Tutorial 16 - Making Electronics Systems

The object of this tutorial is to give you some pointers as to how to make an electronic system.

 

Electronic Systems

Electronic systems have revolutionised our way of life, especially with the advent of computers.  When I was in my teens, a computer was a mysterious machine that occupied a large building.  A computer of the same power is carried about in your pocket in your mobile phone.  One university bought for a vast price one of the biggest hard-drives that had ever been built…100 megabytes!  It was the size of a large commercial washing machine, and had a mass of several hundred kilograms.

 

Now I carry a 4 gigabyte flash memory every day around my neck.  Every day I carry the computer that I use to write these notes to work with me on the train.  It has been said that if aeroplanes had progressed at the rate of computers, a Boeing 747 airliner would cost £300 and do 30 miles to the gallon.

 

If ever there was a discipline that summed up human ingenuity in solving problems, electronics is it.  Our discipline is the fruit of much creative and imaginative thinking, based on a few simple rules.

Electronic systems can be split into two kinds:
Analogue – the signal can be any value you like, positive or negative;
Digital – the signal is ON (1) or OFF (0).

 

 

Digital circuits are used for data storage, transmission, and processing.  They can also be used for security locks and control circuits.

 

Analogue circuits are used in audio amplifiers, TV sets, most radio transmission, and motor control, to name a few. 

 

The internet has many thousands of ideas for you to build up your skills as an electronic engineer.  There are many ways of solving a problem in electronics.  Your task is to decide which approach you are going to make.

 

Circuit Diagrams

Why do we use circuit diagrams?  Have a look at this picture.

 

You can see all the components from the picture, but there is no way that even a highly experienced electronic engineer would be able to build this circuit from this picture, because there is no way of telling how the components are connected together.

 

If the engineer turned the circuit board upside down, then he might have an idea of the connections.  However he would find it difficult to build the circuit without a circuit diagram.  His first thing to do would be to draw a circuit diagram.  Once he has that diagram, he would be able to make the circuit easily.  That assumes, of course, that the circuit board is made up of just one layer. In computers, the motherboard has several layers…

 

 

A Circuit diagram tells us:

·         How the components are laid out and wired up.

·         Engineers use circuit diagrams to assemble the circuit.

·         It uses standard symbols with which you need to be familiar.

·         The lines represent the copper tracks on the circuit board. Where the lines are connected there is a black dot, to represent a blob of solder.

 

The interpretation of circuit diagrams is an important skill.  Electronic engineers use standard symbols in their circuit diagrams and you should be familiar with most of these.

 

Circuit diagrams are essential to communicate your design with other engineers. They are often called schematics. Although you don't have to be a good artist to do good circuit diagrams, it is essential that they are clear. You can buy specialist circuit design software, which is eye-wateringly expensive, but will design printed circuit boards. You can draw circuit symbols and diagrams using graphics from MS Word, or a graphics package like Adobe Photoshop.  The circuit below is drawn using the old-fashioned low-tech approach:

 

 

Although it’s usual to use software nowadays, you too may need to do it the old-fashioned, low-tech, way:

 

A bad circuit diagram is use to neither man nor beast.

 

A good circuit diagram is one that another electronic engineer can pick up and build the circuit that you have designed. And it will work in exactly the same way as your original circuit.

 

Some books (or software) will have symbols that are different to the ones you see in these tutorials.

 

 

Systems Approach

Look at this circuit:

 

 

 

As you can see it is complex, and would be very scary to build as one complete circuit.  The chances are that a novice electronic engineer would give up rapidly.  Even if he or she persisted, the chances are that the thing would not work.  Testing every connection would take for ever.

 

A better approach is to break the circuit down into sub-systems.   The advantages of this approach are:

·         It is simpler to build several subsystems, and then bring them together as a whole.

·         Each subsystem can be tested separately.

·         Fault-finding is so much easier.

·         Complex circuits can be made from standard modules.

·         If a sub-system fails, only that sub-system needs to be replaced.

·         Often the faulty system board can be pulled out and a new one inserted – a job that can be done very quickly.

 

If you look at the diagram above, you can see that there are subsystems that can be identified, for example:

 

 

Here is a simple circuit diagram that is broken up into sub-systems. This circuit diagram is for a simple burglar alarm.

 

Systems Approach

Electronic systems consist of:

 

There may be feedback to control the response of the circuit.

 

We can represent this very basic idea with a block diagram.  Each block represents a sub-system.

Block diagrams

The terms input, processor, and output are not necessarily that helpful.  It is much better to name a sub-system with a description of its function, e.g.

·         Voltage comparator (compares voltages);

·         Microphone input;

·         Set volume;

·         Power amplifier.

 

The idea of a block diagram is to show the parts of a circuit as simple “black” boxes.  Here is the block diagram for an electronic voting machine.

 

 

This system is made up of 9 different sub-systems, each of which would be developed on a separate board.  Information flows are shown with the arrows.  A number of different sub-systems all feed into or are fed from the microcontroller.  For example the voter would need to place her finger onto the finger print module.  These data are then fed to the microcontroller that checks that that voter is eligible to vote.

 

The voter makes her choice on the key pad. These data are then fed to the micro-controller which logs the details of the voter’s vote into the memory. Also the fact that the voter has voted will be registered. Should she fail to register her finger print, her vote would not be counted. On the other hand, if she tries to vote twice, an alarm would sound.

 

Building Circuits

When an electronic engineer is intending to build a circuit, the first step must be the design work.  There are often many alternative solutions to a problem.  The internet contains a vast library of these, but each must be evaluated, as there is no guarantee that they will work.  An experienced engineer will use his knowledge of the theory to make a good judgement as to which is the most effective solution.

 

Then he will define the system parameters (values of components, input voltages, load resistance, etc.) and design the sub-systems.  Design software is available which will test the circuit and give a computer model of the system.  But at this stage the system is virtual.  To be of any use, it needs to be a real artefact, something that can be used.

 

Electronic engineers test their circuits on breadboard or prototype board. It used to be called breadboard, because the early engineers used the breadboard from the kitchen, sticking nails in it. What other family members had to say is not recorded.

 

The horizontal lines take the power supplies. The components need to go horizontally between the vertical lines. If you use the same vertical line, you will short the component out. You can put components vertically across the channel in the middle.

 

 

It is worth planning how you are going to arrange your components. You can get proformas to help you to do this.

 

It is also well worth testing each component with a multimeter.  All multimeters have a scale to measure resistance.  More sophisticated instruments allow you to test:

·         Capacitance of a capacitor;

·         The correct function of diodes;

·         The gain of a transistor.

 

Here is a multimeter testing a transistor.  You can see that the multimeter reads 140, which means that the gain of the transistor is 140. 

 

 

 

If the instrument read 0, and the transistor had been put in properly, then the chances are that the transistor is a dud (technical term for defective).  Throw it out!

When we have got our circuit working, we can solder it onto strip board. It is helpful to draw out the circuit on a strip board planner.

Be careful with soldering:

·         Heat both the wire and the board sufficiently so that the solder runs in.

·         Be careful not to overheat the component. Some components are easily damaged. Use a crocodile clip as a heatsink if necessary.

·         Do not apply too much solder, otherwise you could bridge the tracks.

 

 

For more help with soldering, go to the video clip on How to Solder.

 

Once we have found that the circuit works properly, we can design a printed circuit board (PCB), especially if we are going to construct several copies of the board.

 

 

 

It is possible to get software that will design the PCB. In large factories PCBs are printed in large quantities and components are placed by computer-controlled assembly machines which can operate at very high speed, producing circuit boards very cheaply.

 

Printed circuit boards can only carry a limited current. Excessive currents will cause overheating. Large currents need thick wires.

 

Electronic components, especially chips, are sensitive to high voltages and strong electric fields. If you are handling circuit chips, you need to earth yourself, using an earthing strap. Otherwise static electricity could wreck the chips. A metal case will protect electronic components from high electric fields.

 

 

Fault-finding

Faults are the bane of the electronic engineer’s life, especially the novice.  A non-functional circuit is neither use nor ornament (although some non-functioning computer components have been mounted and hung off walls).  It is also extremely disheartening for the novice engineer who has spent hours putting together a circuit to find that when he switches it on, nothing happens.  Some walk away in disgust vowing never to do it again.

 

Sometimes a fault is obvious – smoke rises from a component or there is a sharp bang as a component explodes.  In this case, the engineer needs to identify what he has done wrong.  Components rarely fail due to manufacturing defects.  Such faults arise because:

·         A component has been wired up with the wrong polarity (+ and – swapped over);

·         The component could not carry the current passing through it;

·         A short circuit has occurred.  Always check that the orientation of tracks in the prototype board.

 

Check the circuit carefully, before replacing the component.

 

Intermittent faults are very tedious.  They are almost always due to a bad connection somewhere, caused by a dry joint.  Good soldering will prevent this.

 

In some circuits, for example with computers, it is easier and cheaper to replace the entire sub-system.  Where there are several layers with a PCB, a repair may be totally impossible.

 

Other times the circuit sits there doing nothing.  This can result from:

·         The circuit being made up incorrectly;

·         A faulty component;

·         Connections not being made properly.

 

It is worth checking that the circuit has been put together correctly.  If needs be start again.  When using prototype board, it’s best to insert wires and components with bottle-nosed pliers.  That is certainly true if your fingers are like bananas.

 

This is why it’s a good idea to build a circuit with subsystems and test it as such. The best engineers do this and test their circuits with voltmeters and milli-ammeters to record what voltages are expected when the circuit is working correctly. If you find that the voltmeter is reading a correct value at a particular point, you can be pretty sure that that part of the circuit is working properly. Therefore you can narrow down where the fault lies.

 

Using a voltmeter

It is better to use a voltmeter rather than an ammeter.  This is because the ammeter has to be placed in series with the components.  This requires links to be broken, which could cause problems.  Voltmeters measure voltage across components.  A voltmeter on a multimeter is almost perfect, in that its resistance is very high indeed, almost infinite.  A CRO is the same, but will display AC waveforms.  This can be very useful, especially if the waveform is not as expected.

 

In many circuits, a zero reference point is shown connected to earth, or grounded.  In many devices the 0 V point is connected to the metal frame or chassis. You can see this in the picture of a computer power supply.  You connect the negative lead of the voltmeter here.

 

There may be voltages that are below zero where there is a split power supply.  In which case, the voltages are negative.

 

 

 

In this circuit, the point Y has been connected to ground, and is at 0 V. Therefore point X could be considered to be at +9 V relative to Y and Z is –9 V relative to Y.

A good circuit designer will put expected voltages at different test points around the circuit.  This circuit diagram shows these:

Image from www.extremecircuits.net

Notice that the negative line is grounded.  This means it’s a datum (reference) point of 0 V from which all other voltages are measured.  8V4 is a way of writing 8.4 V.  This will help the novice engineer as he struggles to find out what is wrong.

 

Using data-sheets to design circuits

Data sheets are very useful for the electronic engineer, but contain a great deal of information, some of which you need to know, but much of it is not that critical.  Much of the data are marginal, and for most purposes you may get away with using a particular component that is not perfectly matched with another component.  However, if you are looking for ultimate performance in speed, or power handling, you may well have to look more critically at the datasheets.

 

For example, a particular transistor might take 1 ms to turn on, while another transistor might take 1 ns.  If you are dealing with a low-frequency circuit, the 1 ms transistor would be perfectly good enough.  However, if you were dealing with frequencies of, say, 10 MHz, you would need the 1 ns transistor.

Here is a real data sheet:

The packaging of this transistor is called TO-92 (Transistor Outline Package, Case Style 92).  There are many other transistors that have this packaging as well.

 

The advantages are that this style of transistor can be made very cheaply indeed.   The disadvantage is that the transistors cannot be made to handle large amounts of power.  Also the pin-out can vary, depending on whether the transistor is made in America, or Japan.  A wrongly wired transistor might not last very long.

 

Here are some transistors that use the TO-92 case:

·         BC548, NPN

·         BC558, PNP

·         2N3904, General purpose NPN

·         2N3905, PNP

·         2N3906, General purpose PNP

 

 

The picture shows some other styles of transistor case. These particular styles have metal cases so that they can be attached to a heatsink. Heat-sinks are important where transistors are handling high powers.

 

Graphical Data

These data are showing as graphs the electrical performance of a particular type of transistor.  Here are some graphical data for a BC546. 

 

 

The advantage of having data as graphs is that they show the behaviour of the component over a range of values.  We need to be careful when we use these graphs, since they show the average of a range of transistors tested.

It is entirely possible that an individual transistor would vary from these ideals.  Therefore if it’s critical that a transistor has a particular characteristic, it’s important that you test it before you insert it into your circuit.

 

Pin-out Diagrams

If you are using integrated circuits, it’s even more important to wire the component correctly, otherwise it will do nothing.  Or it will smoke.  555-timers in particular are sensitive and go belly-up readily.

 

The data-sheet for an integrated circuit will always have the pin-out diagram.  The power supply may not be shown on your circuit diagram, so it’s important that you make sure it’s connected or nothing will happen.

 

Soldering of integrated circuits can be difficult.  You can damage them easily by overheating them, so it’s preferable to mount them in a socket.

 

Using Datasheets in your project

In your project you will be making a working electronic circuit using sub-systems.  It is important that you know the way that components work, and the expected output voltages and currents.

 

Suppose you have a voltage divider that goes into the base of a transistor.  It needs to give out a base current of 15 mA to turn on a specific transistor.  You will need to select input components that are capable of passing a current of 15 mA.  Similarly, you might need to have a transistor that can handle 10 W.  A signal transistor will give you the right voltages, but it will not give you the current.  If you tried to use the signal transformer, it would almost certainly overheat, and the component it was driving would not work.

 

Digital electronic gates give out small currents.  These data are for a 74AC08 chip which has 4 AND gates:

 

 

The chip gives out a high voltage of 5.5 V.  It will give out sufficient current to light an LED, but not to drive a motor.  A MOSFET could be used to drive the motor.

 

 

Doing an Electronics Project

These are notes that I gave out to my Pre-University Access Level 3 Students.  They use the old AQA Electronics Project requirements and I adapted them for my course.  If you are having to do an electronics project as part of your course, you may find them helpful. 

 

You can’t get marks simply because you made a great effort with your project (although in doing so you will probably hit most of the marking points).  Nor will you get a higher mark because you did a more sophisticated project than someone else.

 

Follow this guidance and keep it simple!

 

What you must do:

 

 

 

 

 

 

 

 

 

 

It is perfectly acceptable for parts of your coursework to be adapted from other sources so long as all such cases are clearly identified in the text and fully acknowledged in the supporting evidence. If you quote directly from a source, e.g. circuit diagrams or sentences, you need to use quotation marks and provide the reference in a footnote at the bottom of the page.

 

You must declare that your work is your own.

 

 

What you must NOT do:

 Produce a computer project.  Virtual projects get virtual marks.  This does NOT prevent you from using a computer simulation to help you with your project. 

 

 

Marking Criteria

There are 60 marking points for which you must show evidence to get the marks.

 

To achieve syllabus statements 4.1 and 5.1 you need to meet the following outcomes that are marked as such.  You may use this list to check you have met the criteria.

 

Each outcome will be marked as follows:

0 marks – Not attempted or not met;

1 mark - Partially met;

2 marks – fully met,

 

1.      Planning

Marks

a.

Chose a project to do.

 

b.

Explain what job the circuit is going to do.

 

c.

Explain how the circuit works

 

d.

Compare the circuit with other circuits that do the same job.

 

e.

Explain why you have chosen that particular circuit

 

f.

Carried out a risk assessment for the project.

 

2.      Choosing components (4.1)

a.

Download a data sheet for at least two components and show evidence in your report.

 

b.

Write down three different numerical parameters from the data sheet and explain what they mean

 

c.

Compare your chosen components with at least two other similar components

 

d.

Justify your choice using data from the data sheet to explain your choice.

 

e.

Test two components to see if they fit the data given for them.

 

3.      Developing the circuit

a.        

Devised details of at least one sub-system

 

b.

Shown at least one calculation

 

c.

Converted circuit diagrams into a circuit on proto-board

 

d.

Put two sub-systems together

 

e.

Make the circuit work.

 

f.

Make a neatly constructed circuit

 


 

 

4.      Testing the circuit (4.1)

a.        

Devise a testing plan.

 

b.       

Record results.

 

c.        

Compare observed results with expected result.

 

5.      Further development

a.

Explain any limitations of the circuit.

 

b.

Suggest improvements.

 

c.

Carry out improvements.

 

d.

Report on the improvements.

 

6.      Reporting (5.1)

a.

Write well-organised report

 

b.

Use technical terms correctly.

 

c.

Show good technical authorship

 

d.

Show evidence using clear photographs of all stages of development.

 

e.

Show clearly a bibliography…

 

f.

…with all acknowledgements.

 

 

Total =

60

 

Grade Descriptors

The following criteria will be used to assess students for merit and distinction:

 

GD1: Understanding of the subject

 

Merit

Distinction

At least 42 marks

At least 50 marks

Good understanding demonstrated of the electrical, electronic, and physics principles behind the project.

Excellent understanding demonstrated of the electrical, electronic, and physics principles behind the project.

Calculations carried out clearly and with appropriate units.

Calculations carried out clearly and with appropriate units and appropriate numbers of significant figures.

 


 

 

GD3: Application of skills

 

Merit

Distinction

Circuit works with minimal assistance

Circuit works with no assistance

Circuit is skilfully constructed

Circuit is neatly constructed

Evidence of planning of testing

Full test plan devised

Relevant measurements taken

Measurements taken with appropriate precision and skill.

 

GD7: Quality

 

Merit

Distinction

Clearly written report…

Outstanding report…

…in good technical English.

…in excellent technical English.

Photographic evidence of stages in development.

Photographic evidence is clear, with sharp images.

Measurements taken in a methodical manner.

Measurements made with appropriate precision.

Bibliography shown with sources.

Bibliography is complete.

 


 

 

Evidence to Support the Award of Marks

The development of the coursework project should be fully documented in the report which forms the basis for the assessment.

 

The report should be such that it would enable someone else to carry out the same work and to know what to expect in terms of the artefact’s function and performance. It should be presented in a logical order that is easy to read and understand. It must contain:

 

Marks will only be awarded when there is clear supporting evidence.  

 

Guidance by the Tutor

The work assessed must be solely that of the candidate concerned.

 

It is the teacher’s task to ensure that appropriate project work is undertaken by the candidate and to provide the candidate with appropriate guidance. The supervisor should also provide additional guidance and assistance if requested but this must be taken into account when the work is assessed.

 

You need to understand the distinction between guidance and assistance given to candidates.

 

 

Work

 You should do the work in college.  It shouldn’t be necessary to do the work elsewhere. 

 

 

The button marked exemplar gives a link to a good report done by one of my pre-university students.

 

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