Electronics Digital Processes 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.

Electronics Digital

Processes

Transistor as a Switch1) (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 Systems3) (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 Gates5) (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 Circuits9) (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 Signals10) (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.

Counters11) (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.

Learning Outcomes

This is the symbol for an NPN transistor.

Transistors are process devices.

Transistor Terminals

Transistors have three terminals:

Collector

Emitter

Base

Transistor as a Switch

Transistors can be used as switches.

Transistors can either conductconduct or not conductnot conduct current.

ie, transistors can either be onon or offoff.

Transistor

Switch

How Transistors Work

Switching is controlled by the voltage between the Base 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.

Transistor Switching Example

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

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 a pre determined temperature.

Input = Voltage DividerProcess = TransistorOutput = LED

1) LED = Off.2) Heat the Thermistor.3) RThermistor .4) VThermistor .5) Voltage across 10k resistor .6) Transistor switches on.7) LED = On.

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 DividerProcess = TransistorOutput = LED

1) LED = Off.2) Cover LDR.3) RLDR .4) VLDR .5) Transistor switches on.6) LED = On.

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.

Where would this circuit be found in a car?

Input = Voltage DividerProcess = TransistorOutput = LED

1) Switch closed.2) VC = 0V.3) Transistor switches off.4) LED = Off.5) Open Switch.6) VC .7) Transistor switches on after a short

delay.8) LED = On.

Summary of Transistor Switching Circuits

• In each of the three circuits the input device is: A Voltage Divider using a

In each of the three circuits the output device is: an LED

Thermistor

LDR

Capacitor

Remember that digital signals have only two values,

“1” and “0”, or“High Voltage” and “Low Voltage”, or

“On” and “Off”,

Off

OnHigh Voltage

Low Voltage

Revision: Digital Signals

1

0

Introduction to Logic

Many digital electronic processes are designed around “logic” circuits.

The Inputs and Outputs in logic have only two values:

0 & 1High & LowOn & Off

Logic is ideally suited to help design digital electronic circuits because of its binary nature.

We will look at some fundamental logic circuits.

Logic: Switches in Series

The bulb will light only under certain conditions: what conditions?

The bulb will turn on only when switches S1 AND S2 are closed, for all other combinations the bulb is off.

S1 S2 Lit

0 0 00 1 01 0 01 1 1

S1 S2

Logic: Switches in Parallel

The bulb will light under certain conditions: what conditions ?

The bulb will turn on when switches S1 OR S2 are closed, for all other combinations the bulb is off.

S1

S2

S1 S2 Lit

0 0 00 1 11 0 11 1 1

Logic: Opposites!

The bulb will light under certain conditions: what conditions?

The bulb will turn on when switch S is OFF, and turn off when switch S is ON.

S Lit

0 11 0

S

Truth Tables

The tables on the previous slides 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

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

Logic Gates: AND

Two-Input AND Gate ANDTruth Table

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.

Logic Gates: OR

Two-Input OR Gate

ORTruth Table

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.

Logic Gates: NOT

NOT Gate NOTTruth Table

A Q

0 11 0

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

Note that NOT gates have only one input.

Summary of Logic Gates and Truth Tables

Truth Tables list:Every OutputEvery Output for everyevery combination combination of inputs.

AND GateA B Q

0 0 00 1 01 0 01 1 1

OR GateA B Q

0 0 00 1 11 0 11 1 1

NOT GateA Q

0 11 0

Combinational Logic Circuits

Combinational Logic Circuits are simply circuits using a

combination of AND, OR and NOT gates.

You are expected to design Logic Circuits andTruth Tables of simple combinational logic circuits.

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.

Truth TableIgnitionTemperature OutputSwitch Sensor LED

Off Cold OffOff Hot OffOn Cold OffOn Hot 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”.

LED

IgnitionSwitch

Temperature

Sensor

1

1

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 = 1CHS is Off = 0; CHS is On = 1

House CHS Pump

0 0 00 1 11 0 01 1 0

CentralHeating

Pump

1

10

Temperature

Sensor

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

Heater

Green D/NHeater

0 0 10 1 01 0 01 1 0

LightSensor 0

10

1

Temperature

Sensor

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” outputthen 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!