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.

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 first to come to mind (and to hand sitting at my laptop ) is a program called Circuit Wizard, I’ve already used this in previous posts to create my circuit diagrams. You can build a circuit using symbols or pictures of the components, (it’s really helpful to have a go at the pictures version with some simple circuits first if you have trouble reading circuit diagrams) this makes it easy to build up larger circuit diagrams quickly as you don’t have to spend time drawing out and labeling loads of components.

This programme also has an amazingly useful function where by you can simulate applying power to your circuit. It gives you real time results of what your circuit will do (or won’t if you’ve made a mistake ) You can check that a circuit you have designed will work and do what it is intended to do without ever having to pick up a soldering iron.

The other prototyping method I’ve been exploring is solderless Breadboards, these allow you to easily build and dismantle temporary circuits. They have hundreds of little sockets called contact holes that you can push components or wires into, these connect to metal strips running under the surface that connect groups of these contact holes into groups:

The yellow lines on the photo above show where the metal strips run: the top and bottom strips run horizontally and connect every socket in that row (they break where the red and blue lines stop), the vertical strips in the main body of the board connect up 5 sockets in a numbered row. The gap in the middle is designed for ICs to straddle, this gives you independent connections for each pin. When you poke a component lead or a wire into a socket it makes contact with the metal strip, by plugging in components and connections in the right way and running a power supply to your breadboard you can build a working circuit and because none of the components are permanently fixed it’s easy to tweak component values without the hassle of un-soldering and re-soldering. Once you’ve got your circuit working the way you want it to you can transfer it to a PCB. It’s also best to keep any circuit inputs on the left side of the board and any out puts on the right side, this keeps everything nice and neat and can be a big help when it comes to fault finding. Oh and only ever use single core wire on a breadboard, not only will multi core bend and wiggle when your trying to push it in the sockets but small bits can break off and cause shorts!

As with a perminant circuit you need to carry out a fault finding procedure before applying power. This can help prevent disasters like frying the last 555 timer you have thus condeming you to the circuit purgatory of not knowing if it would have worked after all… (shudders)

I’ve written out a little list to follow to save yourself any heartache (and possible singed fingers… eyebrows…)

• 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?!?

Finally you can switch on, make sure your power supply is set to the correct voltage (never use mains on a breadboard, it could melt and you could be electrocuted!). Once power is flowing perform a powered inspection:

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

Hopefully you now have a working circuit, if not you need to move into the realms of circuit analysis which I’ll go into later.

The soldering was quite easy, some of my GCSE knowledge came drifting back up from where it was buried in my brain plus a couple of the guy’s here have worked as electrical engineers (thanks Steve!)

Tips:

• Make sure that you heat up the lead and the track before touching the solder to the iron, this helps the solder flow to both. (you can see this in the photo to the right)
• ‘Wet’ the end of the iron first by touching the solder lightly to the tip of the iron so just a little adheres to the end.

Right now I’ve finished soldering my components (note that I did not have a terminal block so my battery leads are just soldered straight to the board) I need to see if it will work. First thing I need to do is to perform a visual inspection, this is crucial as a bad/crossed connection can cause your battery to blow up! Here’s the procedure I followed to make sure there were no unexpected bangs:

VISUAL INSPECTION

Looking for:

Soldering errors

Board errors

Component errors

BACK

1)      Ensure all pins are soldered

2)      Are there any solder bridges? (too much solder flowing over to another contact)

3)      Are there any dry joints? (where the solder is not joined to the track and the lead)

4)      Looking for continuity of copper tracks and bridges (checking the mask, was it exposed properly?, was it too long in the solution?)

5)      Are the holes drilled in the right place? (not breaking the tracks)

6)      Are the leads trimmed properly? (don’t bend leads at right angles when soldering!)

7)      Has the board been produced with the artwork the right way round?!?

FRONT

1)      Check the values and placement of components

2)      General condition of components (no cracks, corrosion etc.)

3)      Are the polarised components in the right way round? (anodes and cathodes)

4)      Is anything loose? (links to solder inspection on the back)

Right now I can be as sure as I can be that nothing untoward will happen when I connect up my circuit to the battery pack, here goes…

POWERED INSPECTION

1)      Is anything heating up? (Be careful!)

2)      Does it smell of burning?

Well I still have all my fingers and both eyebrows are intact, now all I need to do is to plug it into a computer and try to programme it.