Wow, what a journey. From landing in OZ (being given my skills audit), traveling the yellow brick road (filling in the gaps in my knowledge) to escaping the evil flying monkeys (procrastination, confusion and misinformation) I think I’ve finally arrived…

My attitude towards the whole of electronics at the start of this process was one of irrational panic and dislike. I’d never felt any inclination towards learning anything about electronics or how any of the devices I use day to day work. Things have changed so much! This subject is immense and so exciting. It’s no longer incomprehensible and intimidating, I understand how current flows through a circuit and what it will do when it reaches a component, I can now look at a circuit or circuit diagram and either know what it will do or have a good enough idea about the components used to work it out.

Right at the beginning of this project I defined competence for myself, my aim being to reach this rarefied state by the end of this semester. So the question is, am I there yet?

Here is an extract from my work on competence, it’s where I finally defined what I think it means to be competent:

So, what is competence? In doing this research my original definition now seems deficient. Competence is not just about having knowledge or understanding the theory of a subject, or even about being able to pass on that knowledge, it’s about being able to ‘do’ as well as ‘say’. To be competent in a subject you need to understand it but also to be able to apply your understanding, to make a product or to be able to understand why something isn’t working and how to fix it.

To be able to get to this point I have gone through the process of research and experimentation detailed in this project, gathering knowledge and working out how to use it. I had really underestimated my abilities in understanding electronics in general and the more work I put into the research and investigations at the start of this process the more a very surprising thing became obvious… It’s fun and I’m really interested in it! I had it in my head from the start of this year that I would be specialising in resistant materials but part way through this project that no longer seems like a concrete concept. I was picking up knowledge much faster and understanding concepts more quickly than I was expecting. The real breakthrough came when I started my first circuit experiments, actually making a circuit, even if it’s just a prototype, has been the best way to work out if I know my stuff. I started working my way towards being competent in reading and understanding circuit diagrams, able to put a circuit together and be confident that it would work on powering up.

At this stage in my process I had made headway into understanding my chosen subject and applying my understanding. Going further I moved on to more complex circuits previously far beyond my capabilities, and conquered them. Each one of my circuits worked (with only a couple of hiccups) and more than that I understood why and how they worked. I started to help other students in building their own circuits and relaying my gathered information on digital electronics to make things clearer. By the time I got to building and testing my binary to decimal counter I was already feeling competent, at least in interpreting the diagrams, prototyping the circuits and understanding the signals involved.

It was at this point that I was considering what I needed to do to actually prove competence, my natural inclination was to design and make a product but on reflection what would be the point? Any product I could come up with would be dealing with only part of my gathered knowledge so why make one? The light bulb moment came on talking to Dean and actually asking myself ‘Do you feel competent?’ the answer being ‘sort of’. The only area I was not comfortable with was in fault finding and troubleshooting, in each instance of one of my circuits not functioning it had taken either Dean or Steve to help me fix it and as I had stated in my definition I needed to be able to ‘understand why something isn’t working and how to fix it’. 

Without taking this step I would not be fully competent, it was also a major sticking point with me as the most frustrating thing about electronics has been powering up a circuit and not knowing why it’s not working. In going through a process of solving the issues in a series of circuits I now have a fuller understanding of the circuits themselves, not just being able to spot and correct issues. Looking at signals going to and from my ICs really presses home all the knowledge I’ve gathered, it makes it more present and accessible. 

Now that I’ve come to the end of this project I can confidently say that I consider myself to be competent in digital electronics but it goes further than that. In doing these investigations it has given me a wider appreciation for the subject of electronics and a thirst to continue my explorations. I no longer have that panicky feeling of not understanding when facing anything electronic, instead it has been replaced by a strong curiosity, wanting to understand this area that has so much to do with moder life and modern design.

I cannot emphasise enough how useful this process has been and how much it has added to my knowledge as a designer. I can carry the work done here not only through to my future career in teaching but also into my way of viewing the world, my decisions as a consumer and my creativity. I think I’ve arrived at the Emerald Castle and who knows… In a couple of years I may be replacing the wizard ;)

Each panel interacts with the next to produce a final outcome of a binary to decimal counter that counts from 0-9 then resets. You’ve got the 555 panel with adjustable components to produce the initial signal pulse, this then feeds into two NOT gates forming a buffer to boost the signal from the 555. It then hooks up to a ripple counter made up of d-type flip flops and on into the 4 outputs representing the binary count, using a 4028 chip this output is then decoded into a decimal count. It doesn’t stop there, using the AND gate panel you hook up a gate to two of the binary outputs to get it to reset after counting 9. Phew! It’s a really comprehensive look at digital electronics, you have to be able to understand the 555 timers states, what various logic gates do, how to wire up a ripple counter form flip flops and why the whole thing works the way it does.

I’d love to have had this at the start of this project, it puts everything I’ve been looking at into one package

There’s only one problem… It’s not working properly:

Being a fault finding master now I volunteeredto help him work out what was wrong with it. The next morning I set about trying to work out what had gone wrong (Steve had already worked it out but it’s a good test of my new found confidence with circuitry so he let me have a play) The problem was the binary to decimal counter was not working correctly, after checking out the leads connecting all the components up it’s definitely a problem in the binary to decimal panel…

Using the output from the monostable 555 I tested each of the binary outputs in turn to see what lit up. No wonder the count is mangled! the binary inputs to the 4028 chip are wired in the wrong order! Steve needed to fix his project then so I didn’t have time to carry the test through to see which input was lighting which output to correct it but with a little more time I wouldn’t have any issues completing the fix.

Right, last visual inspection:

• Double check your circuit matches your diagram.
• Is everything pushed in properly?
• Are all polarised components in the right way round?
• Are your stripped wire ends the right length? (5-10mm)
• Have any connections snapped?
• Are the values of your components correct? (resistors, capacitors etc.)
• Is your IC in the right way round?!?

Astable circuit: the capacitor was not pushed into the right place, all resistor values are ok, capacitor value is ok, all polorised components are placed correctly and the 555 is in the right way round.

Binary counter: I’ve checked the connections to and from the 4510 using the datasheet found here: datasheet HCF4510BE checking the pinout against the board:

1. Pin 16 going high – check
2. Pin 15 clock input taken from the output of the 555 timer – check
3. Pin 14 going to LED output (4s) – check
4. Pin 13 going to ground – check
5. Pin 12 going to ground – check
6. Pin 11 going to LED output (2s) – check
7. Pin 10  going high – check
8. Pin 9 going to ground through pull down resistor – check
9. Pin 8 going to ground – check
10. Pin 7 not connected – check
11. Pin 6 going to LED output (1s) – check
12. Pin 5 going to ground – check
13. Pin 4 going to ground – check
14. Pin 3 going to ground – check
15. Pin 2 going to LED output (8s) – check
16. Pin 1 going to ground – check

 All resistor values are OK, all LEDs are in the right way round, all transistors are connected up correctly and the chip is in the right way round.

Binary to decimal counter: the chip is in the wrong way round!

Using the datasheet found here: datasheet HCF4028BE I’ve checked the pinout against the board:

1. Pin 16 going high – check
2. Pin 15 going to LED output (3) - check
3. Pin 14 going to LED output (1) – check
4. Pin 13 input taken from the binary circuit output (2s) – check
5. Pin 12 input taken from the binary circuit output (4s) – check
6. Pin 11 input taken from the binary circuit output (8s) – check
7. Pin 10 input taken from the binary circuit output (1s) - check
8. Pin 9 going to LED output (8) – check
9. Pin 8 going to ground – check
10. Pin 7 going to LED output (6) – check
11. Pin 6 going to LED output (5) - check
12. Pin 5 going to LED output (9) - wrong! it’s going to 7!
13. Pin 4 going to LED output (7) - wrong! it’s going to 9!
14. Pin 3 going to LED output (0) – check
15. Pin 2 going to LED output (2) – check
16. Pin 1 going to LED output (4) – check

The resistor values are all OK but two of the LEDs are the wrong way round. That, I think, is everything, time to power up… 

• Is anything heating up? (Be careful!)
• Does it smell of burning?

Arrggg hot chip!! The 555 is roasting! I really don’t understand why it’s not working… Steve hint’s to take a closer look… 

The 555 isn’t a 555! it’s a 741CN or an op-amp in English. Right swap it out and try again…

It works! Kind of The LED output representing the 1s on the binary counter isn’t working. Time to tool up.

• Top down, bottom up or split depending on type and size of circuit.
• Multimeter to check continuity and voltage values.
• Oscilloscope to check signal types and changes.

I’m going for the split method as I know that the input is working and some of the output is working so potentially I might have a misfiring pin. Using the oscilloscope I checked out the signals going from the 4 outputs:

As it’s the output corresponding to the 1s column it should switching high and low at a frequency half that of the pulse from the astable output. Hmmm… all the outputs are acting correctly, it must be something in the resistor, transistor LED array.

After the incident with the monstable LED I switched it out to rule out a blown LED which didn’t fix it so… It’s either a problem with the wires or the transistor. I checked the continuity of the wires using the multimeter, no problems there so it must be the transistor. On lifting it out to check the connections I found that sneaky Steve had cut the base lead off to just the right length that it looked correct from a external perspective, clever

It’s not all plain sailing though, after switching out the transistor I loudly announced that it was fixed, sounding ever so pleased with myself… that is until Steve said ‘are you sure?’ Turns out I’d missed another blown LED the one corresponding the the 0 on the decimal side. Oh well, missing one blown bulb isn’t so bad, I won’t be so cocky next time!

• Double check your circuit matches your diagram.
• Is everything pushed in properly?
• Are all polarised components in the right way round?
• Are your stripped wire ends the right length? (5-10mm)
• Have any connections snapped?
• Are the values of your components correct? (resistors, capacitors etc.)
• Is your IC in the right way round?!?

Straight off I can see that although the circuit diagram is correct the connection to the resistor and on to the LED has been moved:

As this is a breadboard circuit the connection needs to be on the same vertical line of connections as the resistor to make the connection.

All polorised components ok, all resistor values ok, the chip is in the right way round, time to power up…

• Is anything heating up? (Be careful!)
• Does it smell of burning?

No explosions Hmmm… still not working. time to do the top down checks

• Top down, bottom up or split depending on type and size of circuit.
• Multimeter to check continuity and voltage values.
• Ocilloscope to check signal types and changes.

I’m using the multi meter to check the power going to and coming from each stage of the circuit using the Top Down method:

1. Voltage is traveling through the switch ok
2. The capacitor is charging properly
3. The readout from Pin 8 is 9V as expected
4. the readout from Pin 4 is 9V as expected
5. Pin 1 is at 0V as expected
6. Pin 2 goes high when the switch is closed
7. Pin 7 follows the capacitor charge as expected
8. Pin6 follows the capacitor charge as expected

This only left Pin 3 which is the output going to the LED, surprisingly this is also going high when the switch is closed, power is getting to the LED… Steve must have fried it!

Bingo, after swapping out the LED for a new one my circuit now works

On to Number 4.

• Double check your circuit matches your diagram.
• Is everything pushed in properly?
• Are all polarised components in the right way round?
• Are your stripped wire ends the right length? (5-10mm)
• Have any connections snapped?
• Are the values of your components correct? (resistors, capacitors etc.)
• Is your IC in the right way round?!?

Hmmm… I can’t see any faults. I wonder if this is a red herring, I’ll double check. I’ve checked the position of the base, collector and emmiter using the transistors datasheet found here: NPN datasheet BC109.

I still can’t find anything, all the polarised components are ok, no loose or broken connections, the values are ok… time to power up.

It works! definitely a red herring then.

On to Number 3.

• Double check your circuit matches your diagram.
• Is everything pushed in properly?
• Are all polarised components in the right way round?
• Are your stripped wire ends the right length? (5-10mm)
• Have any connections snapped?
• Are the values of your components correct? (resistors, capacitors etc.)
• Is your IC in the right way round?!?

On comparing the circuit to the diagram the first fault is found!

An extra wire has been added connecting pin 1 to ground.

After removing the additional wire the circuit matches the diagram. All components are pushed in, the LED is the right way round, no snapped or short connections, all resistors are the correct values and the chip is the right way round, I’ve also double checked the pinout of the chip using the datasheet found here: TC4011BP data sheet. Time to power up…

• Is anything heating up? (Be careful!)
• Does it smell of burning?

Nothing melted and I still have all my fingers but it doesn’t work This is frustrating I’ve double checked the circuit diagram, the connections, the pinout of the chip and by all rights it should work!!

Note to self… Steve is sneaky. As all my circuits are made on breadboards I had removed all references to soldering from my original fault finding document. For this circuit I used two SPST switches, as they don’t fit straight into the board I had to solder on lead wires to the correct points. After examining the switches I can see that he’s changed the points that my wires are attached to, moving one on each off the common!

I’ve adjusted my procedure accordingly

On to Number 2.

The transistor circuit is one that I’ve not tried out before, I’ve simulated it using Circuit Wizard in my previous post so now time to bring it into the real world:

 Next is the monostable:

And lastly the beast… The binary to decimal counter:

I’ve built and tested each one, they’re functional and ready to be maliciously tampered with Steve has volunteered to be the saboteur and after handing over my lovingly put together breadboards *sob* he has made a total of 10 faults. Steve has kept a record of everything he’s done so I can compare notes with him once they’re all working again. On with my protocol then…

Sorry couldn’t resist,  I love the IT Crowd

Ok, what am I fixing and what are my procedures?

I need to remake some of my circuits from my experimentation posts and give them to one of my lovely colleagues to sabotage. I should start with something relatively simple and work up to the complex stuff so…

• Start off with a simple logic gate circuit
• Step it up to a transistor circuit
• Move on to a basic monostable circuit with an LED output
• Finish it off with the binary to decimal counter

4 circuits of increasing complexity… working my way through these and returning them to a working state will prove my competency

I have my circuits but what are my procedures?

I’ve already done some work on fault finding and analysis but I think, considering my past performance, that I need a little help. Dean has highlighted 3 methods of circuit analysis that I can try:

• Top Down – starting at the input, working through the process and finishing with the output
• Bottom Up – starting at the output, working through the process and finishing with the input
• Split – starting with the process and working outwards

Looking at these three methods I can see that they would be suited to different circuits. You might use the top down or bottom up method on a simple circuit but it would take ages to go through a large or complex circuit all the way from input to output. For this type you’d use the split method, identifying the signals going into and coming out of your process section, this narrows it down a little faster.

I’m getting a little ahead of myself here, the first thing I need to do (considering the mischievous nature of my fellow students) is to make sure I’m not going to have my eyebrows singed off first! . If I’m going to hand someone a circuit and ask them to break it there is the distinct possibility that they might put some of my components in the wrong way round, this could result in said eyebrow singing if I just powered it up straight away. Back when I first started to work with electronics I put together a fault finding document, I then added to it for use with prototyping methods. Both procedures included a visual inspection, this has to be the first thing that’s carried out. The next would be a powered inspection and finally using the tools I’ve investigated for analysis using the new methods I’ve learned above.

So my final… awesome… circuit-in-distress-saving  procedure is:

Visual

• Double check your circuit matches your diagram.
• Is everything pushed in properly?
• Are all polarised components in the right way round?
• Are your stripped wire ends the right length? (5-10mm)
• Have any connections snapped?
• Are the values of your components correct? (resistors, capacitors etc.)
• Is your IC in the right way round?!?

Powered

• Is anything heating up? (Be careful!)
• Does it smell of burning?

Tooled Up

• Top down, bottom up or split depending on type and size of circuit.
• Multimeter to check continuity and voltage values.
• Ocilloscope to check signal types and changes.

Time to get building those circuits and choosing someone to break them…

The Wizard has spoken!

After reviewing my progress to date with Dean I now have a way forward to prove my competence. I stated in one of my first posts what I deem to be competence:

 ’To be competent in a subject you need to understand it but also to be able to apply your understanding, to make a product or to be able to understand why something isn’t working and how to fix it.’

Throughout this project I have been working on making functioning digital circuits. Learning about how and why they work the way they do. I consider myself competent in this area. The only thing I have trouble with is the ‘why it doesn’t work and how to fix it’ bit. Each time I’ve had a circuit non functional after powering up I have a bit of a brain hiccup and all my analysing skills seem to fly out of my head. I have needed Dean or Steve to help me get things up and running again.

To complete my work and become competent I need to be able to show that I can do this bit myself. So… I’m going to make up some prototype circuits and get one of my lovely fellow students to break them (carefully checking for any underlying stresses that may make them take the work ‘break’ to extremes ). I’ll then sit down with my arsenal of analysing techniques and equipment and try and fix them.

The ever helpful Steve has aslo been building a binary counter but he has done it in a very different way to mine, he has used a chain of 4 linked D type flip flops.

To give you a brief explanation of what I mean by D type flip flop I have to go back to my logic gates:

This OR gate looks perfectly normal but what if we…

…link it’s output back through one of it’s inputs? This turns it into a digital logic with feedback device. Any digital circuit using feedback is called a multivibrator, the example I used above is a bistable multivibrator as it is stable in one of two states, you can also get monostable (one stable state) and astable (no stable state) just like the 555 timer!

The logic circuit for a D type flip flop looks like this:

The latch in the truth table looks a bit odd though… We know that a high input to a NOR gate will gove a low output, the circuit will ‘latch’ in the state after anytime that the inpit is high.  When the input is low the output could be either high or low depending on the circuits prior state.

It’s quite quite simple but a little longwinded to explain fully but if you fancy finding out exactly how they work have a look here.

So a D type flip flop is a bistable multivibrator, a digital logic device using feedback with two stable states. 

As I said before Steve has been using these to build binary counting circuits so how does that work? First you need to start by looking at the input and output of a flip flop. The input can come from something like a 555 timer astable circuit, this high, low, high, low wave form goes into the clock input and then is fed back through the data input, this results in the second wave form from output Q. The second wave form is half the frequency of the first, the flip flop divides it by two. This is really important for making the counting circuit.

When you feed the output from one chip into the clock input of another it divides the frequency in half again so the following signals will look like…

…this. Look familiar? It’s the same as the pattern of the outputs from the 4510 chip Each output fed into each subsequent chip divides the signal forming the pattern of a binary counter. 

So whats the difference between the two? As I have mentioned in my post on counting circuits there are two types of counting circuit, ripple and synchronous. The arrangement of flip flops I have described above is an example of a ripple counter. It is vary simple and does work but it has it’s disadvantages. The first is there is a tiny delay as the original clock signal ‘ripples’ through the circuit and the second is that if you are using it to trigger a logic circuit it can give glitchy signals, for a ripple counter to count from 0111 (7) to 1000 (8) it will go through 0110, 0100 and 0000 before it settles on 1000, this happens too fast for us to see but not for a logic circuit. 

A synchronous counter is like the one I built in my previous post.It has a much more complex structure which does the job of making sure that all the outputs change at exactly the same time on each clock pulse. It also avoids the tiny false counts so you could use them to trigger a logic system.