Lisnagarvey High School
TECHNOLOGY AND DESIGN
GCSE
Electronic Systems Handbook
Name:
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1
Systems:
Understand that systems include a function, performance and may have feedback.
Input, process and output:
Understand that electronics systems may contain input-process-output.
Units and Measurements:
Understand and apply electrical units to measure current, voltage and resistance. Use of
multimeter for direct measurement of current, voltage and resistance.
Voltage and Current
Voltage and Current are vital to understanding electronics, but they are quite hard to grasp
because we can't see them directly.
Voltage is the Cause, Current is the Effect
Voltage attempts to make a current flow, and current will flow if the circuit is complete. Voltage
is sometimes described as the 'push' or 'force' of the electricity, it isn't really a force but this may
help you to imagine what is happening. It is possible to have voltage without current, but current
cannot flow without voltage.
Voltage and Current
Voltage but No Current
No Voltage and No Current
The switch is closed making a
complete circuit so current can flow.
The switch is open so the circuit is
broken and current cannot flow.
Without the cell there is no source
of voltage so current cannot flow.
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A. Voltage (V)
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Voltage is a measure of the energy carried by the
charge.
The proper name for voltage is potential difference or
p.d. for short, but this term is rarely used in electronics.
Voltage is supplied by the battery (or power supply).
Voltage is used up in components, but not in wires.
We say voltage across a component.
Voltage is measured in volts, V.
Voltage is measured with a voltmeter, connected in
parallel.
The symbol V is used for voltage in equations.
Connecting a voltmeter in parallel
Voltage at a point and 0V (zero volts)
Voltage is a difference between two points, but in electronics
we often refer to voltage at a point meaning the voltage
difference between that point and a reference point of 0V (zero
volts).
Zero volts could be any point in the circuit, but to be consistent it
is normally the negative terminal of the battery or power
supply. You will often see circuit diagrams labelled with 0V as a
reminder.
You may find it helpful to think of voltage like height in geography. The
reference point of zero height is the mean (average) sea level and all heights are measured from that point. The
zero volts in an electronic circuit is like the mean sea level in geography.
B. Current (I)
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Current is the rate of flow of charge.
Current is not used up, what flows into a component must
flow out.
We say current through a component.
Current is measured in amps (amperes), A.
Current is measured with an ammeter, connected in series.
To connect in series you must break the circuit and put the ammeter
across the gap, as shown in the diagram.

The symbol I is used for current in equations.
1A (1 amp) is quite a large current for electronics, so mA (milliamps) are often used. m (milli)
means "thousandth":
1mA = 0.001A, or 1000mA = 1A
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Conductors and Insulators
Understand the use of conductors and insulators.
Conductors – allow the flow of electrical current through them e.g all metals, water, graphite
(the lead in a pencil). The best conductors are aluminium and copper. Although water is not a
very good conductor, it is very dangerous to touch electrical items with wet hands.
Insulators – do not allow the flow of electrical current through them e.g rubber, plastic, wood.
Insulators are used to protect people from electricity which is why electrical cables are coated
with plastic or rubber.
Resistors - (see table later for what they are for)
Use of colour coding system to identify values of individual resistors.
Resistor Colour Codes
The Resistor
Colour Code
Colour
Number
Black
0
Brown
1
Red
2
Orange
3
Yellow
4
Green
5
Blue
6
Violet
7
Grey
8
White
9
Resistance is measured in ohms, the symbol for ohm is an omega
1 is quite small so resistor values are often given in k and M .
1 k = 1000
1 M = 1000000 .
.
Resistor values are normally shown using coloured bands.
Each colour represents a number as shown in the table.
Most resistors have 4 bands:
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The first band gives the first digit.
1 2 3 4
The second band gives the second digit.
The third band indicates the number of zeros.
The fourth band is used to shows the tolerance (precision) of the resistor,
this may be ignored for almost all circuits.
Ex1. A resistor with value 470 would be coloured yellow, violet, brown
Ex2. A resistor with value 10k (10000) would be brown, black, orange
Ex3. A resistor coloured red, red, red would have a value 2200 (2.2k)
Tolerance of resistors (fourth band of colour code)
The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the
precision of the resistor and it is given as a percentage. For example a 390 resistor with a
tolerance of ±10% will have a value within 10% of 390 , between 390 - 39 = 351 and 390 +
39 = 429 (39 is 10% of 390).
A special colour code is used for the fourth band tolerance:
silver ±10%, gold ±5%, red ±2%, brown ±1%.
If no fourth band is shown the tolerance is ±20%.
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Calculate the resistance of two or more resistors in series using: Rtotal = R1+R2+R3
Resistors connected in Series
When resistors are connected in series their combined resistance is equal to the individual
resistances added together. For example if two resistors R1 and R2 are connected in series
their combined resistance, R, is given by:
Combined resistance in series: R = R1 + R2
This can be extended for more resistors:
R = R1 + R2 + R3 + R4 + ...
Note that the combined resistance in series will always be
greater than any of the individual resistances.
e.g if R1 = 470 and R2 = 1k then R = 470 + 1000 = 1470
Ohm's Law (Relationship between Voltage, Current and Resistance)
To make a current flow through a resistance there must be a voltage across that resistance.
Ohm's Law shows the relationship between the voltage (V), current (I) and resistance (R). It can
be written in three ways:
V=I×R
where:
or
I=
V = voltage in volts (V)
I = current in amps (A)
R = resistance in ohms ( )
V
R
or:
or
R=
V
I
V = voltage in volts (V)
I = current in milliamps (mA)
R = resistance in kilohms (k )
For most electronic circuits the amp is too large and the ohm is too small, so we often measure
current in milliamps (mA) and resistance in kilohms (k ).
1 mA = 0.001 A and 1 k
= 1000
.
The Ohm's Law equations work if you use V, A and
, or if you use V, mA and k .
You must not mix these sets of units in the equations so you may need to convert between mA
and A or k and .
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The VIR triangle
You can use the VIR triangle to help you remember the 3 versions of Ohm's Law.
Write down V, I and R in a triangle like the one in the yellow box on the right.
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To calculate voltage, V: put your finger over V,
this leaves you with I R, so the equation is V = I × R
To calculate current, I: put your finger over I,
this leaves you with V over R, so the equation is I = V/R
To calculate resistance, R: put your finger over R,
this leaves you with V over I, so the equation is R = V/I
V
I
R
Ohm's Law
triangle
Ohm's Law Calculations (It should be Very Easy Now!)
Use this method to guide you through calculations:
1. Write down the Values, converting units if necessary.
2. Select the Equation you need (use the VIR triangle).
3. Put the Numbers into the equation and calculate the answer.
V
I
Example 1
3 V is applied across a 6
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Example 2
R
resistor, what is the current?
Values: V = 3 V, I = ?, R = 6
Equation: I = V/R
Numbers: Current, I = 3/6 = 0.5 A
A lamp connected to a 6 V battery passes a current of 60 mA,
What is the lamp's resistance?
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Example 3
Values: V = 6 V, I = 60 mA, R = ?
Equation: R = V/I
Numbers: Resistance, R = 6/60 = 0.1 k = 100
(using mA for current means the calculation gives resistance in k )
A 1.2 k
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resistor passes a current of 0.2 A, What is the voltage?
Values: V = ?, I = 0.2 A, R = 1.2 k = 1200
(1.2 k is converted to 1200 because A and k
together)
Equation: V = I × R
Numbers: V = 0.2 × 1200 = 240 V
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must not be used
Components:
Identify the following components circuit symbols: Identify and select appropriate components
to meet the requirements of a circuit diagram.
Batteries (Power Supplies)
Power Supplies
Component
Circuit Symbol
Function of Component
Supplies electrical energy.
The larger terminal (on the left) is positive (+).
Cell
A single cell is often called a battery, but strictly a battery is
two or more cells joined together.
Supplies electrical energy. A battery is more than
one cell.
The larger terminal (on the left) is positive (+).
Battery
Pictures of 9V PP3 Battery (left) and 1.5V AA Battery (right)
Resistors
Component
Resistor
Variable Resistor
Circuit Symbol
Function of Component
A resistor restricts the flow of current, for example
to limit the current passing through an LED. A
resistor is used with a capacitor in a timing circuit.
This type of variable resistor with 2 contacts (a
rheostat) is usually used to control current.
Examples include: adjusting lamp brightness,
adjusting motor speed, and adjusting the rate of
flow of charge into a capacitor in a timing circuit.
Picture of Resistor:
Pictures of Variable Resistors
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Diodes and Light Emitting Diodes (LEDs)
Diodes
Understand the use of a diode as a one way conductor. Use of a protective diode in circuits
containing inductive devices, for example, motors.
Use LEDs in circuits.
Use current limiting resistors in conjunction with LEDs.
Diodes
Component
Circuit Symbol
Function of Component
Diode
A device which only allows current to flow in one
direction, in the direction of the arrow. It is mainly
used in projects to protect transistors etc from
back EMFs form motors etc.
LED
A special type of diode which converts electrical
energy to light. Only works in one direction. Used
generally to indicate power on in circuits
Light Emitting Diode
Picture of Diodes:
Picture of LED:
Thermistors and LDRs (Input Devices)
Sensors (input devices)
Component
Circuit Symbol
Function of Component
LDR
A transducer which converts brightness (light) to
resistance (an electrical property).
LDR = Light Dependent Resistor. Used in projects
such as automatic night light for a child. Its
resistance normally increases as it gets darker.
Thermistor
A transducer which converts temperature (heat) to
resistance (an electrical property). Used in projects
which sense temperature such as freezer alarm,
automatic fans etc. Its resistance normally
increases as it gets colder.
Picture of LDR
Picture of Thermistor
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Transistors and Thyristors
Transistor
Component
Circuit Symbol
Function of Component
Transistor NPN
A transistor amplifies current. It can be used with other
components to make an amplifier or switching circuit.
Used mainly in projects as a sensitive electronic switch. It
has 3 legs
1. The Base (B) – connected to the sensing part of the
circuit (Potential divider)
2. The Collector (C) – connected to the +ve side of the
circuit
3. The Emitter (E) – Connected to the – ve side of the
circuit.
To switch on, the transistor needs at least 0.7V at its base
Thyristor
Eg C106D
This is very similar to the Transistor in that it has 3 legs.
1.The Gate (G) – similar to the base of a transistor
2. The Anode (A) – similar to the collector of a transistor
3. The Cathode (C) – similar to the emitter of a transistor
A thyristor is more useful than a transistor in alarm
circuits, as it stays latched on even when the voltage is
removed from the gate. To turn it off requires the power
supply to be removed from the anode/cathode.
Transistors
Use an NPN transistor as a switch in a sensing circuit. Identify the base,
collector and emitter of a transistor from a diagram.
BC108
TIP type
Pictures of Transistors:
Thyristors look similar in appearance to The TIP type transistor.
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Buzzers Bulbs and Motors (Output Devices)
Output Devices: Lamps, Heater, Motor, etc.
Component
Circuit Symbol
Function of Component
Lamp
A transducer which converts electrical energy to
light. This symbol is used for a lamp which is an
indicator, for example a warning light on a car
dashboard.
Motor
A transducer which converts electrical energy to
kinetic energy (motion).
Buzzer
A transducer which converts electrical energy to
sound.
Pictures of Buzzers
Pictures of Bulbs
Picture of Motor
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Switching and Switches
Understand the action of common switches by recognising and selecting according to
application:
Toggle Switch
Push Button Switch (PTM and PTB)
Microswitch
Membrane Switch
Slide Switch
Reed Switch
SPST Switch DPDT Switch.
Selecting a Switch
There are three important features to consider when selecting a switch:
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Contacts (e.g. single pole, double throw)
Ratings (maximum voltage and current) – not important for the course
Method of Operation (toggle, slide, key etc.)
Awareness of the different types switches used in familiar products.
For example, control a model vehicle in forward and reverse directions.
Understand the concept of “normally open” and “normally closed”.
Switch Contacts
Several terms are used to describe switch contacts:
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Pole - number of switch contact sets.
Throw - number of conducting positions, single or double.
Way - number of conducting positions, three or more.
Momentary - switch returns to its normal position when released.
Normally Open - off position, contacts not conducting.
Normally Closed - on position, contacts conducting, there may be several on positions.
For example: the simplest on-off switch has one set of contacts (single pole) and one switching
position which conducts (single throw). The switch mechanism has two positions: open (off) and
closed (on), but it is called 'single throw' because only one position conducts.
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Standard Switches
Type of Switch
Circuit Symbol
Example
ON-OFF
Single Pole, Single Throw = SPST
A simple on-off switch. This type can be used to
switch the power supply to a circuit.
When used with mains electricity this type of switch
must be in the live wire, but it is better to use a
DPST switch to isolate both live and neutral.
SPST toggle switch
Push-to-make = SPST Momentary (ON-OFF)
A push-to-make switch returns to its normally open
(off) position when you release the button, this is
shown by the brackets around ON. This is the
standard doorbell switch.
Push-to-make switch
PTM
Push-to-break = SPST Momentary (ON-OFF)
A push-to-break switch returns to its normally
closed (on) position when you release the button.
Push-to-break switch
PTB
Dual ON-ON
Double Pole, Double Throw = DPDT
A pair of on-on switches which operate together
(shown by the dotted line in the circuit symbol).
DPDT slide switch
A DPDT switch can be wired up as a reversing
switch for a motor as shown in the diagram.
Wiring for Reversing Switch
A membrane switch is similar in how it works to a PTM. It is used in the sense
of a pressure pad, which could be positioned under, for example, a mat to
indicate that some-one is standing on it, or a car is driving over something.
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Special Switches
Type of Switch
Example
Push-Push Switch (e.g. SPST = ON-OFF)
This looks like a momentary action push switch but it is a standard
on-off switch: push once to switch on, push again to switch off. This
is called a latching action.
Microswitch (usually SPDT = ON-ON)
Microswitches are designed to switch fully open or closed in
response to small movements. They are available with levers and
rollers attached.
Reed Switch (usually SPST)
The contacts of a reed switch are closed by bringing a small
magnet near the switch. They are used in security circuits, for
example to check that doors are closed. Standard reed switches
are SPST (simple on-off) but SPDT (changeover) versions are also
available.
Warning: reed switches have a glass body which is easily broken!
Potential Dividers Use a potential divider to control voltage in a circuit.
Potential divider (also known as a voltage divider)
A voltage divider consists of two resistances R1 and R2 connected in
series across a supply voltage Vs. The supply voltage is divided up
between the two resistances to give an output voltage Vo which is the
voltage across R2. This depends on the size of R2 relative to R1:

If R2 is much smaller than R1, Vo is small (low, almost 0V)
(because most of the voltage is across R1)
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If R2 is about the same as R1, Vo is about half Vs
(because the voltage is shared about equally between R1 and R2)
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If R2 is much larger than R1, Vo is large (high, almost Vs)
(because most of the voltage is across R2)
One of the main uses of voltage dividers is to connect sensors such as
LDRs, Thermistors etc into circuits... (see next page)
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Vo =
Vs × R2
R1 + R2
Using an input transducer (sensor) in a voltage divider
Most input transducers (sensors) vary their resistance and usually a voltage divider is used to
convert this to a varying voltage which is more useful. The voltage signal can be fed to other
parts of the circuit, such as the input to a chip or a transistor switch.
The sensor is one of the resistances in the voltage divider. It can be at the top (R1) or at the
bottom (R2), the choice is determined by when you want a large value for the output voltage Vo:
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Put the sensor at the top (R1) if you want a large Vo when the sensor has a small
resistance.
Put the sensor at the bottom (R2) if you want a large Vo when the sensor has a large
resistance.
Then you need to choose a value for the resistor...
Choosing a resistor value
The value of the resistor R will determine the range of the output
voltage Vo. For best results you need a large 'swing' (range) for Vo
and this is achieved if the resistor is much larger than the sensor's
minimum resistance Rmin, but much smaller than the sensor's
maximum resistance Rmax.
You can use a multimeter to help you find the minimum and
maximum values of the sensor's resistance (Rmin and Rmax).
There is no need to be precise, approximate values will do.
Then choose resistor value: R = square root of (Rmin × Rmax)
Choose a standard value which is close to this calculated value.
For example:
An LDR: Rmin = 100 , Rmax = 1M , so R = square root of (100 × 1M) = 10k .
swapping over the resistor and sensor
The resistor and sensor can be swapped over to invert the action of
the voltage divider. For example an LDR has a high resistance
when dark and a low resistance when brightly lit, so:
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If the LDR is at the top (near +Vs),
Vo will be low in the dark and high in bright light.
If the LDR is at the bottom (near 0V),
Vo will be high in the dark and low in bright light.
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OR
Using a variable resistor
A variable resistor may be used in place of the fixed resistor R. It
will enable you to adjust the output voltage Vo for a given
resistance of the sensor. For example you can use a variable
resistor to set the exact brightness level which makes a chip
change state.
The variable resistor value should be larger than the fixed resistor
value. For finer control you can use a fixed resistor in series with
the variable resistor. For example if a 10k fixed resistor is suitable
you could replace it with a fixed 4.7k resistor in series with a 10k
variable resistor, allowing you to adjust the resistance from 4.7k to
14.7k .
If you are planning to use a variable resistor connected between the +Vs supply and the base of
a transistor you must include a resistor in series with the variable resistor. This is to prevent
excessive base current destroying the transistor when the variable resistor is reduced to zero.
The use of a potential divider in light and temperature sensing circuits.
Below are 4 useful circuits which use potential dividers connected to a transistor to turn on an
output device (bulb, buzzer, motor, LED etc). (Remember that it is the voltage across the bottom resistor
in the voltage divider which activates the circuit)
1. Low Temperature Sensing Circuit:
When the temperature drops, the resistance of the thermistor
rises and so does the voltage across it. When this voltage rises
above 0.7 V, the transistor switches on, which in turn brings on
the buzzer. This circuit could be used in projects such as an ice
alarm, an old persons temperature alarm etc. The variable
resistor allows adjustment to the temperature at which the
buzzer goes off.
2. High Temperature Sensing Circuit:
In this circuit the thermistor and variable resistor have been
swapped over, therefore this time as the temperature drops, the
resistance across the thermistor again increases and so does the
voltage. However the voltage across the variable resistor has
decreased and it is this voltage which switches on the transistor
this time (always the bottom voltage), so as the temperature
drops the buzzer switches off and as the temperature increases
the buzzer switches on. (Opposite to circuit 1). This circuit
could be used as an automatic fan which switches on when it
gets too warm and off again when it cools down.
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3. Low Light Level Sensing Circuit
This is similar in ways to circuit 1 with the LDR sensor in the
bottom of the potential divider, therefore the circuit will activate
when the light level drops (Resistance of and voltage across the
LDR increases as light levels drop). This could be used as a
child’s night light where the light switches on when it gets dark
and turns off when it gets light again. The variable resistor is
used to adjust how dark it needs to be when it switches on and
off. (Note a bulb has been used instead of a buzzer)
4. High Light Level Sensing Circuit
Similar to circuit where the LDR and variable resistor have
been swapped. Therefore the circuit will do the opposite to
circuit 3 above i.e. the circuit will activate when it gets light.
This probably is not so useful as circuit 3, but a possible use
might be a dawn alarm, where a buzzer or alarm comes on
when it begins to get light.
Construction Techniques Understand the design and manufacture of printed circuit boards.
Translate a circuit diagram for a light sensing circuit to a PCB layout.
Wires and connections
Component
Wire
Wires joined
Wires not joined
Circuit Symbol
Function of Component
To pass current very easily from one part of a
circuit to another.
A 'blob' should be drawn where wires are
connected (joined), but it is sometimes omitted.
Wires connected at 'crossroads' should be
staggered slightly to form two T-junctions, as
shown on the right.
In complex diagrams it is often necessary to draw
wires crossing even though they are not
connected. I prefer the 'bridge' symbol shown on
the right because the simple crossing on the left
may be misread as a join where you have forgotten
to add a 'blob'!
Identify constructional faults on Components located incorrectly a PCB. and/or dry joints.
Printed Circuit Boards (PCBs) – are generally well made and the most common faults are either
components have been put in the wrong way or the components have not been soldered to the
board correctly creating a dry joint.
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