Design & Technology

dehisceforkElectronics - Devices

Nov 2, 2013 (4 years and 6 months ago)


Design & Technology

Materials and components

There are three types of materials used in

making devices which operate using the flow of electrons through
integrated circuits
such as TVs, radios and computers
: electrical
conductors, electrical insulators and semi

Electronic components are divided into two groups: discrete
electronic components

such as diodes, transistors, capacitors
and resistors

and integrated circuits. You need to know what
the common discrete components are used for, and to
understand ohms and resistance values. You also need to know
how to represent

components using symbols when drawing
circuit diagrams.


There are three types of materials used in electronic components:


Electrical conductors

are materials that allow electricity to flow
through them easily.


An electrical insulator

Electrical insulators

are materials that prevent electrical flow. In
the diagram below the insulating material (plastic) surrounds the
conducting material (copper wires)


conducting materials

exhibit both conducting and
insulating properties. The way i
n 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 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:



Integrated circuits

The simplest kind of semiconductor device is a diode. In a diode the
lectrical 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

A diode

Another semi
conducting material

, but this material is
used less widely than silicon.

The ease with which electricity flows through a material is called its
. The value of resistivity is measured in
. The
higher a material's resistivity, the more
difficult it will be for electricity
to flow through it:


have very high resistivity values.


have low resistivity values.


Electronic components can be divided into two groups:

1. 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

components which resists or 'slows
down' the current in a circuit by acting against the flow of electrons.
Resistance is measured in ohms.


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.


circuit components which store and discharge electrical current. It is
made from two parallel metal plates separated by
an insulator (called
a dielectric).


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.


light emitting diodes or
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

circuit) having two states, neither of which is stable. An astable circuit
therefore oscillates between the two states, giving a constant

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 etched on to a piece of silicon or
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, but the most common type of configuration is called the
line or DIL

package, which has two rows of connecting 'legs',
one on each side.

A Surface Mount package integrated circuit

far smaller than a finger

You don't
need to understand how the circuit inside a silicon chip

there's some quite complicated physics involved. It's best to
think of ICs simply as input
output process blocks, as shown below:

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 res
t of the


A diode is the simplest form of semiconductor. Diodes are a discrete
component that allows current to flow in one direction only. The
direction that current is allowed to pass is called the
forward bias
The direction that current is not allowed to pass is called the
. A diode has two leads: for forward bias, the current comes in at

(positive lead) and out at the

(negative lead).


A common use for diodes is rect

that is, the changing of
alternating current into direct current. (An alternating current (AC) is
one which flows alternately in opposite directions around a circuit,
while a direct current (DC) is one that always flows in one direction

A rectifying circuit can be found in the transformers used with many
types of equipment that require a mains alternating current to be
converted into a smaller direct current

eg electronic keyboards or
mobile phone chargers.

The circuit diagrams show the

two methods of rectification.

Full wave rectification:

Half wave rectification:

Light emitting diode

Light emiting diode (LED)

A light
emitting diode or LED is a special kind of diode that glows
when electricity passes through it. The LED is made from a semi
conducting material called gallium arsenide phosphide. LEDs can be
bought in a range of colours. In common with all diodes,
the LED will
only allow current to pass in one direction. The current required to
power an LED is usually 25 mA.

segment LED displays

segment LED display

A seven
segment LED is a special type of LED display used in digital
clocks, video recorders and microwave ovens.


Electronics began with the development of the transistor in the 1950s.
Transistors are essentially tiny semiconductor amplifiers
switches, several thousands of which can be put on a 1mm

piece of

Transistors have three leads: the

. The
base lead controls the transistor: applying an electrical current to the
base lead switches the transistor

on. When the transistor is on,
current flows from the collector to the emitter

but when it is off no
current will flow.

A transistor and its three leads

Transistors are easily damaged, so it is important not to mix up the
three leads. To help
identify the leads some transistors have a dot
near the collector, and/or a tab near the emitter. Each type of
transistor is identified by a code printed on the side.

Darlington pairs

Some transistors can take a very low current flowing in the base and
lify it to give a much higher current in the collector (called

the amount of amplification of the input voltage


how much
bigger the output voltage is compared to the input voltage

). Other
transistors can output a large current at the collector

but without very
much gain. Single transistors cannot have both high gain and high
collector current.

To overcome this
problem, a high
gain transistor is paired up with a
current transistor in what is called a Darlington pair. The
combined transistors allows both a higher gain and a larger amount of
current to flow than would be possible with a single transistor.
ington pairs are often used to drive motors.

Circuit diagram for a Darlington pair


are often used as interface devices

that is, devices
which ensure that the right amount of current is supplied to power
another device, such as an output co
mponent. Examples of output
components that might require transistors are direct current motors,
solenoids and meters.



is a discrete component which can store an electrical
charge for a period of time. The larger the capacitance the more
charge it can store.

The unit of measurement of a capacitor is the
. Often you will
see capacitors of much less than a farad. These

will be measured in

(one millionth of a farad or 1/1,000,000) or

(one million
millionth of a farad or 1/1,000,000,000,000).

There are two types of capacitor:

polarised or electrolytic capacitors, and

polarised or
electrolytic capacitors

Polarised capacitors

Polarised (electrolytic) capacitor

These generally have larger capacitance values. Polarised capacitors
have a positive pole and a negative pole, so you have to connect
them to a circuit the correct way
round. The polarity and value of a
capacitor are usually shown like this:

Mounting of polarised capacitors

Image shows two electrolytic capacitors. One is axially mounted, one
is radially mounted.

Electrolytic capacitors may be either axially mounted (on their side,
connected at each end) or radially mounted (upright with both
connections at the bottom).

polarised capacitors

These are usually much smaller than the polarised type, and have
smaller capacitance values ranging from a few picofarads to a few
microfarads. Because they have no positive or negative poles these
capacitors can be connected to a circuit either way round.

There are
four types of non
polarised capacitor, each named after the material
they are made from:





Applications of capacitors


rectified alternating current voltages into steady direct
current voltages


direct current signals whilst allowing alternating signals
to pass

Filtering out

unwanted portions of a fluctuating signal



Storing charge

to keep a transistor turned on or off


Resistors are components which restric
t or resist the flow of current.
The ability of a material or component to resist current flow is
measured in

units of electrical
resistance, usually
shown by the symbol R. 1 volt will force a current of 1 amp through a
resistance of 1 ohm.

. There are three main types of resistor:

Fixed resistors

Variable resistors, and

Special resistors, such as thermistors

and light
resistors (LDRs)

Fixed resistors

These are the most common type of resistor. They are found in nearly
every electronic circuit. Their three most important uses are:

A light
emitting diode (LED) protected by a fixed resistor

Protecting other components (such as an LED) from damage by too
much current.

A circuit diagram for a temperature detector

potential divid

potential dividers
components which split a
circuit's voltage into two. Potential dividers consist of two resistors in


(or voltage dividers).

A fixed resistor is used to split voltage
between different parts of the circuit. Potential dividers are used, for
example, with LDRs in circuits which detect changes in light.

A circuit diagram for a timing application

In timing applications. In this
role a fixed resistor is used with a
capacitor in series.

Variable resistors or potentiometers

There are two types of variable resistor:

The first type of variable resistor can be altered continually as they
work. For example the volume control in a radio.

The second type is called a pre
set potentiometer. It has a
resistance control that is adjusted and then fixed. These resistors
would n
ormally be adjusted once only.

The main difference between the two types of Potentiometers is their
size. The pre
set potentiometers tend to be smaller and are usually
adjusted with a screwdriver. A variable resistor is generally provided
with a long spind
le onto which an operating knob is attached.

Special resistors


change resistance as temperatures change. Most
thermistors have a negative temperature coefficient

meaning their
resistance falls as temperature increases. Thermistors are used i
sensing circuits.

dependent resistors (LDRs)

have a resistance which changes
in response to changes in light levels, as detected by a photo
sensitive plate on the resistor. Most LDRs have a negative light

meaning that thei
r resistance falls as the amount of light
falling on them increases. LDRs are used in light
detection circuits.

Ohms and resistance values


The ohm is the unit of resistance. Larger values are measured in kilo
ohms (1000 ohms) and mega
ohms (1,000,000
ohms). Resistors are
marked, using a code specified in British Standard 1852, as follows:

The letter

. Numbers coming before the R indicate a
value more than one. So 1R (or 1R0) = 1 ohm; 47R = 47 ohms;
and 4R7 = 4.7 ohms. Numbers coming after the R indicate a value
less than one

so R56 = 0.56 ohms.

The letter

. Numbers coming before the

indicate a value more than one, while numbers coming after the k
indicate a value less than one. So 1k8 = 1.8 kilo
ohms and 5k6 =
5.6 kilo

The letter

. Numbers coming before the M
indicate a value of more than one, while numbers a
fter the M
indicate a value less than one. So 2M = 2 mega
ohms, and 2M2 =
2.2 mega

Resistance values

The resistance value of a resistor is shown by a series of coloured

The first band denotes tens, and the second band units. Each
colour stands for a different unit: black is zero, brown is one, red is
two; orange is three; yellow is four; green is five; blue is six; violet
is seven; grey is eight; white is nine. So the seq
uence red

denotes the value 22.

The third band is the multiplier. Black denotes a multiplier of one;
brown 10; red 100; orange 1000 and so on. So the sequence red


red denotes a value of 22 x 100, or 2.2 kilo

The fourth band is the tol
erance. Manufacturers of resistors
cannot guarantee the exact resistance figure shown by the first 3
bands, so they give a percentage value by which the resistance
may be higher or lower than the resistance quoted. A red band
denotes a tolerance of 2 perce
nt; gold a tolerance of 5 percent;
and silver a tolerance of 10 percent. Thus a 100 ohm resistor of
10 per cent tolerance has an exact resistance value falling
somewhere between 90 ohm and 100 ohm.

Graphical table summarises the colour coding found on
the four
bands on a resistor.

Potential dividers

Potential dividers are used for dividing up the voltage, so that a part or
parts of a circuit only receive the voltage they require. Potential
dividers consist of two or more components (usually

components which resists

or 'slows down' the current in a
circuit by acting against the flow of electrons. Resistance is measured
in ohms.

) arranged
in series

in ser
connected to a circuit in such
a way that the same current flows through each component in turn.
Opposite of in parallel


across a power supply.

The circuit diagrams below show three common types of potential
divider: two
fixed resistors

fixed resistors
type of resistor whose
resistance remains constant. Opposite of a variable resistor


series, a fixed resistor and

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.


in series, and a

type of resistor that changes resistance with tempe

also called
a Temperature
Dependent Resistor. Usually the resistance decreases
with an increase in temperature (and vice versa)



variable resistor
type of resistor whose resistance can be
varied to change the amount of current flowing through it. Opposite of
a fixed resistor


in series. (Note that the resistors are usually drawn
vertically on a circuit diagram.)

a circuit diagram showing two fixed resistors arranged one above the

circuit diagram shows an LDR and fixed resistor arranged one above
the other

A circuit diagram, a Thermistor and variable resistor

Common uses of potential dividers

Potential dividers are important in both transistor
switching circuits
and op
comparator circuits

comparator circuits
circuit with a

a component which compares two voltages or currents,
amplifies the difference between them, and changes its output
depending on the result of the comparison

. The diagram shows a
darkness sensor circuit with a

which do not conduct electricity unless they are turned on by a
(different) electrical current. This means they can be used as
switches, ampl
ifiers and in other ways.


used as a switch. When the
LDR senses a drop in light, the LED is switched on.

When the LDR has light falling on it, its resistance is low

around 400 Ohms. When the LDR is covered up the resistance
increases, (often t
o many kilo
Ohms). When the resistance of the LDR
is small its share of the voltage supply is small too, so Vout from the
voltage divider is small, and the transistor is switched off.

In the dark the large resistance of the LDR takes a large share of the
oltage supply, so Vout is large and the transistor and LED both
switch on.

Standard symbols guide

The standard symbols for the key components used in electronic
circuits are shown in the tables below. You may find it useful to print
off copies of these
tables to use for reference.

Electronic circuit diagram components

Some more common symbols, including output components and logic
gates, are shown in the table below.

Graphical table showing some more standard symbols for the key
components used in el
ectronic circuits

Now try a
Test Bite

Back to Revision Bite