Digital Processes

8/15/2019 Digital Processes

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Page Title2) Contents Page3) Header Page4) Learning Outcomes

6) Introducing Transistors7) Transistor Terminals8) Transistor as a Switch9) How Transistors Work10) Transistor Switching Example11) Transistor Circuit #1: Temperature-Controlled Circuit12) Transistor Circuit #2: Light-Controlled Circuit13) Transistor Circuit #3: Time-Controlled Circuit14) Summary of Transistor Switching Circuits

16) Logic17) Revision: Digital Signals18) Introduction to Logic19) Logic: Switches in Series20) Logic: Switches in Parallel21) Logic: Opposites!22) Truth Tables23) Logic Gates: AND24) Logic Gates: OR

25) Logic Gates: NOT 26) Summary of Logic Gates and Truth Tables

28) Combinational Logic Circuits29) Logic Circuit #1: Cars Hot Engine30) Logic Circuit #2: Central Heating Pump31) Logic Circuit #3: Greenhouse Heater32) Summary of Combinational Logic Circuits

ContentsPage Title

3) Clocks4) A Simple Oscillator Circuit5) How an Oscillator Circuit Works

6) How an Oscillator Circuit Works (Alternative)7) How to Change a Clocks Frequency

9) Counters10) Counting in Decimal11) Devices Using Counters


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Electronics Section 4.4Digital Processes

Electronics 4.4: Digital Processes


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Transistor as a Switch (G) State that a transistor can be used as a switch. (G) State that a transistor may be conducting or nonconducting, ie on or off.

Simple Switching Systems (G) Draw and identify the circuit symbol for an NPN transistor. (G) Identify from a circuit diagram the purpose of a simple transistor switching circuit. (C) Explain the operation of a simple transistor switching circuit.

Digital Logic Gates (G) Draw and identify the symbols for two-input AND, OR and NOT gates. (G) State that logic gates may have one or more inputs and that a truth table shows the

output for all possible input combinations.

(G) State that high voltage = logic 1, low voltage = logic 0. (G) Draw the truth tables for AND OR and NOT gates. (C) Identify the following gates from truth tables: AND, OR, NOT.

Combinational Logic Circuits (G) Explain how to use combinations of digital logic gates for control in simple situations. (C) Complete a truth table for a simple combinational logic circuit.

Clock Signals

(G) State that a digital circuit can produce a series of clock pulses. (C) Explain how a simple oscillator built from a Resistor, Capacitor and Inverter operates. (C) Describe how to change the frequency of a clock.

Counters (G) Give an example of a device containing a counter circuit. (G) State that there are circuits which can count digital pulses. (G) State that the output of the counter circuit is in binary. (G) State that the output of a binary counter can be converted to decimal.

4.4 Digital Processes: Learning Outcomes


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Introducing Transistors

This is the symbol for an NPN transistor.3

Transistors are process devices.


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Transistor Terminals

Transistors have three terminals:

Collector

Emitter

Base


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Transistor as a Switch

Transistors can be used as switches.1

Transistors can eitherconductconduct ornot conductnot conduct current.2

ie, transistors can either be onon oroffoff.2

Transistor Switch


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How Transistors Work

Switching is

controlled bythe voltagebetween theBase and the

Emitter.

Collector

Emitter

Base

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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Transistor Switching Example15

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.

X

Variable

VoltageSupply

12V


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Transistor Circuit #1: Temperature-Controlled Circuit

This transistor circuit containsa Thermistor.

Because of the thermistor, thiscircuit is dependent ontemperature.

The purpose of this circuit is to

turn on the LED when thetemperature reaches . . .

Input = Voltage DividerProcess = TransistorOutput = LED

LED = Off. Heat the Thermistor.

R

Thermistor

. VThermistor . Voltage across 10k resistor . Transistor switches on. LED = On.


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Transistor Circuit #2: Light-Controlled Circuit

This transistor circuit containsa Light-Dependent Resistor.

Because of the LDR, this circuitis dependent on light.

The purpose of this circuit is toturn on the LED when the light

reaches a certain intensity.

Input = Voltage DividerProcess = TransistorOutput = LED

LED = Off. Cover LDR. RLDR .

VLDR

. Transistor switches on. LED = On.


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Transistor Circuit #3: Time-Controlled Circuit

This transistor circuit containsa Capacitor.

Because of the capacitor, thiscircuit is dependent on the timetaken to charge and dischargeof the capacitor.

The purpose of this circuit is toturn on the LED a short timeafter the switch is opened.

Where would this circuit befound in a car?

Input = Voltage DividerProcess = Transistor

Output = LED

Open Switch. VC . Transistor switches on after

a short delay.5) LED = On.

Switch closed. VC = 0V.

Transistor switches off. LED = Off.


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Summary of Transistor Switching Circuits

You are expected to know the purpose of a transistor switchingcircuit: the last three ages should hel .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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Logic


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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 _____.

Off

On High Voltage

Low Voltage

Revision: Digital Signals

1

0


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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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Logic: Switches in Series

The bulb will light only under certain conditions: what?

S1 S2

Complete the following: The bulb will turn on only when switches S1___S2 are

closed, for all other combinations the bulb is off.

S1 S2 Lit

0 0 00 1 01 0 01 1 1

ll l


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Logic: Switches in Parallel

S1

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.

S1 S2 Lit

0 0 00 1 11 0 11 1 1

!


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Logic: Opposites!

S

The bulb will light under certain conditions: what?

Complete the following:

The bulb will turn on when switch S is____, and turnoff when switch S is______.

S Lit

0 11 0

This circuit is for illustration only! If this was a real circuit, what would happen to the battery

when switch S was closed?

T h T bl


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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 whichcan be used in control situations.

S1 S2 Lit

0 0 00 1 01 0 01 1 1

S1 S2 Lit

0 0 00 1 11 0 11 1 1

S1 Lit

0 11 0

L i G AND


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Logic Gates: AND

Two-Input AND Gate5

AND

Truth Table8,16

A B Q

0 0 00 1 01 0 01 1 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.

L i G OR


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Logic Gates: OR

Two-Input OR Gate5

OR

Truth Table8,16

A B Q

0 0 00 1 11 0 11 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.

L i G t NOT


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Logic Gates: NOT

NOT Gate5

NOT

Truth Table8,16

A Q

0 11 0

The output of a NOTgate is the opposite of the input.

When Input A is 0, the output is 1.When Input A is 1, the output is 0

Note that NOT gates have only one input.

See page Logic: Opposites!.

S f L i G t d T th T bl


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Summary of Logic Gates and Truth Tables

Truth Tables list:6

Every OutputEvery Output for everyevery combinationcombination of inputs.

AND GateA B Q

0 0 0

0 1 01 0 01 1 1

OR GateA B Q

0 0 0

0 1 11 0 11 1 1

NOT GateA Q

0 1

1 0

Logic gates may have one or more inputs.6

C bi ti l L i Ci it 9 17


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Combinational Logic Circuits9,17

Combinational Logic Circuits are simply circuits using acombination of AND, OR and NOT gates.

You are expected to design Logic Circuits and

Truth Tables of simple combinational logic circuits.

L i Ci it #1 C H t E i


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Logic Circuit #1: Cars Hot Engine

When a cars engine becomes too hot an LED should lightbut only when the ignition is switched on.

LED

IgnitionSwitch

TemperatureSensor

Truth TableIgnition Temperature Output

Switch Sensor LED

Off Cold OffOff Hot OffOn Cold Off

On Hot On

Here, the truth table is simplythat for an AND Gate.

For the LED to light, theIgnition Switch must be onandthe Temperature Sensor mustbe hot.

1

1

L i Ci it #2 C t l H ti P


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Logic Circuit #2: Central Heating Pump

Derive a logic circuit that will turn on a Central Heating Systemspump when the house is cold and the Central Heating System isturned on.

This time lets find the truth table first: House is Cold = 0 ; House is Hot = 1 CHS is Off = 0; CHS is On = 1

Truth TableHouse CHS Pump

Cold Off Off

Cold On OnHot Off OffHot On Off

House CHS Pump

0 0 00 1 11 0 01 1 0

CentralHeating

Pump

1

10

TemperatureSensor

L i Ci it #3: G h H t


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Heater

Logic Circuit #3: Greenhouse Heater

Derive a logic circuit that will turn on a heater in a greenhouse onlywhen it gets cold at night.

Truth Table: Greenhouse Cold = 0 ; Hot = 1 Dark = 0; Light = 1

Truth TableGreen Day/ Heaterhouse Night

Cold Night On

Cold Day OffHot Night OffHot Day Off

Green D/N Heater

0 0 10 1 01 0 01 1 0

LightSensor 0

10

1

TemperatureSensor

Summ f C mbin ti n l L ic Ci cuits


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Summary of Combinational Logic Circuits

Combinational Logic Circuits are simply combinationsof AND, OR and NOT gates.

Constructing Logic Circuits2) Make a Truth Table.

3) Get the logic circuit from the Truth Table.

Tip: If the circuit has only one high outputthen the circuit will probablyuse an AND Gate.

Tip: If the circuit has more than one high outputthen the circuit will probablyuse an OR Gate.

Tip: Note how useful NOTgates are!

Clocks


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Clocks

Clocks are normally square waves.

Clocks are regular waves of pulses,just like the ticking of a conventional clock:

Digital circuits can be used to producea series of clock pulses.10

The circuits which produce clock pulses are sometimescalled oscillators because they constantly oscillate

between on and off.

A Simple Oscillator Circuit


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A Simple Oscillator Circuit

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.

Oscillator Circuits change between two valuesin a regular cyclical pattern: a clock output.

How an Oscillator Circuit Works18


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How an Oscillator Circuit Works18

The Invertors Input is 1, so its Output = 0:the Capacitor starts to dischargethrough the Resistor.

Capacitor C chargesand dischargesthrough Resistor R.

Start:ASSUME THECAPACITOR IS

FULLY CHARGED.

As the Capacitor dischargesthe Invertors Input eventually falls to0, so its Output becomes 1:

the Capacitor starts to chargethrough the Resistor.

As the Capacitor chargesthe Invertors Input eventually rises to 1,so its output becomes 0: the Capacitor discharges again.

1) This sequence of charging and discharging continues ad infinitum

to produce a series of clock pulses.

How an Oscillator Circuit Works (Alternative)


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How an Oscillator Circuit Works (Alternative)

Capacitor NOT NOT V2 V1 LEDInput Output

Charged 1 0 0V 5V OnDischarged 0 1 5V 0V Off

Charged 1 0 0V 5V On

Discharged 0 1 5V 0V Off

How to Change a Clocks Frequency19


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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 sothe clock fre uenc is decreased.

If the value of the Resistor is increased,charging and discharging takes longer sothe clock frequency is decreased.

C then f

R then f

Counters


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Counters

Counters are electronic circuits whichcan count digital pulses from a clock.12

Counters count the clock pulses in binary.13

1 2 3 4

Counting in Decimal


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Counting in Decimal

Binary Decimal

0000

00010010

etc

011110001001

etc

Circuits called Binary-to-Decimal ConvertorsBinary-to-Decimal Convertorsconvert a counters binary output into decimal.14

Devices using Counters


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Devices using Counters

You will be expected to name a devicewhich uses a counter.

The most common device to usea counting circuit is an electronic clock or watch.11

Electronic timing devices work

with great accuracy.

An electronic watchs clock circuitgenerates regular pulses and aCounter simply counts these.

The watchs microprocessoris programmed to know howmany clock pulses correspond to a second(and minute,hour etc) and will update the 7-segment display

accordin l : thus dis la in the time!

Future Improvements


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Future Improvements

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Digital Processes