electrical current or prevent it from flowing.
The most common semi-conducting material is silicon. Silicon needs
to have very small amounts of other elements such as boron and
phosphorous added to it in order to become a semi-conductor. This is
called doping. Doped silicon is used to make components such as:
 Transistors
 Diodes
 Integrated circuits
A diode
The simplest kind of semiconductor device is a diode. In a diode the
electrical current can be made to flow in one direction only (see
diagram below). If the diode is reversed the flow of current is stopped.
This behaviour is due to the semi-conducting property of the doped
silicon.
Another semi-conducting material is germanium, but this material is
used less widely than silicon.
The ease with which electricity flows through a material is called its
resistivity. The value of resistivity is measured in ohms. The higher a
material's resistivity, the more difficult it will be for electricity to flow
through it:
 Insulators have very high resistivity values.
 Conductors have low resistivity values.
Components
Electronic components can be divided into two groups,Discrete
electronic components and Integrated circuits (ICs).
Discrete electronic components
These are separate components that you can combine together to
make a circuit on a breadboard, printed circuit board or veroboard
(discrete means separate). Examples are resistors [resistors:
components which resists or 'slows down' the current in a circuit by
acting against the flow of electrons. Resistance is measured in ohms.
], transistors [transistors: components which do not conduct
electricity, unless they are turned on by a (different) electrical current.
This means they can be used as switches, amplifiers and in other
ways. ], capacitors [capacitors: circuit components which store and
discharge electrical current. It is made from two parallel metal plates
separated by an insulator (called a dielectric). ], relays [relays: type of
switch which which uses an electromagnetic coil to connect two or
more contacts, which close when the coil is energised. Use of relays
enables a very small voltage in the relay circuit to control a much
larger one in a separate circuit. ] and light emitting diodes or LEDs
[LEDs: stands for Light-Emitting Diodes. LEDs glow when current
passes through them. ].
Discreet components
These components are called discrete because you can select them
individually and combine them to make up the circuit you require.
Discrete components can also be used as external components of an
integrated circuit system. For example a 555 astable [astable: (a
circuit) having two states, neither of which is stable. An astable circuit
therefore oscillates between the two states, giving a constant on/off
digital output. Used, for example, to make LEDs flash continuously. ]
integrated circuit requires two discrete resistors and a discrete
capacitor to make it work.
Integrated circuits (ICs)
These are miniature circuits incorporating large numbers of
components, all etched on to a piece of silicon or chip. These chips
are encapsulated inside a protective plastic package, and nowadays
are manufactured in vast numbers. The circuits inside the package
are arranged in different configurations for particular purposes, for
building circuits in schools the DIL package is used, it is big enough to
handle. To save space on commercial products surface mount chips
are used.
A Surface Mount package integrated circuit - far smaller than a fingertip
You don't need to understand how the circuit inside a silicon chip
works - there's some quite complicated physics involved. It's best to
think of ICs simply as input-output process blocks, as shown below:
Input-output process block
When using ICs you need to know which pins need to be connected,
the function of each pin and how the IC is connected to the power
supply. A circuit diagram that includes one or more ICs should show
the pin numbers and how the pins are connected to the rest of the
circuit.
Programmable components
Many electronic devices on the market, such as those in the photo
below, are now designed using components that can be programmed
[programmed: instructed to perform a function or set of functions ] to
function in different ways. The advantage is that the same key
component used in one product can also be used in something
completely different. This reduces costs, as expensive customised
integrated circuits do not need to be designed and manufactured for
every new product.
Devices using programmable components
A microprocessormicroprocessor: An integrated circuit which
contains the processor of a computer all on one chip. is a multifunction programmable device. Microprocessors enable computers to
work, and they can also be used to control many types of electronic
system.
An alternative type of programmable component is the
microcontroller or PIC [PIC: Programmable Interface Controller - a
device which can be programmed via a computer to control complex
circuits. ]. Microcontrollers are essentially single-purpose
microprocessors, and they enable designers to use the same device
to control a wide range of situations, while being cheaper than a
computer control system. The rate at which the PIC works is
controlled by an internal clock [clock: a pulsed "clock" signal is used
to determine how fast a microcontroller (such as a microprocessor in a
computer) processes a programme. ].
For example, the various systems in a car, including the one pictured
below, could be controlled either by a single microprocessor
controlling a number of different functions at once, or by a series of
microcontrollers, each controlling specific functions such as fuel
consumption.
Car engine controlled by microprocessors
The advantage of the microprocessor is that one device can control
everything. The disadvantage comes if the microprocessor
malfunctions: all the systems could be affected and the replacement
cost is high. With microcontrollers, on the other hand, if one is
damaged it can be easily replaced and when one fails the others
continue to function.
Electronic circuits
In electronic systems, many components work together in circuits
[circuits: closed loops through which current flows - from a power
source, through a series of components, and back again ]. The photo
shows a printed circuit board with components on the uper side
connected by copper wires on the underside of the board..
PCB showing copper connections running beneath the board
Rather than drawing the components as they look in real life, symbols
are used to produce circuit diagrams or schematics, showing the
components and connections between them. These symbols are
recognised universally around the world and avoid confusion between
components which look similar.
The diagram below shows a 555 astable circuit with components
and the standard symbols for each of them.
Circuit diagram with 555 timer IC
On the next page is a table of the standard component symbols you
need to know.
Circuit components
Electronic circuit components are drawn as symbols so that they are
easy to draw and recognise all around the world.
The following guide shows the main symbols used in electronic
circuits.
Electronic circuit diagram components
Inputs and outputs in electronic circuits
The input is what sets an electrical circuit in action. Any of the
following components can be used as inputs:
 a switch (eg push-switch, microswitch)
 a key pad
 a Light Dependent Resistor (LDR [LDR: Light Dependent
Resistor, or LDR, is a type of resistor which is affected by
changes in light levels. A cadmium sulphide layer causes a
decrease in resistance in the light and increase in the dark. ])
 a thermistor [thermistor: type of resistor that changes resistance
with temperature - also called a Temperature-Dependent Resistor.
Usually the resistance decreases with an increase in temperature
(and vice versa) ]
 a photodiode [photodiode: a light-sensitive diode; a
semiconductor which detects light and emits an electric current ]
 a phototransistor [phototransistor: a light-sensitive transistor,
which switches itself on or off when it detects light ]
 an opto-isolator [opto-isolator: component which sends a signal
between two circuits via a light-path, thus maintaining isolation
between the two circuits ]
 a proximity switch [proximity switch: type of sensor which can
detect its relationship to a metal target without making physical
contact ] or reed switch [reed switch: a magnetically-controlled
switch which is activated by detection of movement ].
The output is what results from an electrical circuit. Any of the
following components may be used as outputs:
 an LED [LED: stands for Light-Emitting Diode. LEDs glow when
current passes through them. ]
 a lamp
 a buzzer
 a piezo [piezo: component made of crystal, ceramic or polymer
which changes shape when a current is applied, or emits a current
when its shape is changed ]
 a motor or stepper motor [stepper motor: type of motor which
rotates in fixed, precise and often very small steps. Used to turn
the hands in some watches ]
 a solenoid [solenoid: type of electromagnetic switch which
creates a magnetic field when a current is applied ]
 a relay [relay: type of switch which which uses an electromagnetic
coil to connect two or more contacts, which close when the coil is
energised. Use of relays enables a very small voltage in the relay
circuit to control a much larger one in a separate circuit. ]
 a seven-segment display [seven-segment display: type of
electronic feedback display for numerical data, which displays all
digits using combinations of lines in seven different positions.
Common in digital alarm-clocks.. ].
Logic gates
Logic gates are a family of digital devices which compare two or more
inputs and give a specific output. Each logic gate (NOT, AND, NAND,
OR, NOR etc) acts in a different way, and will always act so. The
action of any logic gate is shown by a Truth Table, eg the AND gate
will only give a high output, when all the inputs are high.
Some more common symbols are shown in the diagram below,
including output components and logic gates.
Circuit component symbols
Integrated circuits 1: 555 timer
Integrated circuits (ICs) are complex, highly-miniaturised circuits,
incorporating hundreds or even thousands of components, etched on
to a tiny piece of silicon - a chip.
The 555 timer chip
The 555 timer IC, shown in the diagram below, has many functions.
Two of them are: monostable and astable circuits.
Pin-out diagram for a 555 timer chip
Monostable mode
A 555 monostable timer is usually, unless a circuit turns it on by
applying a voltage to pin 2. When the voltage at pin 2 goes above a
level the 555's output will go high, but only for one pulse, after which
the 555 will return to the low stable state. So this type of circuit is used
where a single, timed output is required, either as an on-for-a-period
or as a timed delay.
In the diagram below, the length of the time period - ie how long the
output is on for - is determined by a resistor/capacitor network (RC
network [RC network: stands for Resistor/Capacitor network - a
combination of resistors and capacitors that are used to control a
circuit, such as a timer. The resistor 'slows-down' the current, charging
the capacitor. If the value of either component increased, the time
period will also increase. ]), which is connected to pins 6 and 7. The
RC network is a combination of resistors [resistors: components
which resists or 'slows down' the current in a circuit by acting against
the flow of electrons. Resistance is measured in ohms. ] and
capacitors [capacitors: circuit components which store and discharge
electrical current. It is made from two parallel metal plates separated
by an insulator (called a dielectric). ] used to control this time period.
The resistor 'slows-down' the current, which charges the capacitor.
Monostable timer circuit
Astable mode
In an astable timer the output is not stable in either the on or the off
state, but instead is pulsed on and off continuously. The frequency
(number of pulses per second) is determined by the RC network [RC
network: stands for Resistor/Capacitor network - a combination of
resistors and capacitors that are used to control a circuit, such as a
timer. The resistor 'slows-down' the current, charging the capacitor. If
the value of either component increased, the time period will also
increase. ] connected to the 555 timer. When connected to an LED an
astable timer gives a continuously flashing light. When set to a very
high frequency and connected to a loudspeaker it will generate a tone.
Astable timer circuit
Integrated circuits 2: Op-Amp
The operational amplifier or Op-Amp amplifies [amplifies: changes a
small input current or voltage into a larger output current or voltage ]
the difference between the two inputs to produce a voltage gain
[voltage gain: the amount of amplification of the input voltage - ie
how much bigger the output voltage is compared to the input voltage.
] as high as 100,000 times the difference. The output voltage cannot
be any greater than the power-supply voltages. It cannot output a
voltage more than two volts above or below its power connections. If
the power supply is + 12 and -12 volts the maximum values at the
output are likely to be +10 and -10 volts. An estimate of 85% of the
supply may also be used. The diagram below shows the pin set-up of
an Op-Amp.
Operational amplifier
The circuit below is a typical application using the 741 Op-Amp. Can
you identify the inputs and the outputs?
More electronics
You will find lots more on electronics - including information on
switches, resistors, capacitors, transistors, integrated circuits, logic
gates and output devices - in the Electronics section of this subject
module. Go to GCSE Bitesize Design and Technology: Electronics
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