Transcript Electronics 4.4: Digital Processes

Electronics 4.4: Digital Processes
Contents
Page Title
1)
2)
3)
Contents Page
Header Page
Learning Outcomes
4)
5)
6)
7)
8)
9)
10)
11)
12)
Introducing Transistors
Transistor Terminals
Transistor as a Switch
How Transistors Work
Transistor Switching Example
Transistor Circuit #1: Temperature-Controlled Circuit
Transistor Circuit #2: Light-Controlled Circuit
Transistor Circuit #3: Time-Controlled Circuit
Summary of Transistor Switching Circuits
13)
14)
15)
16)
17)
18)
19)
20)
21)
22)
23)
Logic
Revision: Digital Signals
Introduction to Logic
Logic: Switches in Series
Logic: Switches in Parallel
Logic: Opposites!
Truth Tables
Logic Gates: AND
Logic Gates: OR
Logic Gates: NOT
Summary of Logic Gates and Truth Tables
24)
25)
26)
27)
28)
Combinational Logic Circuits
Logic Circuit #1: Car’s Hot Engine
Logic Circuit #2: Central Heating Pump
Logic Circuit #3: Greenhouse Heater
Summary of Combinational Logic Circuits
Page Title
29)
30)
31)
32)
33)
Clocks
A Simple Oscillator Circuit
How an Oscillator Circuit Works
How an Oscillator Circuit Works (Alternative)
How to Change a Clock’s Frequency
34)
35)
36)
Counters
Counting in Decimal
Devices Using Counters
1
Electronics 4.4: Digital Processes
Electronics 4.4: Digital Processes
Electronics Section 4.4
Digital Processes
2
Electronics 4.4: Digital Processes
4.4 Digital Processes: Learning Outcomes
Transistor as a Switch
1) (G) State that a transistor can be used as a switch.
2) (G) State that a transistor may be conducting or nonconducting, ie on or off.
Simple Switching Systems
3) (G) Draw and identify the circuit symbol for an NPN transistor.
4) (G) Identify from a circuit diagram the purpose of a simple transistor switching circuit.
15) (C) Explain the operation of a simple transistor switching circuit.
Digital Logic Gates
5) (G) Draw and identify the symbols for two-input AND, OR and NOT gates.
6) (G) State that logic gates may have one or more inputs and that a truth table shows the
output for all possible input combinations.
7) (G) State that high voltage = logic 1, low voltage = logic 0.
8) (G) Draw the truth tables for AND OR and NOT gates.
16) (C) Identify the following gates from truth tables: AND, OR, NOT.
Combinational Logic Circuits
9) (G) Explain how to use combinations of digital logic gates for control in simple situations.
17) (C) Complete a truth table for a simple combinational logic circuit.
Clock Signals
10) (G) State that a digital circuit can produce a series of clock pulses.
18) (C) Explain how a simple oscillator built from a Resistor, Capacitor and Inverter operates.
19) (C) Describe how to change the frequency of a clock.
Counters
11) (G) Give an example of a device containing a counter circuit.
12) (G) State that there are circuits which can count digital pulses.
13) (G) State that the output of the counter circuit is in binary.
14) (G) State that the output of a binary counter can be converted to decimal.
3
Electronics 4.4: Digital Processes
Introducing Transistors
• Transistors are process devices.
• This is the symbol for an NPN transistor.3
4
Electronics 4.4: Digital Processes
Transistor Terminals
• Transistors have three terminals:
Collector
Base
Emitter
5
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
6
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.
7
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.
8
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.
9
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.
10
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.
11
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
12
Electronics 4.4: Digital Processes
Logic
13
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
14
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.
15
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.
16
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.
17
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?
18
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
19
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”.
20
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”.
21
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”.
25
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!
28
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”.
29
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.
30
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
32
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.
33
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
34
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
35
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!
36
Electronics 4.4: Digital Processes
Future Improvements
• Clip Art
• Imagination
• Summarise
37