Rosshall Academy Physics Department

Standard Grade Physics

Unit 4: Electronics
Notes

Section 1: Overview

G1.State that an electronic system consists of three parts:

Input, process and output.

G2.Distinguish between digital and analogue outputs.

G3.Identify analogue and digital signals from waveforms viewed on an oscilloscope.

Electronic Systems

Electronics is a science that uses special electrical components such as transistors or silicon chips to control the behaviour of electrons is a circuit. These circuits can then be used to form the electronic systems that are all around us, for example, televisions and telephones.

All electronic systems consist of three parts: input  process  output.

Consider a loudhailer used to address large crowds.

Input-The person speaks into the mouthpiece of the loudhailer. The input energy is therefore ______energy. The input device is a ______. This changes the ______energy into ______energy.

Process-The signal from the ______is very weak and has to be made larger. This process is achieved by an ______.

Output-The purpose of the loudhailer is to broadcast a loud sound signal. The output device therefore has to convert the ______signal from the ______into a ______signal. This is done by a ______.

The loudhailer system contains two devices known as transducers. These are devices that take in one form of energy and change it into another. An input transducer changes one form of energy into an electrical signal and an output transducer changes an electrical signal into some other form of energy.

For the loudhailer:Input transducer - ______

Output transducer - ______

Block Diagrams

Electronic systems can be shown on block diagrams.

The block diagram for the loudspeaker would be as follows:

Analogue or Digital?

There are two types of signals used within electronics – analogue and digital.

  • Analogue signals can have any value between a maximum and minimum.

Looked at on an oscilloscope an analogue signal may look like the following:

  • Digital signals on the other hand have only two possible values – these are known as ‘high’ and ‘low’ or ‘1’ and ‘0’.

On an oscilloscope a digital trace would look like the following.

Section 2: Output Devices

G1.Give examples of output devices and the energy conversions involved.

G2.Give examples of digital output devices and of analogue output devices.

G3.Draw and identify the symbol for an LED.

G4.State that an LED will light only if connected one way round.

G5.Explain the need for a series resistor with an LED.

G6.State that different numbers can be produced by lighting appropriate segments of a 7-segment display.

C7.Identify appropriate output devices for a given application.

C8.Describe by means of a diagram a circuit that will allow an LED to light.

C9.Calculate the value of the series resistor for an LED.

C10.Calculate the decimal equivalent of a binary number in the range 0000 – 1001.

Output Devices

An output device takes in ______energy and converts it to some other form of energy.

  • The Loudspeaker

Energy Change ______energy  ______energy

Analogue or digital device? ______

Practical Use ______

  • The Electric Motor

Energy Change ______energy  ______energy

Analogue or digital device? ______

Practical Use ______

  • The Relay

Energy Change ______energy  ______energy

Analogue or digital device? ______

Practical Use

A relay is an electrically operated switch. It can sometimes be useful to be able to operate a switch by remote control; for example in a nuclear power station. This can be done by sending an electrical signal to the relay which causes the electromagnet to become magnetized which in turn closes the switch activating the remote circuit.

  • The Solenoid

Energy Change ______energy  ______energy

Analogue or digital device? ______

Practical Use

A solenoid consists of a coil of wire wrapped round a metal core. If there is no current flowing through the coil a spring pushes the metal core away from the coil. When a current flows the coil becomes magnetized and this attracts the metal core into the coil. Solenoids are used in the central locking systems found in cars.

  • The Filament Lamp

Energy Change ______energy  ______energy

Analogue or digital device? ______

Practical Use ______

  • The Light Emitting Diode (LED)

Energy Change ______energy  ______energy

Analogue or digital device? ______

Practical Use ______

The LED in the circuit shown is connected the correct way round. What would happen if it was connected the other way round?

______
______

  • The 7-Segment Display

Energy Change ______energy  ______energy

Analogue or digital device? ______

Practical Use

7-segment displays make use of 7 LEDs connected in parallel. They are arranged as shown below and can be found in many electronic devices such as ______and ______.

By illuminating different combinations of LEDs the numbers 0 – 9 can be displayed.

LEDs Again

LEDs have many advantages over filament bulbs.

List 3 advantages that LEDs have when compared with filament bulbs.

  • ______
  • ______
  • ______

LEDs will only light if they are connected properly to the power supply.

Draw a circuit to show an LED connected correctly to a battery and a resistor.

LEDs are semiconductor devices that require a very small current. For this reason they must be protected by a series resistor when they are connected to a power supply. The series resistor limits the amount of current flowing in the circuit and ensures that it doesn’t exceed the value that the LED can handle safely.

In general the voltage across an LED should not exceed 2 or 3 V and the maximum current shouldn’t be greater than 10 mA.

LEDs – Worked Example

The LED shown below operates from a voltage of 3V and a current of 10mA. What size of series resistor is needed to allow the LED to be connected to a 5V supply?

Binary and Decimal Numbers

  • Decimal Numbers

All the numbers that we use everyday are made up of 10 digits from 0 to 9 this is known as a decimal code.

Once we pass 9 we place a 1 in front of the number to represent 1 set of 10 – hence 11 is 10 + 1 and so on.

  • Binary Numbers

Computers and other devices that use digital components count in a code known as binary. Binary code made up of 1s and 0s.

It is possible to convert from decimal into binary and vice versa.

When working in binary it is easiest to think of columns of numbers and each of the columns has a heading as shown below:

8s / 4s / 2s / 1s

To convert a decimal number into binary always start at the left hand column.

For example: Convert the decimal number 5 into binary.

5 isn’t big enough to include a group of 8 so that column has a zero placed in it. 5 will contain 1 group of 4s, 0 2s and 1 1s.

8s / 4s / 2s / 1s
0 / 1 / 0 / 1

Hence decimal 5 is equivalent to binary 0101.

To go from binary to decimal the process is reversed.

For example: Convert 1001 into a decimal number.

This number has 1 set of 8s, 0 4s, 0 2s and 1 1s. It is therefore 8 + 1 = 9.

8s / 4s / 2s / 1s
1 / 0 / 0 / 1

Complete the table shown on the next page by converting from binary code into decimal code and vice versa.

Binary and Decimal Numbers – Worked Example

Complete the following table.

Decimal Number / Binary Number
0
0001
7
0101
2
0110
4
1000
0011
9

Section 3: Input Devices

G1.Describe the energy transformations involved in the following devices:

Microphone; thermocouple; solar cell

G2.State that the resistance of a thermistor changes with temperature and the resistance of an LDR decreases with increasing light intensity.

G3.Carry out calculations using V = IR for the thermistor and LDR.

G4.State that during charging the voltage across a capacitor increases with time.

G5.Identify from a list an appropriate input device for a given application.

C6.Carry out calculations involving voltages and resistances in a voltage divider.

C7.State that the time taken to charge a capacitor depends on the values of the capacitance and the series resistance.

C8.Identify appropriate input devices for a given application.

Input Devices

An input device takes in a form of energy and converts it to ______energy.

  • Microphone

Energy Change ______energy  ______energy

Practical Use ______

  • Thermocouple

Energy Change ______energy  ______energy

When the junction of a thermocouple is heated it produces ______energy. The greater the temperature of the junction the ______the ______energy produced.

Practical Use ______

  • The Solar Cell

Energy Change ______energy  ______energy

When the solar cell is exposed to light energy it produces ______energy. The greater the light intensity falling on the solar cell the ______the ______energy produced.

Practical Use ______

  • The Thermistor

When the thermistor is heated the ohmmeter reading ______.

The ______of a thermistor changes with temperature. As the temperature increases, the ______of the thermistor decreases.

Temperature  Resistance ___

  • The Light Dependent Resistor (LDR)

When the LDR is exposed to light the ohmmeter reading ______.

The ______of an LDR changes with light intensity. As the light intensity increases, the ______of the LDR decreases

Light  Resistance ___

Thermistors and Light Dependent Resistors – Worked Examples

1.Look at the circuit diagram and the data table for a thermistor.

Temperature (C) / Resistance ()
20 / 6000
30 / 4500
40 / 3000

(a)What is the size of the current in the circuit at each temperature?

(b)Estimate the resistance of the thermistor at a temperature of 45C.

2.In an experiment to find out how the resistance of an LDR changes with light intensity a group of pupils obtained the following results.

Light Conditions / Voltage across LDR (V) / Current through LDR (A) / Resistance of LDR ()
Darkness / 10 / 0.001
Dim light / 10 / 0.025
Bright light / 10 / 0.500

(a)Complete the last column of the table.

(b)What happens to the resistance of the LDR as the light intensity decreases?

  • The Capacitor

A capacitor is a device that stores electric charge. It consists of two metal plates separated by a layer of insulating material.

A capacitor has a capacitance – an ability to store charge – and the unit of capacitance is the farad (___).

When there is no charge on the plate of the capacitor it is said to be uncharged. If a voltmeter is connected across an uncharged capacitor it will read 0V.

If the voltage across the capacitor is monitored over a period of time then the graph of voltage against time would look like the following trace:

It is normal when using a capacitor to include a series resistor.

Add a second trace to the graph above to show the effect of increasing the size of either the capacitor or resistor on the charging time.

Capacitors are input devices that can be used to introduce a time delay into a circuit.

Practical Use ______

  • The Potentiometer

At position A the resistance is ______ and the voltmeter will therefore read ______V.

At position B the resistance is now at a maximum and the voltmeter will read _____ V.

A potentiometer makes it possible to ‘tap’ different voltages from a supply.

Voltage Dividers

A voltage divider circuit usually consists of two resistive components in series with a power supply.

For a voltage divider

This can be rearranged

i.e., the ratio of the resistances is equal to the ratio of the voltages in the circuit.

Voltage and Resistance Ratios – Worked Example

Find the unknown in each of the following.

(a)(b)

Voltage Dividers 2

Look at the circuit below and calculate the reading on the voltmeter.

2.The current in the circuit should be found.

V=IR

12=I x 30

I=12/30

=0.4 A

3.The voltage across the appropriate resistor should be calculated.

V=IR

=0.4 x 20

=8 V

Therefore the voltmeter reading is 8V.

By looking at the results above we can see that 20 is of the total resistance (=). Looking at the voltages we see that the voltage across the 20 resistor is of the total voltage ( = ). In other words, whatever fraction the resistor is of the total resistance it takes the same fraction of the supply voltage.

We can use this fact to build another equation that allows us to calculate the voltmeter reading in the above example in a much more straightforward manner.

V= x Vs

Using this on the above example:

V= x Vs

=8V

The voltmeter reading is 8V.

Voltage Dividers – Worked Example

Calculate the voltmeter reading in each of the following circuits.

(a)

(b)

Temperature Sensor in a Voltage Divider

In the circuit shown the thermistor forms part of a voltage divider. The other element in the voltage divider is a fixed resistor.

If the temperature of the thermistor increases then its resistance decreases. Its share of the supply voltage will therefore decrease and hence the voltmeter reading will go down.

Light Sensor in a Voltage Divider

In the circuit shown the light dependent resistor forms part of a voltage divider. The other element in the voltage divider is a fixed resistor.

If the light intensity falling on the LDR increases then its resistance decreases. Its share of the supply voltage will therefore decrease and hence the voltmeter reading will go down.

Sensors – Worked Example

(a)Explain what will happen to the reading on the voltmeter if the temperature of the circuit shown below increases.

(b)Explain what will happen to the reading on the voltmeter if the light intensity increases.

Switch as an Input Device

A switch can be used as an input device. It provides a simple way of changing the voltage in a circuit.

Using a switch like this allows us to create a digital input – 5 V is ‘high’ or ‘1’ and 0 V is ‘low’ or ‘0’.

Section 4: Digital Processes

G1.State that a transistor can be used as a switch.

G2.State that a transistor may be conducting or non-conducting, i.e. On or Off.

G3.Draw and identify the circuit symbol for an NPN transistor.

G4.Identify from a circuit diagram the purpose of a simple transistor switching circuit.

C15.Explain the operation of a simple transistor switching circuit.

Up until this point we have looked at input and output devices. To make any use of these we need a process device. The first process device we shall examine in the transistor.

Transistors

Transistors can be used as an electronic switch. The symbol for a transistor is shown below:

Transistors have three terminals, known as the base, the emitter and the collector. The particular transistor shown above is known as an NPN transistor.

Consider the electronic system shown below:

The potentiometer is used to change the voltage across the base-emitter junction. When the voltage across this junction is greater than ______V the transistor switches on it is said to be conducting. If the voltage falls below ______V then the transistor switches off – it is said to be non-conducting. The transistor switch is an example of a digital process – it can either be on or off.

  • Light Controlled Switches

If the light intensity decreases then the resistance of the LDR will ______. This means that the voltage across the LDR will also ______. If the voltage across the LDR ______then the voltage across the variable resistor must ______. The voltage across the variable resistor is the voltage across the base-emitter junction of the transistor. When this voltage falls below ______V the transistor switches ______and the LED switches ____.

Practical Use ______

Explain what happens in the circuit shown below if the light intensity decreases.

______

______

______

______

______

Practical Use ______

  • Temperature Controlled Switches

Explain what happens in this circuit if the temperature decreases.

______

Practical Use ______

Explain what happens in this circuit if the temperature decreases.

______

Practical Use ______

  • Time Delayed Switch

Explain what happens in this circuit when the switch is closed.

______

Practical Use ______
G5.Draw and identify the symbols for two-input AND and OR gates,

and a NOT gate.

G6.State that logic gates may have one or more inputs and that a truth table

shows the output for all possible input combinations.

G7.State that

High voltage = logic ‘1’

Low voltage = logic ‘0’.

G8.Draw the truth tables for two-input AND and OR gates and a NOT gate.

G9.Explain how to use combinations of digital logic gates for control in simple situations.

C16.Identify the following gates from truth tables:

Two-input AND

Two-input OR

NOT (inverter).

C17. Complete a truth table for a simple combinational logic circuit.

Logic Gates

Logic gates are tiny integrated circuits built on a single silicon chip. They are digital devices and both the input to and the output from a logic gate is in the form of a digital signal i.e. a ‘1’ or ‘0’.

We will look at three types of gates: AND gates, OR gates and NOT gates. AND and OR gates have two inputs and one output whereas a NOT gate has one input and one output. A table, known as a truth table, shows the possible outputs for all possible combinations of inputs.

  • NOT gate

SymbolTruth Table

Input / Output
0
1
  • AND Gate

SymbolTruth Table

Inputs / Output
A / B
0 / 0
0 / 1
1 / 0
1 / 1
  • OR gate

SymbolTruth Table

Inputs / Output
A / B
0 / 0
0 / 1
1 / 0
1 / 1

Combinational Logic Circuits

AND, OR and NOT gates can be combined to form more complicated circuits that can be used to switch something on or off.

When logic gates have been combined, completing the truth table should be taken in steps.

A / B / C / D / E / F
0 / 0
0 / 1
1 / 0
1 / 1

Consider the following examples. In these examples asssume that a light sensor gives out a logic 1 in light conditions and a logic 0 in dark conditions;and that a temperature sensor gives out a logic 1 when the temperature is high and a logic 0 when the temperature is low.

Example 1

Draw a logic diagram and the truth table for a warning LED to light when a motorbike’s engine gets too hot. The LED should only go on when the ignition of the motorbike is switched on (logic level 1) and the engine is too hot.

Inputs / Output
Temperature Sensor / Ignition Switch
Cold (0) / Off (0)
Cold (0) / On (1)
Hot (1) / Off (0)
Hot (1) / On (1)

Example 2

Draw a logic diagram and truth table which will switch on the pump of a central heating system when the house is cold and the central heating is switched on (logic level 1)

G10.State that a digital circuit can produce a series of clock pulses.

G11.Give an example of a device containing a counter circuit.