Materials and components
There are three types of materials used in electronics: electrical conductors, electrical insulators and semi-conductors.
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.
Materials
There are three types of materials used in electronic components:
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 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
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:
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 resistors, transistors, capacitors, relays and light emitting diodes.
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 integrated circuit requires two discrete resistors and a discrete capacitor to make it work.
Discreet components
Integrated circuits (ICs)
These are miniature circuits 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, but the most common type of configuration is called the dual-in-line or DIL package,
which has two rows of connecting 'legs', one on each side.
A DIL package integrated circuit - far smaller than a finger-tip
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.
Diodes
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 reverse bias. A diode has two leads: for forward bias, the power comes in at the anode (positive lead) and out at
the cathode (negative lead).
Rectification
A common use for diodes is rectification - 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 only.)
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 - e.g. electronic keyboards or mobile phone chargers.
The circuit diagrams show the two methods of rectification.
Full wave rectification:
Half wave rectification:
Light emitting diode
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.
Seven-segment LED displays
A seven-segment LED is a special type of LED display used in digital clocks, video recorders
and microwave ovens.
Transistors
Electronics began with the development of the transistor in the 1950s. Transistors
are essentially tiny semiconductor amplifiers and/or switches, several thousands of
which can be put on a 1mm2 piece of silicon.
Transistors have three leads: the emitter, collector and base. 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.
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 amplify it to
give a much higher current in the collector (called gain). 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 high-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. Darlington pairs are often used to drive motors.
Transistors 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 component. Examples of output components that might require transistors
are direct current motors, solenoids and meters.
Capacitors
A capacitor 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 farad. Often you will see capacitors of much less than a farad. These will be
measured in microfarads (one millionth of a farad or 1/1,000,000) or picofarads (one million-millionth of a farad or
1/1,000,000,000,000).
There are two types of capacitor:
" polarised or electrolytic capacitors, and
" non-polarised or non-electrolytic capacitors
Polarised capacitors
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:
Polarised (electrolytic) capacitor
Mounting of polarised capacitors
Electrolytic capacitors may be either axially mounted
(on their side, connected at each end) or radially mounted (upright with both
connections at the bottom).
Image shows two electrolytic capacitors.
One is axially mounted, one is radially
mounted.
Non-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:
" Polyester
" Polystyrene
" Mica
" Ceramic
Applications of capacitors
" Smoothing rectified alternating current voltages into steady direct current voltages
" Blocking direct current signals whilst allowing alternating signals to pass
" Filtering out unwanted portions of a fluctuating signal
" Timing applications
" Storing charge to keep a transistor turned on or off
Resistors
Resistors are components which restrict or resist the flow of current. The ability of a material or component to resist current
flow is measured in ohms. There are three main types of resistor:
" Fixed resistors
" Variable resistors, and
" Special resistors, such as thermistors and light-dependant 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:
Protecting other components (such as an LED) from damage by too much current.
As potential dividers (or voltage dividers). A fixed resistor is used to split current
between different parts of the circuit. Potential dividers are used, for example, with
LDRs in circuits which detect changes in light.
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 normally 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 spindle onto which an operating
knob is attached.
Special resistors
Thermistors change resistance as temperatures change. Most thermistors have a negative temperature coefficient - meaning
their resistance falls as temperature increases. Thermistors are used in temperature-sensing circuits.
Light-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 coefficient - meaning that their resistance falls as the
amount of light falling on them increases. LDRs are used in light-detection circuits.
Ohms and resistance values
Ohm
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 R means ohm. 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 k means kilo-ohm. Numbers coming before the k 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-ohms.
" The letter M means mega-ohm. Numbers coming before the M indicate a value of more than one, while numbers
after the M indicate a value less than one. So 2M = 2 mega-ohms, and 2M2 = 2.2 mega-ohms
Resistance values
The resistance value of a resistor is shown by a series of
coloured bands.
" 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 sequence red - 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 - red denotes a
value of 22 x 100, or 2.2 kilo-ohms.
" The fourth band is the tolerance. 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 percent; 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.
Potential dividers
Potential dividers are used for dividing up the current, so that a part or parts of a circuit only receive the current they require.
Potential dividers consist of two or more components (usually resistors) arranged in series across a power supply.
The circuit diagrams below show three common types of potential divider: two fixed resistors in series, a fixed resistor and
LDR in series, and a thermistor and variable 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 other
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-amp comparator circuits. The diagram shows a
darkness sensor circuit with a transistor 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 - usually around 400 Ohms. When the LDR is covered up the
resistance increases, often to many kilo-Ohms). The reduction in voltage causes the transistor to switch, completing the LED
circuit and lighting up the LED.
Standard symbols guide
The standard symbols for the key components used in electronic circuits are shown in the tables below.
Electronic circuit diagram components
Some more common symbols, including output components and logic gates, are shown in the table below.