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…

Rapid look through equivalent chips with Dean and it looks like I can use a 4028 chip instead. A 4028 is officially known as a BCD (binary coded decimal) to decimal (1 of 10) decoder, this should perform the same function as the decade counter would in my counting circuits.

First I want to see if I can build a binary counter running off pulses from an astable 555 circuit, here goes…

The circuit diagram is very simple, the pulses generated by the astable 555 circuit feed into a 4510 binary counter chip, this converts the input signal into the 4 output signals needed to generate the binary count, it’s much easier to see in a diagram:

The chip uses it’s network of tiny logic gates to interpret the input from the 555 and turn it into a binary count. I find it almost easier to understand looking at these wave forms rather than truth table below, but that’s just me, I get on better with visual representations :):

The 4510 is a BCD chip, this means that it will count a binary sequence representing the decimal numbers 0-9 hence binary coded decimal counter :) 

So I’ve calculated my resistor values to give me a good frequency of pulses, I’ve hooked up the 4510 according to the pinout below found in it’s data sheet (more on data sheets here) time to test.

Clang! not working! Hmmm… everything is in the right place, the two chips are hooked up properly what could be wrong… at this point enter Dean, saviour of all malfunctioning circuitry…  

I hate it when it’s something really simple that I haven’t spotted, my reset switch was pushed into the board the wrong way round, *smacks head with palm of hand* and this is after Steve showed me how to use a multimeter to see how components should be connected up.

Now that’s fixed here’s the circuit working:

An extension of this would be to link up lots of timers together. To do this you connect the carry out (pin 7) to the carry in/enable (pin5) of another timer, this second counter now counts up 10s. By cascading them in this way you can count much larger numbers.

Note (07/04/11) There are two types of counting circuit, ripple and synchronous. The linked counters above form a synchronous counting circuit, a ripple counter is made up of linked flip flop chips. More on them here along with a more detailed look into ripple and synchronous counters.

Now I can have a go a converting the digital binary signal I have to a decimal format…

I get to have a go using the 4028 chip :) All you need to do is to hook up the 4 outputs from the binary counter to the 4 inputs of the 4028, then, carefully reading the pinout (they’re not in order as you can see below!) attach your LED outputs.

If you were using a divide by 16 binary counter you’d need to add in an extra step to limit the counter to 0-9. To do this you need to connect an AND gate using outputs B and D as the gates inputs. This would have the effect of triggering the reset every time the counter got to 10 (B and D going high together) keeping it counting from just 0 to 9.

The truth table for the 4028 is really simple, once you get past the volume of 0s and 1s the information is easy to extract:

Right, that’s the board put together, safety checks done and schematic double checked, power up

Crud! Crud! Crud! another no go! what could be wrong this time? The astable is still going, the signals to the binary are still working but the signals going to the 4028 are really weak… once again, Dean to the rescue. This time I really have had a proper brain has temporarily left the premises moment. I’d got the leads coming off the vss and vdd pins going to the opposite of where they should be. There is no excuse for this (apart from said stepping out of brain) as the data sheet clearly shows the right connections. Must slow down a bit…

Anyway now it works :) 

You can take this circuit another step forward by replacing the 4028 with a seven segment display driver. It’s a very similar set up, the 4 outputs from the 4510 go into the display driver (4511) and the outputs then connect up to the inputs on the seven segment display. I may have a go at this later on but for now I think it’s time I revisited competence…

I’m going to take a look at the 3 types of circuit you can make with a 555 timer, astable, monostable and bistable.

Monostable – one stable state

This is my monostable circuit and I’ve calculated that I need a 100K resistor with my 100µf capacitor to give me a time of 10 seconds…

Taking into account the tolerances of the resistor and the capacitor that’s pretty good going. If I wanted to get exactly 10 seconds I could use a POT or a variable capacitor.

Here you can see the capacitor charging using a multimeter (more on those here):

So a monostable circuit generates a single pulse when triggered, without the trigger the circuit produces a low (zero) voltage which is it’s stable state. I pulled this brilliant diagram from a site that’s been really helpful, it shows the wave forms of the signals involved in this circuit:

(http://www.kpsec.freeuk.com/555timer.htm#monostable site accessed 18/03/11)

I had one problem with this circuit in that my LED output kept burning out after 3 or 4 triggers so I decided to investigate…

My multimeter showed that 6.5 v was getting through to my poor little LED, it shouldn’t get more than 1.7V so my resistor was really not limiting enough of the current.

Using Ohms law I worked out which resistor would be best:

R = (VS-VL)/I

VS – supply voltage

VL – LED voltage

R = (6.5V-1.7V)/2ma

R = 4.8/0.02

R = 240R

So by adding another 240R to my 470R resistor I should have enough resistance to stop the burnout. As there isn’t a resistor with a value of 710R I used the next highest value which is 750R.

Monostable circuits are often used to trigger digital logic devices. Mechanical switches can ‘bounce’ when they are closed, causing voltage spikes that would confuse an IC into thinking it’s gotten a high signal. By putting a monostable in between your mechanical switch and your end device you can stop confusing signals.

 Astable – no stable state

An astable circuit is sort of like an electronic metronome using a combination of two resistors and a capacitor you can configure your circuit so you output, say an LED, will turn on and off at a set frequency. It has no stable state because it keeps changing on its own.

I wanted to make my output LED blink on and off for 5 seconds each, 5 on and 5 off. To do this I needed an (initially) scary range of calculations.

The first is for working out the frequency of your circuit, this is how many times per second your circuit completes one cycle of it’s wave form:

Frequency is measured in Hertz so a frequency of 2Hz would mean that your circuit would complete its wave form twice per second.

The next two are for working out the high (on) and low (off) times:

T high = 0.69(R1+R2)xC

T low = 0.69(R2xC) 

So… you start by working out the resistor needed for the low time first:

T low = 0.69(R2xC) 

5sec = 0.69(R2x100µf)

R2 = 5/0.69×0.0001

R2 = 72,464 or 72K (I tried it with a 68K)

Then you can work out R1:

T high = 0.69(R1+R2)xC

5sec = 0.69(R1+75K)x100µf

R1 = 5/(0.69×0.0001)-68,000

R1 = 4,464K (I used a 3K4)

So…

And here it is in test:

Note (25/03/11) Cool siren effect using two astable circuits and POTs! I think I’ve just made a device for communicating with dolphins… or for just really annoying dogs…

The effect of using two astable circuits feeding into the speaker and controllable from the two potentiometers gives you an enormous range of (really annoying ;)) sounds:

Bistable

Right, we’ve had one stable state, no stable states and now… yes you guessed it two stable states!

If this sounds a little like theintro to a cheap game show then I’m setting the right tone…

A bistable state circuit can be used as the buzzer system on a quiz show. The contestants each have one trigger, to trigger the high state and the quiz master has the other to reset it.

This one is the easiest one by far as there are no resistor values to calculate!!

Note (25/03/11) Mnemonic for remembering resister colours:

This originally sounded like Dean was asking me to something on par with “just run along to the nearest dragons lair and bring me back some rubies would you , oh and careful you don’t get scorched along the way”. Me? Make a PCB? From scratch? Right.

First step, what is a PCB? Well as I found out it stands for Printed Circuit Board and is the basis for almost every  electronic product out there.It serves two purposes, as a stable surface to fix components to and as a means of making an electrical connection between them. As someone who has been on the fringes of several PC builds I know what a PCB looks like but have no idea how you would actually go about creating one.

Turns out, not the crazy, complicated quest I first thought. The process is surprisingly simple as long as your artwork isn’t printed backwards! (more on that later). I’ve detailed the steps below, hope you find them as useful as I do:

Step 1 – Get your artwork, we picked up ours from www.picaxe.co.uk. As some of us are relatively new to this we are making the basic PICAXE-08 circuit, designed for the smallest PIC chip they produce, details here axe001_pcb.

Step 2 – Print (the right way round!) onto acetate using a laser printer. Our first attempt was printed from the component view rather than the track view, this meant that the numbers printed on the board appeared the right way round even though the tracks were wrong! Unfortunately we did not realise this until we had finished the process and were soldering components to them, note to self, always double check artwork!

Step 3 – Warm up the UV light box. Peel off the black backing on the board to expose the photo-reactive chemical film over the copper, place the acetate sheet making sure it’s the right way up (When you hold the printed sheet over the board you need to be able to read any numbers or text) onto the light box and cover with the board, film side down, and expose for 3 minutes. (The guy in the photos is my colleague Steve who very kindly ran the demo for us, thanks Steve!)

Step 4 – You now need to develop the image  that has been transferred to the board. You need at make up a solution of (hmmm, can’t find my notes, will need to update tomorrow) in a shallow tray and place the board so it is entirely covered. Gently swish the solution over the board and you will see threads of blue running off the board, carry on until you can’t see anymore blue running off the board.

Step 5 – Now to insert the prepared board into the Rots-Spray etching machine to strip all of the unwanted copper away (H&S info on this machine found here). This machine also need warming up so make sure to turn it on at the start of this process. Fix the board into the holder shown below and slot into the chamber to the right (this unit has the capabilities to develop the board as well in the chamber to the left but it was  not operational when we were using it) there is a notch in the bottom that the holder sits in. Make sure you close the lid! The chemical used to strip the copper is ferric chloride (COSHH info here), it will stain just about anything and can eat through cloth. Set the machine to etch for 1.5 minutes and you will see it start to spray a brownish, yellow fluid at the board (be warned it smells!).

Step 6 – Once the machine has finished, wash the board in the middle slot in the tank to get rid of any remaining ferric chloride and cut to shape using the board guillotine. Using a little PCB drill make all the holes needed in the board. Tip: the drill will naturally find the middle of the hole if you gently hold the board and lower the drill slowly, it feels a little strange at first, normally when drilling you don’t want your material to move but it does work!

Right there you go. the lair was stormed, the dragon tamed (not slain, I think that’s mean) and the rubies gained and no longer shall I think of this as something beyond my capabilities. Now I just need to know how to use it…

The kit comes from a company called Tech Supplies (http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/CHI008) and contains the basics I need for making my safe.

Before I received my kit I had assumed that the keypad would work by assigning an input to each number on the pad so the microcontroller would know which buttons had been pressed, this would take up the majority of pins on my 18 pin chip leaving very few to use for outputs such as lights and sounds. On arrival I found that the microcontoller used a surprisingly simple method of scanning that reduced the number of pins needed to 7. It manages this by breaking the pad up into 3 columns and 4 rows, assigning the columns to inputs and the rows to outputs. If all the outputs (rows) are switched off the signal to the microcontroller along the inputs is 0-0-0. if a button is pressed while the rows are switched off the signal remains 0-0-0, however if you switch one of the rows high and press a button in that row for example 1 the signal to the microcoltroller will be 1-0-0 telling it that button 1 has been pressed. if the signal had been 0-1-0 it would tell the microcoltroller that button 2 had been pressed. The PIC scans the keypad by continuously switching each of the output rows high in turn, waiting for an input signal back corresponding to the button pressed. The full description of the workings of the kit can be found here chi008.

In putting it together I knew that I would need to reposition elements such as the LED and the keypad itself so I soldered it together using flying leads:

This means that I can easily fit the components into my case design and shorten/neaten the wires as required. 

Time to test… (connects up battery pack with eyes squinched shut in anticipation of failure) It Works!! The bi-colour LED glows red to show that the circuit is in it’s locked position, as the code is entered the buzzer sounds for each button press and once the correct code is used the bi-colour LED changes to green and the output for the solenoid (just a connector block at the moment) registers a 4.5v signal when measured with a multi meter then resets after 5 seconds. So I have a working circuit, now all I need is to create my case, connect up a solenoid for my locking mechanism, add in any additional lights and sounds, re-programe my chip and test it all over again (not much to do then…).

After painstakingly putting the kit together and testing it, my contrary PIC chip has decided to stop working. Today I plugged it in to find that only some of the keypad keys are working, the LED won’t flash and the buzzer is making a constant low level bzzzzzzzzzzzzzzzzzzzzzzzzzz (very annoying when you’re trying to find out what’s wrong with it). None of my connections are loose, I’ve got no solder bridges and no crossed connections, time to ask for help from a higher power…. Dean, are you free?….

OK, time to run some diagnostics, after discussing my problem with Dean he has reccomended that we use an oscilloscope (such a cool name) to try and see what is going on inside my circuit. The oscilloscope will track any signals being sent from the legs of my PIC to the outputs on my board, the blip on the screen should jump up for a live signal or remain flat for no signal. Or in the case of my key pad it should jump up and down rapidly to indicate the PIC is scanning each of the output rows in turn.

Once I had taped over the buzzer (there’s only so long you can listen to it going bzzzzzzzzzzzzzzzzz without having serious thoughts about destroying it in an inventive fashion) we got down to investigating. Using the circuit diagram (see below) we tested each output in turn, it didn’t take long to see that outputs 7 and 12 had swapped places, the constant high/low signal that was supposed to be scanning one of the keypad rows was what was causing the buzzer to be so irritating. We’re still not sure what caused this to occur as the only thing that had happened to the board is that I had carried it home and back to the workshop, the only explanation we can come up with is that it suffered a static shock from the box I was keeping it in and it scrambled the programme. Right, keeping it in a static proof bag now…

One rapid reprogramme later and I’m back to having a working board, only 3/4 of a day lost…

Much neater It makes such a difference when fitting it to the case as well:

Now that I’ve finished my alterations I can show you how it is going to work (please excuse the hiccup, my PIC must have been having a bad day…):