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Gary Plimer 2005
Electrical Circuits / Electronics
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.
Some important terms:
Electric current

Electric current is the name given to the flow of negatively
charged particles called 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.
Gary Plimer 2005
Electrical Circuits
Voltage
In most circuits a battery or voltage supply is used to drive the electrons
through the components. Voltage is measured in volts (V).
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.
Resistance is therefore a measure of how much voltage is required to let a
current flow. Resistance is measured in ohms ().
+
V
_
Conventional
Current
R
Gary Plimer 2005
Batteries & Voltage Supplies
Sing le b a ttery or c e ll
Multip le b a tterie s or c e lls
-ve
+ ve
6 volts
Volta g e sup p ly
Gary Plimer 2005
Components - Resistors
Fixed Resistor Symbol
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.
Although the symbol for ohms is ‘’ it is often shown as a
capital R; that is, 270 ohms can be expressed as either 270  or
270 R.
Gary Plimer 2005
Resistor Colour Code
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
Gary Plimer 2005
Resistor Value Calculation
4 Ba nd Resistor Colour Co d e La yout
If the colours on the resistor
are:
1st band  red
2nd band  violet
3rd band  brown
4th band  gold
1st b a nd
1st d ig it
4th b a nd
to le ra nc e
2nd b a nd
2nd d ig it
Then its value is: 2(red) 7(Violet) x 10(Brown)
with a 5% tolerance (Gold) i.e. 270ohms 5%
tolerance.
3rd b a nd
m ultip lier
Gary Plimer 2005
Pupil Assignment
Calculate the value of the following resistors:
1) blue – violet – brown – silver
2) orange – white – brown – gold
3) brown – black – red – gold
4) brown – black – green – brown
What colours would the following resistors have?
1) 270 R
2) 1K5
3) 33 K
Gary Plimer 2005
Diodes
Diodes are devices that allow current to flow in one direction
only.
C urre nt c a n p a ss th is wa y o nly
Ano d e
Ca thod e
Symbol for Diod e
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.
Gary Plimer 2005
Light Emitting Diode
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
Gary Plimer 2005
Switches
Tog g le
Key
Slid e
Tilt
Roc ker
Reed
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.
Gary Plimer 2005
Switch Contacts
DPDT
SPST
SPDT
DPST
Gary Plimer 2005
Pupil Activity
We have now seen a number of common electronic components.
Lets now try and combine some of these into a working circuit.
6V
Switc h
I
Copy the circuit into your workbook
390R
or
390
LED
Build the circuit using the modular
boards
Your teacher will demonstrate how to
connect the boards
Gary Plimer 2005
Series Circuits
When components are connected end to end, as in the last
activity, we say they are connected in series.
This leads to an important law, Kirchoff’s 2nd Law
The sum of voltages dropped across each component (V1, V2
) is equal to the total voltage supply in the circuit.
6V
6V
6V
VT = V1 + V2 + V3 + …
18 V
Gary Plimer 2005
Measuring Voltage Drops
To measure d.c. voltage:
 Connect the black lead to the ‘COM’
socket
On/Off
Amps 20mA
a.c. d.c.
Volts
10A 50V
10V
1V
2mA
 Connect the red lead to the ‘V’ socket
200k
 Make sure that ‘d.c.’ is selected
 Place the lead probes on the points
where the voltage is to be measured
100mV
200A
20k
200
2k
Ohms
10A
mA
V
COM
Digital Multimeter
Gary Plimer 2005
Measuring Voltage Drops
V
Note how voltage is measured over the components
Make sure you take a note of the symbol for VOLTMETER
Gary Plimer 2005
Pupil Activity (Voltage Drops)
Task:
Measure the voltage drop
over the 2 bulbs. Enter
your findings into a table.
Bulb No.
9V
1
2
Voltage
(v)
Gary Plimer 2005
Pupil Activity (Voltage Drops)
Task:
Measure the voltage drop
over the 2 bulbs and
resistor. Enter your
findings into a table.
Gary Plimer 2005
Prototype Board
Prototype Board is used to test circuits prior to manufacturing
the circuit in large numbers.
Build a series circuit using 2
resistors of different values as
shown by your teacher.
Using the multimeter, check
the voltage drop over each
resistor.
Do the results confirm
Kirchoff’s law?
METALLIC STRIP
CONNECTOR
Gary Plimer 2005
Circuit Simulation
As in Pneumatics, it is possible to simulate electrical circuits.
In this case we will use a program called Crocodile
Technology. Your teacher will demonstrate the use of Croc
Clips to simulate the circuit shown below..
Gary Plimer 2005
Measuring Current
Current is measured
through components or
parts of circuits, as shown
in the circuit diagram
opposite.
6V
Note that it is necessary
to ‘break’ the circuit and
connect the meter in
series with the
components.
Take a note of the symbol
for an Ammeter
A
Gary Plimer 2005
Current measurement
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.
Gary Plimer 2005
Measuring Resistance
Connect two resistors in series on a
prototype circuit board and measure the
overall resistance.
You should find that
Rtotal = R1 + R2
And the general rule for
finding the sum of any
amount of resistors in series is
R1
10 A
Rtotal = R1 + R2 + R3 + Rn
R2
mA
V
CO M
Gary Plimer 2005
OHMS LAW
Ohms law can be used to calculate theoretical Voltage drops,
Current and Resistance in circuits.
V
R =
I
Using the triangle shown opposite,
we can rearrange the formula to
obtain V or I.
V
I
R
Gary Plimer 2005
Ohms Law in Practice
The task is to calculate the
resistance of the lamp.
6 volts
Lamp
V
R =
I
6
R =
0. 06
Current 0.06 amps
R = 100
Gary Plimer 2005
Worked Example
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
Gary Plimer 2005
Worked Example
a)
RT = R1 + R2
= 6 + 18
b)
VS = I C
IC =
R T = 24
IC
c)
RT
VS
RT
12
24
= 0. 5 A
V2 = I C  R 2
 0.5  18
VT = V1 + V2
V2 = 9 V
VT = 12 V
3 + 9
Gary Plimer 2005
Pupil Problems
For the circuit shown below calculate:
a) The total resistance of the circuit
b) The circuit current
12V
Gary Plimer 2005
Pupil Problems
For the circuit shown below calculate:
a)
b)
c)
d)
The total resistance
The circuit current
The voltage drop across each resistor.
Use Kirchoff’s second law to verify your answers to (c).
6V
Gary Plimer 2005
Pupil Problems
For the circuit shown below calculate:
a) The total resistance of the circuit
b) The circuit current.
24V
Gary Plimer 2005
Pupil Problems
A circuit has three resistors in series. Their values are 15 R,
24 R and 60 R. Calculate the total resistance of the circuit.
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
Gary Plimer 2005
Series Circuits
One of the problems with series circuits is if a component
fails, then the whole circuit fails. Consider a set of bulbs
connected in series.
If one of these bulbs fail, then current cannot flow
through the circuit, hence the remaining bulbs will fail
to light also.
Gary Plimer 2005
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.
240 volts
Gary Plimer 2005
Parallel Circuits Activity
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.
Simulate the circuit shown below, then describe its
operation in your note book.
12 vo lts
Gary Plimer 2005
Resistors in Parallel
Connect two resistors in
parallel on a prototype
circuit board and measure
the overall resistance
The formula to calculate the
theoretical value of
resistors in parallel is
shown below.
1
RT
1
1
=
+
R1
R2
R1
R2
10 A
mA
V
CO M
Gary Plimer 2005
Worked Example
Calculate the resistance of the parallel branch and the total
circuit resistance.
The resistance values are R1 = 270 R, R2 = 100 R and for the
buzzer 240 R.
R1
R2
12 volts
Gary Plimer 2005
Pupil Activity
(Parallel Circuits)
Task:
Build the circuit,
Measure the voltage
over each of the
bulbs. Enter your
findings into a table.
Gary Plimer 2005
Current in Parallel Circuits
There are two important points to remember about
resistors in parallel.
1) The voltage drop across each resistor is the same.
2) The sum of the currents through each resistor is equal
to the current flowing from the voltage source.
I
I
1
I
T
I
2
T
Gary Plimer 2005
Worked Example
The resistance values are R1 = 270 R, R2 = 100 R and for the
buzzer 240 R.
R1
R2
12 volts
Your teacher will work through this problem on the white board.
Gary Plimer 2005
Pupil Problems
For the circuit shown below calculate:
(a) The total resistance of the circuit
(b) The circuit current.
9V
Gary Plimer 2005
Pupil Problems
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
Gary Plimer 2005
Pupil Problems
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).
(d) the current flowing through R2 (470 R).
240 V
Gary Plimer 2005
Pupil Problems
A 6 R resistor and a 75 R resistor are connected in parallel
across a voltage supply of 12 V. Calculate the circuit
current.
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
Gary Plimer 2005
Combined Series & Parallel
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.
Gary Plimer 2005
Combined Series & Parallel
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.
Gary Plimer 2005
Worked Example 2
For the combined series and parallel circuit shown,
calculate:




The total circuit resistance (RT)
The circuit current (IC)
The voltage drop across resistor R1 (VR1)
The current through resistor R2 (I2).
48R
24R
10R
12V
Gary Plimer 2005
Pupil Problems
For the circuit shown calculate:
 The resistance of the parallel combination
 The total circuit resistance.
 The branch currents
7.5 V
Gary Plimer 2005
Pupil Problems
For the circuit shown calculate:
 The total resistance
 The circuit current
 The branch current
 The voltage drop across each resistor.
24 V
Gary Plimer 2005
Pupil Problems
For the circuit shown calculate:




The total resistance of the circuit
The circuit current
The current through each resistor
The voltage drop across each resistor.
110 V
Gary Plimer 2005
Power in 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)
Gary Plimer 2005
Worked Example
P = V
An electric household lamp consumes
60 watts from a 240 volt supply.
Calculate the current drawn by the
lamp and the resistance of the lamp.
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
Gary Plimer 2005
Pupil Problems
In the following simplified circuit for a vacuum cleaner
motor, calculate:
 The power consumption of the motor
 The voltage of the lamp
 The total power drawn from the power supply.
3 amp s
240 volts
M
200 volts
Gary Plimer 2005
Pupil Problems
The torch circuit below is supplied with
two 4.5 volt batteries connected in
series, with the current being 20 mA.
Switc h
Determine:
 The resistance of the bulb
100 o hm s
 The voltage across the bulb
 The total power drawn from the
supply
 The power drawn by the bulb.
Bulb
Gary Plimer 2005
Pupil Problems
 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 half- heat setting?
 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.
 An electric drill draws a current of 1.5 amps from a 110 volt
supply. Calculate the power rating of the drill.
Gary Plimer 2005
Pupil Problems
 An emergency power generator has to drive 80 lamps. Each
lamp takes 60 W at 230 V. Calculate the current through each
bulb if:

They are connected in series.

They are connected in parallel.

How many 150 W lamps can be connected in parallel to a
250 V supply through a 5 A fuse?
Gary Plimer 2005
Pupil Problems
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.
Gary Plimer 2005
Voltage Dividers
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.
0V
0V
Gary Plimer 2005
Voltage Dividers Activity
Build a voltage divider
circuit using any 2 values
of resistor.
Using the multimeter
measure the voltage drop
over R2.
Volts
VS
R1
This voltage is known as
Vo or the output voltage
from the divider.
10 A
R2
0V
mA
V
CO M
Gary Plimer 2005
Voltage Dividers Activity
Measure the resistance of the 2 resistors from the last activity.
Enter the values into the formula below and calculate Vo.
Simulate the circuit using croc clips and measure Vo.
Hopefully! The value of Vo should be the same in all three
cases, (within reason).
VO
R2
=
R1 + R 2
VS
Gary Plimer 2005
Worked Example
VS = 12 volts
V2 = VS
R 1 = 80k
R 2 = 40k
0 volts
V2
R2
R1 + R2
40
V2 = 12
40 + 80
V2 = 4 volts
Gary Plimer 2005
Pupil Problems
Calculate Vo in the following exercises
VS = 12 volts
VS = 12 volts
R 1 = 270R
R 2 = 810R
0 volts
R 1 = 390R
R 2 = 10K
V2
0 volts
V2
Gary Plimer 2005
Pupil Problems
Calculate Vo in the following exercises
VS = 6 volts
VS = 9 volts
R 1 = 10K
R 2 = 47K
0 volts
R 1 = 10K
R 2 = 2.2K
V2
0 volts
V2
Gary Plimer 2005
Switches

Switches are useful input devices (transducers).

There are several ways in which the contacts in
mechanical switches can be operated. Such as push
button, key, slide, toggle, magnetic (reed) and tilt.

These switches are digital input devices as they can
only be on or off.

The contacts on a switch can be NO or NC (normally
open, normally closed)
Gary Plimer 2005
Switch Contacts
Types of switch contacts:
SPST (Single Pole Single Throw)
SPDT (Single Pole Double Throw)
Double-pole single-throw switch (DPST)
Double-pole double-throw switch (DPDT)
Gary Plimer 2005
Pupil Activity
Copy the circuit into your note book. Simulate the circuit using
Croc Clips, then describe in your own words how the circuit
operates.
Gary Plimer 2005
Analogue Transducers
 A thermistor is a device whose resistance varies with
temperature. It is a temperature-dependent resistor. There
are two main types.

Negative temperature coefficient (t or NTC) – where
resistance decreases as temperature increases.

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 on the next slide.
Gary Plimer 2005
Thermistor
NTC is the most common
thermistor
Gary Plimer 2005
Data Charts
In your Data book
you should find a
graph which
describes how the
resistance of a
thermistor changes
with temperature.
Your teacher will
work through the
use of the chart.
Thermistor types
Gary Plimer 2005
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
Gary Plimer 2005
Light Dependent Resistor
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).
Gary Plimer 2005
Pupil Activity
Use your Data Book to find the resistance of an ORP12
LDR for the following light conditions:
a) 10 Lux
b) 40 Lux
c) 100 Lux
Gary Plimer 2005
Pupil Activity
1) Copy the circuit shown
below into your note book.
2) Using the Electrical
Modular Boards, construct
the voltage divider circuit.
VS = 9 volts
3) Using a multimeter
measure Vo.
R 1 = 10K
4) Warm the thermistor up
with your fingers and re
measure Vo.
5) Describe the operation of
the voltage divider.
6) Reverse the position of
the thermistor and
resistor. Repeat 3,4 & 5.
-t
VO
0 volts
Gary Plimer 2005
Pupil Activity
1) Copy the circuit shown below
into your note book.
VS = 9 volts
2) Using the Electrical Modular
Boards, construct the voltage
divider circuit.
10K
3) Using a multimeter measure Vo.
Cover the LDR up with your
hand and re measure Vo.
ORP12
4) Describe the operation of the
voltage divider.
5) Reverse the position of the
LDR and resistor. Repeat 3,4 &
5. Describe what is happening.
0 volts
VO
Gary Plimer 2005
Pupil Activity
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.
Build a voltage divider using a potentiometer. Check its
operation by measuring Vo from the voltage divider.
Gary Plimer 2005
Potentiometers
Some more examples of potentiometers.
Gary Plimer 2005
Voltage Divider Sensitivity
VS = 9 volts
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).
ORP12
47K
This allows us to fine tune
the sensitivity of the voltage
divider.
0 volts
VO
Gary Plimer 2005
Pupil Activity
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.
VS = 9 volts
Build the following circuit using
modular circuit boards.
10K
Adjust the variable resistor so
as Vo goes higher when your
hand is about 100mm above the
LDR
ORP12
V
0 volts
Gary Plimer 2005
Pupil Problems
Calculate the voltages that would appear across each of
the resistors marked ‘X’ in the circuits below.
9V
0V
5V
0V
6v
0v
Gary Plimer 2005
Pupil Problems
In each of the following voltage divider circuits determine
the unknown quantity.
12 V
0V
16 V
12 V
0V
0V
Gary Plimer 2005
Pupil Problems
In each of the following voltage divider circuits determine the
unknown quantity.
15 V
20 V
220R
0V
0V
0V
Gary Plimer 2005
Pupil Problems
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: 80C OR 20C.
Gary Plimer 2005
Pupil Problems
What would happen to the voltage across the
thermistor in the circuit shown previously as the
temperature increased?
What would happen to the voltage across the
resistor in the circuit shown previously as the
temperature increased?
Gary Plimer 2005
Pupil Problems
A thermistor (type 5) is used in a voltage divider circuit as shown
below. The characteristics of the thermistor are shown in your
Data Book. 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
Gary Plimer 2005
Voltage Dividers
We have seen that Voltage Dividers, divide the voltage
depending on the value of resistors used. In addition, if we
include a variable resistor, we can alter the sensitivity of the
voltage divider.
If we include a thermistor, we can measure changes in
temperature.
If we include a LDR, we can measure changes in light levels.
If we include a potentiometer, we can measure changes in
position.
Gary Plimer 2005
Transistors
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.
Transistors come in many variations and sizes.
However, they all are reliable, efficient, small and
relatively cheap.
Gary Plimer 2005
Transistors

A transistor is an electronic
switch

Transistors amplify current
which enables them to drive
heavy loads such as motors

A voltage of 0.7V will switch
on a NPN transistor
Base
Collector
Emitter
NPN Bipolar Transistor
Gary Plimer 2005
Transistors Activity
9V
La mp
c
b
A
Simulate the circuit using Croc
Clips. Fill in the table shown
below.
BC 108
1K
e
Base resistor
value (K)
V
2200
0V
1000
470
220
100
47
33
22
10
1
Base/Emitter
Voltage (mV)
Base current
(A)
Lamp
on/off
Gary Plimer 2005
Transistors
Your teacher will work through a
number of problems which will
look at calculating:
a) Base current
M
b) Voltage drop over R
c) Base – Emitter voltage
Vin
R
Gary Plimer 2005
Transistors Activity
 Build the following transistor
circuit using modular boards.
 Adjust the voltage reaching the
transistor base by altering the
value the potentiometer.
 At what voltage does the
transistor switch on?
 Measure the current flowing to
the base.
 Now measure the current flowing
in the collector leg.
 What is the transistor doing?
5V (B)
5V (A)
10K
Buzzer
1k
Gary Plimer 2005
Relays
Although relays are often considered to be
output devices, they are really output
switches from electric or electronic circuits.
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.
Gary Plimer 2005
Relays
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.
Gary Plimer 2005
Relays – Protection Diode
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.
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.
Gary Plimer 2005
Solenoid
A solenoid is another output transducer that has a coil inside.
Circuits containing a solenoid require a protective diode as
well.
Gary Plimer 2005
Pupil Activity - Relay


Build the following circuit
using modular boards
Change the resistance of
the thermistor by heating
it up with your hand
+ 6 volts
-t
Re la y
c
b
1K
BC 108
Listen carefully, you
10K
should hear the relay
contacts opening and
0 vo lts
closing
Now add a bulb and 5V power supply to the output side of the
relay and heat the thermistor up again, (your teacher will
demonstrate the correct connections). The bulb should light when
the thermistor is hot.

e
It might be necessary to replace the 10K with a potentiometer.
Gary Plimer 2005
DPDT Relay
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.

Simulate a sensing circuit using an LDR in a voltage divider

Add a transistor driving circuit and a DPDT relay

Connect the relay up so as the motor drives clockwise and
anticlockwise depending on the amount of light hitting the
LDR
TO SENSOR
CIRCUIT
0V
+V
Gary Plimer 2005
Motor Reversal Circuit
Gary Plimer 2005
Capacitors
Capacitors are electronic components that store electricity for
short periods of time within electronic circuits or networks.
ELECTROLYTIC
AXIAL
CAPACITATOR
RADIAL
CAPACITATOR
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.
Gary Plimer 2005
Capacitors
Polyester capacitors are for small-value uses and can be
connected without regard to polarity.
Capacitance in measured in farads, but because this is a
very large measurement most capacitors are rated in F
(microfarads) or in nF (nanofarads).
POLYESTER
Gary Plimer 2005
Pupil Activity
9V
 Construct the following circuit
 Allow the capacitor to charge up
 Connect the end of the LED to 0V
 The LED should light up for a short
period of time
10
K
+
100uF
0V
Gary Plimer 2005
555 Integrated Circuit
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.
Gary Plimer 2005
Pupil Activity - Monostable
+ 6 volts
Simulate the circuit
shown, press the switch
and observe the circuit
operation.
100K
1K
390R
1K
7
Monostable means one
stable state. The light
comes on but always
goes back to its original
state.
6
100 F
0 volts
8
4
IC 1
555
2
1
3
Gary Plimer 2005
ASTABLE CIRCUIT
Gary Plimer 2005
Pupil Activity - Astable
+ 6vo lts
1K
555
LDR 1 K
3 90 R
+
0 vo lts
1 00 u F
1 uF
LED
Build the following circuit on Breadboard, describe its operation.