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Electronics 4.4: Digital Processes
Electronics 4.4: Digital Processes
Transistors and
Digital Electronics
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Electronics 4.4: Digital Processes
Introducing Transistors
• Transistors are process devices.
• This is the symbol for an NPN transistor.3
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Electronics 4.4: Digital Processes
Transistor Terminals
• Transistors have three terminals:
Collector
Base
Emitter
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Electronics 4.4: Digital Processes
Transistor as a Switch
• Transistors can be used as switches.1
Transistor
Switch
• Transistors can either
conduct or not conduct current.2
• ie, transistors can either be on or off.2
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Electronics 4.4: Digital Processes
How Transistors Work
Collector
• Switching is
controlled by
the voltage
between the
Base and the
Emitter.
Base
Emitter
• When VBE < 0.7V the transistor switches off and
no current flows between the Collector and the Emitter.
• When VBE ≥ 0.7V the transistor switches on and
current flows between the Collector and the Emitter.
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Electronics 4.4: Digital Processes
Transistor Switching Example15
X
12V
Variable
Voltage
Supply
• When VBE is less than 0.7V the transistor is off
and the lamp does not light.
• When VBE is greater than 0.7V the transistor is on
and the lamp lights.
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Electronics 4.4: Digital Processes
Transistor Circuit #1: Temperature-Controlled Circuit
• This transistor circuit contains
a Thermistor.
• Because of the thermistor, this
circuit
is
dependent
on
temperature.
• The purpose of this circuit is to
turn on the LED when the
temperature reaches . . .
Input = Voltage Divider
Process = Transistor
Output = LED
1)
2)
3)
4)
5)
6)
7)
LED = Off.
Heat the Thermistor.
RThermistor .
VThermistor .
Voltage across 10k resistor .
Transistor switches on.
LED = On.
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Electronics 4.4: Digital Processes
Transistor Circuit #2: Light-Controlled Circuit
• This transistor circuit contains
a Light-Dependent Resistor.
• Because of the LDR, this circuit
is dependent on light.
• The purpose of this circuit is to
turn on the LED when the light
reaches a certain intensity.
Input = Voltage Divider
Process = Transistor
Output = LED
1)
2)
3)
4)
5)
6)
LED = Off.
Cover LDR.
RLDR .
VLDR .
Transistor switches on.
LED = On.
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Electronics 4.4: Digital Processes
Transistor Circuit #3: Time-Controlled Circuit
• This transistor circuit contains
a Capacitor.
• Because of the capacitor, this
circuit is dependent on the time
taken to charge and discharge
of the capacitor.
• The purpose of this circuit is to
turn on the LED a short time
after the switch is opened.
Input = Voltage Divider
Process = Transistor
Output = LED
1)
2)
3)
4)
Switch closed.
VC = 0V.
Transistor switches off.
LED = Off.
• Where would this circuit be found
in a car?
5) Open Switch.
6) VC .
7) Transistor switches on after
a short delay.
8) LED = On.
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Electronics 4.4: Digital Processes
Summary of Transistor Switching Circuits
• You are expected to know the purpose of a transistor switching
circuit: the last three pages should help.4
• In each of the three circuits the input device is:
• A Voltage Divider using a
Thermistor
LDR
Capacitor
• In each of the three circuits the output device is:
an LED
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Electronics 4.4: Digital Processes
Logic
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Electronics 4.4: Digital Processes
Revision: Digital Signals
• From Section 4.2 Output Devices, remember
that digital signals have only two values,7
• “1” and “_”, or
• “High Voltage” and “___ _______”, or
• “On” and “___”, or
• “True” and “_____”.
On
1
Off
0
High Voltage
Low Voltage
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Electronics 4.4: Digital Processes
Introduction to Logic
• Many digital electronic processes are
designed around “logic” circuits.
• The Inputs and Outputs in logic have only two values:
• 0 & 1;
• High & Low;
• On & Off;
• True and False.
• Logic is ideally suited to help design
digital electronic circuits because of its binary nature.
• We will look at some fundamental logic circuits.
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Electronics 4.4: Digital Processes
Logic: Switches in Series
S1
S2
S1
S2
Lit
0
0
1
1
0
1
0
1
0
0
0
1
• The bulb will light only under certain conditions: what?
Complete the following:
• The bulb will turn on only when switches S1 ___ S2 are
closed, for all other combinations the bulb is off.
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Electronics 4.4: Digital Processes
Logic: Switches in Parallel
S1
S1
S2
Lit
0
0
1
1
0
1
0
1
0
1
1
1
S2
• The bulb will light under certain conditions: what?
Complete the following:
• The bulb will turn on when switches S1 ___ S2 are
closed, for all other combinations the bulb is off.
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Electronics 4.4: Digital Processes
Logic: Opposites!
S
S
0
1
Lit
1
0
• The bulb will light under certain conditions: what?
Complete the following:
• The bulb will turn on when switch S is ____, and turn
off when switch S is ______.
• This circuit is for illustration only!
• If this was a real circuit, what would happen to the battery
when switch S was closed?
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Electronics 4.4: Digital Processes
Truth Tables
• The tables on the previous pages are truth tables.
Truth Tables list:
• All combinations of all possible inputs,
• Every Output for each combination of inputs.
• There are special circuits called logic gates which
can be used in control situations.
S1
S2
Lit
S1
S2
Lit
S1
Lit
0
0
1
1
0
1
0
1
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
1
0
1
1
0
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Electronics 4.4: Digital Processes
Logic Gates: AND
AND
Truth Table8,16
Two-Input AND Gate5
A
B
Q
0
0
1
1
0
1
0
1
0
0
0
1
The output of an AND gate is 1 only when all inputs are 1.
Only when Input A AND Input B are 1, the output is 1.
• See page “Logic: Switches in Series”.
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Electronics 4.4: Digital Processes
Logic Gates: OR
OR
Truth Table8,16
Two-Input OR Gate5
A
B
Q
0
0
1
1
0
1
0
1
0
1
1
1
The output of an OR gate is 1 when any input is 1.
When Input A OR Input B is 1, the output is 1.
• See page “Logic: Switches in Parallel”.
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Electronics 4.4: Digital Processes
Logic Gates: NOT
NOT
Truth Table8,16
NOT Gate5
A
Q
0
1
1
0
Note that NOT gates have only one input.
The output of a NOT gate is the opposite of the input.
When Input A is 0, the output is 1.
When Input A is 1, the output is 0
• See page “Logic: Opposites!”.
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Electronics 4.4: Digital Processes
Summary of Logic Gates and Truth Tables
• Logic gates may have one or more inputs.6
AND Gate
A
B
Q
0
0
1
1
0
1
0
1
0
0
0
1
OR Gate
A
B
Q
0
0
1
1
0
1
0
1
0
1
1
1
NOT Gate
A
Q
0
1
1
0
Truth Tables list:6
• Every Output for every combination of inputs.
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Electronics 4.4: Digital Processes
Combinational Logic Circuits9,17
• Combinational Logic Circuits are simply circuits using a
combination of AND, OR and NOT gates.
• You are expected to design Logic Circuits and
Truth Tables of simple combinational logic circuits.
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Electronics 4.4: Digital Processes
Logic Circuit #1: Car’s Hot Engine
• When a car’s engine becomes too hot an LED should light
but only when the ignition is switched on.
Ignition
Switch
1
LED
Temperature 1
Sensor
Truth Table
Ignition Temperature
Switch
Sensor
Off
Off
On
On
Cold
Hot
Cold
Hot
Output
LED
Off
Off
Off
On
• Here, the truth table is simply
that for an AND Gate.
• For the LED to light, the
Ignition Switch must be on and
the Temperature Sensor must
be “hot”.
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Electronics 4.4: Digital Processes
Logic Circuit #2: Central Heating Pump
• Derive a logic circuit that will turn on a Central Heating System’s
pump when the house is cold and the Central Heating System is
turned on.
This time let ’ s find the truth table
first:
• House is Cold = 0 ; House is Hot = 1
• CHS is Off = 0; CHS is On = 1
Central
Heating
1
0
Temperature
Sensor
1
Pump
Truth Table
House CHS
Pump
Cold
Cold
Hot
Hot
Off
On
Off
On
Off
On
Off
Off
House
CHS
Pump
0
0
1
1
0
1
0
1
0
1
0
0
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Electronics 4.4: Digital Processes
Logic Circuit #3: Greenhouse Heater
• Derive a logic circuit that will turn on a heater in a greenhouse only
when it gets cold at night.
Truth Table:
• Greenhouse Cold = 0 ; Hot = 1
• Dark = 0; Light = 1
Light
Sensor
Temperature
Sensor
0
1
0
1
Truth Table
Green Day/ Heater
house Night
Heater
Cold
Cold
Hot
Hot
Night
Day
Night
Day
On
Off
Off
Off
Green
D/N
Heater
0
0
1
1
0
1
0
1
1
0
0
0
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Electronics 4.4: Digital Processes
Summary of Combinational Logic Circuits
• Combinational Logic Circuits are simply combinations
of AND, OR and NOT gates.
Constructing Logic Circuits
1) Make a Truth Table.
2) Get the logic circuit from the Truth Table.
• Tip: If the circuit has only one “high” output
then the circuit will probably use an AND Gate.
• Tip: If the circuit has more than one “ high ”
output
then the circuit will probably use an OR Gate.
• Tip: Note how useful NOT gates are!
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Electronics 4.4: Digital Processes
Clocks
• Clocks are regular waves of pulses,
just like the ticking of a conventional clock:
• Digital circuits can be used to produce
a series of clock pulses.10
• Clocks are normally square waves.
• The circuits which produce clock pulses are sometimes
called oscillators because they constantly oscillate
between “on” and “off”.
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Electronics 4.4: Digital Processes
A Simple Oscillator Circuit
• Oscillator Circuits change between two values
in a regular cyclical pattern: a clock output.
• Supply Voltage VS = V1 +V2
• When the NOT-Gate outputs a 0,
V2=0V and V1=5V:
the LED lights.
• When the NOT-Gate outputs a 1,
V2=5V and V1=0V:
the LED does not light.
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Electronics 4.4: Digital Processes
How an Oscillator Circuit Works18
• Capacitor C charges and discharges through Resistor R.
•Start:
ASSUME THE
CAPACITOR IS
FULLY CHARGED.
1) The Invertor’s Input is 1, so its Output = 0:
the Capacitor starts to discharge through the Resistor.
2) As the Capacitor discharges the Invertor’s Input eventually falls to
0, so its Output becomes 1:
the Capacitor starts to charge through the Resistor.
3) As the Capacitor charges the Invertor’s Input eventually rises to 1,
so its output becomes 0: the Capacitor discharges again.
4) This sequence of charging and discharging continues ad infinitum
to produce a series of clock pulses.
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Electronics 4.4: Digital Processes
How an Oscillator Circuit Works (Alternative)
Capacitor
NOT
Input
NOT
Output
V2
V1
LED
Charged
1
0
0V
5V
On
Discharged
0
1
5V
0V
Off
Charged
1
0
0V
5V
On
Discharged
0
1
5V
0V
Off
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Electronics 4.4: Digital Processes
How to Change a Clock’s Frequency19
• The frequency of clock pulses can be altered:
High Frequency
Low Frequency
• If the value of the Capacitor is increased,
charging and discharging takes longer so
the clock frequency is decreased.
C then f
• If the value of the Resistor is increased,
charging and discharging takes longer so R then f
the clock frequency is decreased.
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Electronics 4.4: Digital Processes
Counters
• Counters are electronic circuits which
can count digital pulses from a clock.12
1
2
3
4
• Counters count the clock pulses in binary.13
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Electronics 4.4: Digital Processes
Counting in Decimal
• Circuits called Binary-to-Decimal Convertors
convert a counter’s binary output into decimal.14
Binary
Decimal
0000
0001
0010
etc
etc
0111
1000
1001
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Electronics 4.4: Digital Processes
Devices using Counters
• You will be expected to name a device
which uses a counter.
• The most common device to use
a counting circuit is an electronic clock or watch.11
• Electronic timing devices work
with great accuracy.
• An electronic watch’s “clock circuit”
generates regular pulses and a
Counter simply counts these.
• The watch’s microprocessor is programmed to know
how many clock pulses correspond to a second (and
minute, hour etc) and will update the 7-segment display
accordingly: thus displaying the time!
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