Building the TARDIS - (Pulsating LED Circuit)

Hello readers! In this Instructable, I am going to be detailing how to make your very own pulsating LED, just like the light on the top of the TARDIS! I am going to do my very best to explain this process, and why certain aspects of this circuit are designed in that particular way, as well as hopefully providing you with the knowledge that enables you to alter certain things in this build to match your liking. The inspiration for this project came when designing a clock face for my Resistant Materials GCSE. I wanted to make my clock look like a TARDIS, however, didn't want it to compose of just boring plastic. So, I decided to replace the top of my acrylic TARDIS with a pulsating LED. I then designed a circuit that could perform that very task (see the diagram above). So, without any further ado, let's get straight into it!

Before proceeding, I would like to just quickly recommend that you pay careful attention to the illustrative pictures provided on each step! They have detailed notes and will definitely help you with building and understanding this circuit!

Step 1: Gather All the Parts!

OK, so let's start by gathering all the parts we are going to need. You should be able to pick these up on the cheap from your local big electronics store (US: RadioShack UK: Maplin Online: farnell.com)

For this build, you will need:

The Circuit:

Veroboard (Stripboard) (about a 12cm x 12cm area will suffice)
A switch
5 x 330Ω resistors
3 x 10kΩ resistors
An 8-pin IC holder
A 555-Timer chip
A 100uF (microFarad) capacitor
An LED (of any colour)
A BC108 transistor

For Power:

A 9 volt battery
A 9 volt battery clip

The LED:

Wire (and plenty of it: red and black)
Heat-shrink tubing of different diameters

Also (optional):

Squares of Velcro (optional - for finishing touches)

Step 2: Gather All the Equipment!

Now we have got all of our parts, let's get on with putting them together! We can't just stick them on the board and hope for the best though: we need some equipment! On the equipment side of things, you will need:

The Absolute Basics:

A soldering iron or soldering station
Solder
A pair of pliers (or basically anything that will be able to cut the leads off our parts soldered into the Veroboard)
A stripboard track cutter
Matches
This Instructable
YOU - the most important tool of all!

Some Welcome Additions (SUGGESTIONS):

A 'helping hand' tool - usually consisting of two alligator clips and a magnifying glass
A 'circuit board holder' (I don't have one of these but it should be a very helpful tool!)
A heat-gun (in replace of the matches - will be extremely useful and do a much better job on the heat-shrink tubing than matches!)

Step 3: Place the Parts on the Veroboard!

So, now it's time to get to work - and this is perhaps the most difficult step of this entire project: placing the components on the bare circuit board! First get your Veroboard and position it with the lines on the rear pointing vertically. Place your collected components on the Veroboard as shown in the photographs above! Yes, these diagrams ARE of the completed circuit: somehow these pictures were corrupted when taken, so I have had to make do with pictures from later steps! You're clever! I have no doubt you will be able to work it out from the above illustrations! One cannot just put parts anywhere on the board and hope for it to work! I have put these parts in these particular places for a reason: it will conserve as much space as possible using the least jumpers as possible!

All in all, the positioning of the components matters and is a hard skill to learn - however, I hope you see now why I have placed them in those positions. If you're knew to Veroboard/stripboard then do make sure to spread your components out. It will certainly help!

When placing the parts on the board, make sure:

There is a flow of current through the switch from the positive lead of the battery connector to the output of the switch. If your switch is like mine and has 3 pins, this can be a bit confusing. So check with a multimeter that, when the switch is flicked on, there is a flow of electricity through the two pins you are going to use to allow electricity to flow into the main circuit. (I do not have a multimeter so I had to do a few test circuits to try and understand how the switch worked.)
You bend the leads under the components very slightly before soldering them in, to stop them falling out when you turn the board over.
You put the transistor in the correct orientation: with the tab on the side of the circuit on the right.
You put the capacitor (the little blue, tall cylindrical component) the right way around: with the cathode (also marks by a white stripe on that side on the left).
You place your 8 pin chip cradle with the notch pointing left.

Step 4: Solder Them On!

Now it's time to utilize our trusty soldering iron! Set up your soldering iron (or soldering station if you have one). It should need about 3 mins to heat up properly. Take out your solder and let's get to it!

All of the parts we've put in the board so far need to be soldered in. This is is pretty easy - a very simple step that doesn't need much explaining, yet if you are new to it - follow the tips below!

Practise A Good Soldering Technique:

Always have a steady surface to work on. We wouldn't want to burn our fingers if something slipped (like I have done, many, many times).
Tin and clean your soldering iron regularly. Tinning your soldering iron involves periodically coating the tip of your soldering iron with solder. Cleaning is very simple as you should have received a sponge that fits into your soldering iron holder when you bought your it. Make sure this is wet and available to clean your iron on.
It is a good idea to bend the connections on the rear of the board when you place them in the Veroboard in the first place. This restrains the terminal you will be soldering from moving or even falling out!
Always heat the terminal you are soldering to the board a little first instead of just going straight in with solder and your iron. If you weren't to do this, the solder would just form a bubble around the cold terminal and very likely not make a solid connection to the tracks on the Veroboard. This is known as a 'cold solder joint'.

Step 5: Now Time for Some Jumpers!

Now it's time to include some 'jumpers' in our build. Basically, these are small pieces of wire that allow electricity to flow to different places in the circuit. In the diagram above you can see that I have utilized 'jumpers' to connect the positive current coming from the switch to different parts of the 555 Timer that I usually wouldn't be able to connect to. You may also notice that some jumpers come from different parts of the circuit to the track in line with the negative end of our battery (thus, grounding the circuit).

To make a jumper is very simple, it's just a small piece of wire. In this build, I stripped these wires to illustrate to you what indeed they were.

In this circuit you will need a jumper from:

The positive track to pin 8 on the 555 Timer (see diagram)
The positive track to pin 4 on the 555 Timer (see diagram)
The negative track to pin 1 on the 555 Timer (see diagram)
The negative track to the cathode on the capacitor (see diagram)
The middle connection from the transistor to the anode of the capacitor (see diagram)
The middle connection from the transistor to the 10kΩ resistor (see diagram)
The far right connection from the transistor to the far right connection of the 330Ω resistor series (see diagram)
Pin 2 to Pin 6 on the 555 Timer (see diagram 2)

I hope this is legible enough! Anyway, the simple process is to:

Take your piece of wire
Strip the ends (or the whole thing)
Place the ends in the appropriate terminals
Solder them in!

For connecting Pin 2 to Pin 6 on the 555 Timer, it is best to put the jumper on the rear of the board (as displayed in diagram 2).

Simple! - Huh?

Step 6: Cut the Tracks Under the Veroboard!

SEE IMAGES FOR WHERE TO CUT! We cut the tracks of our Veroboard to restrict current from flowing to places we don't want it to - even when the terminals are in the same track. To cut the tracks is one of these steps that should be paid a great deal of attention and care when doing as if it is done wrong then it cannot be undone like a solder joint can. It is irreversible so get it right! You will need your stripboard track cutter for this bit! You will need to cut the tracks:

Under the 555 Timer socket (see photograph 1) - we do this to disconnect the pins from each other on the board. If we didn't Pin 1 would be connected to Pin 8, Pin 2 would be connected to Pin 7 and Pin 3 to Pin 6, etc.
Under our resistor series - thus, isolating the resistors from the rest of the circuit!
Under the vertical 10kΩ resistor - if we didn't current would bypass the resistor and the circuit wouldn't work properly.
ONCE on the same track as Pin 7 of the 555 Timer
ONCE under the far left of our resistor series on the same track as the final connection yet leaving some space to connect the LED.

After that, your done with cutting the Veroboard tracks! It's now time to sort out our LED - proceed onto the next step!

Step 7: Prepare the LED!

Of course to make a pulsating LED, we need an LED! In Step 2, I told you that you could use one of any colour: I am using a high intensity, high efficiency white LED! For this project, I am installing the LED in a remote location via a wire rather than merely soldering it to the board (although you can do that if you want) so I am going to need to extend it with some wires. Firstly, take your LED and lie it on your work surface. Notice how around one connection is longer than another. The longer connection is called the anode and is the positive connection for the LED. The shorter side is called the cathode and (you guessed it!) is the negative connection of the LED! Familiarise yourself with this concept!

Furthermore, take some wires, I am using red (marking positive) and black (marking negative). Cut them as long or as short as you need depending on how far your LED needs to reach from your main circuit board! Strip the wires with wire strippers (or your teeth). Now solder the red wire to the anode and the black wire to the cathode.

Then slip some thin heat-shrink tubing over the connections. This prevents them from shorting the circuit if they were to touch! Take a match (or ideally a heat gun) and move it up and down the tubing until you can clearly see that you have a tight fit around the connection. REMEMBER: You CAN overheat this tubing so don't go at it for ages as it will melt and you may have to start again!

The next bit is completely optional but I like to twist the wires - it makes it not only look a lot neater but cooler as well! I then bound my wires with an elastic band - again to keep it nice and neat!

Once you have done that - you have successfully prepared your LED. You are ready to proceed to the final stage of this project: soldering everything together! If you have got this far - WELL DONE!

Step 8: Solder the LED to the Board!

To solder the LED to the board, you need to solder the positive connection of the LED, (called the anode) on the same track as the end of our series of resistors. You can only go either above the resistor connection (moving away from the 555 Timer) or you can solder to the single open terminal underneath the resistor connection (moving closer to the 555 Timer). However you have already cut the track very close to that resistors connection so be very careful where you go - REMEMBER: it MUST be in the same track as the last connection from the last resistor, and also on the same side of our line of cut tracks as the resistor connections themselves!

You must also solder the negative connection of the LED, (called the cathode) on the same track as the negative connection from our battery (the black wire from our battery clip). It is a good idea to draw lines in red and black Sharpie as to which tracks you are lining up! After you have done that - YOUR CIRCUIT IS COMPLETE!

Step 9: Done! - What You Can Change & How!

Congratulations! You have successfully completed the circuit! Now, flicking the switch should activate our pulsating LED - just like mine in the pictures above! Proceed through the following steps to see some finishing touches I added to my board, and also how this entire circuit works! Anyway, if you don't fancy that and want to take well deserved rest I wouldn't blame you! However, please could you just take the time to 'VOTE!' for this Instructable! I worked so very hard to produce it to the standard that is, so please show me that you appreciate my lengthy efforts! Thanks again for reading!

Resolving Possible Errors: If the above doesn't happen, common errors that one might have made, include: incorrect orientation of such components such as the: capacitor, LED, transistor or 555 Timer chip. Other errors may include not cutting the tracks under the Veroboard properly. To fix these errors, I suggest looking back on this Instructable, there is always a chance one might have missed something - we're only human!

What You Can (and cannot) Change:

You can change the speed of the pulsating very easily! You can alter the value of the 10kΩ resistors. You could even put in a variable resistor so you always have the ability to change the speed slightly!
You can also change the value of the capacitor slightly to alter this same aspect of the flashing/pulsating!
You cannot change the 555 Timer chip to any other chip!
You cannot change the transistor!
Do not change the voltage: it will very likely not work as the circuit won't have enough supply voltage - so stick to a healthy 9 volts!

Step 10: Add Some Finishing Touches (OPTIONAL)

So, now it's time to add some finishing touches! I always like to do this with my projects, especially if they are going to be put into use in actual products. This particular circuit I will be incorporating into TARDIS (hence the title) clock project that I am doing for my Resistant Materials GCSE. I will be attaching this circuit to the back of my acrylic TARDIS with some Velcro squares as I think hot-glue is too messy and if anything goes wrong with circuit, Velcro will allow me to simply pull it off and correct any issues!

Step 11: Conclusion: How Does It Work?

OK then: How It Works...

See Figure 1: If the capacitor is initially uncharged and then at a time (t = 0), we switch the switch on: the output of Figure 1 (which is the voltage across the capacitor) rises. If we were to measure the voltage with respect to time, it would rise like the diagram in Figure 2.

See Figure 2: This diagram shows: the time is takes for the voltage to rise is dependent on the value of C (the capacitor) and R (the resisitor).

Creating An Oscillator: So, to create an oscillator (i.e. our pulsating LED), what we want to do is, when charging the capacitor, detect when the voltage has risen to a particular value, then discharge the capacitor until it falls to a particular value, and then repeat. Simple! This is just what our 555 Timer can be configured to do!

The 555 Timer: Basically, the 555 Timer has internally two voltage components and a 'flip-flop' (a sort of switch).

See Figure 3: Figure 3 shows the names of the pins on the 555 Timer. What the 555 Timer does is compare the voltage on the THRESHOLD pin. If it is less than 1/3 of the supply voltage (1/3 of 9V is 3V), it sets the OUTPUT voltage to +9V (the supply voltage). If the THRESHOLD pin is greater than 2/3 of the supply voltage (2/3 of 9V is 6V), it resets the OUTPUT to zero volts. So lets take a look at the completed circuit: Figure 4.

See Figure 4: So, at POWER ON: the TRIGGER pin (pin2) is at zero volts as the capacitor is uncharged! This sets the OUTPUT voltage to +9V. The capacitor now begins to charge through the 2 10kΩ resistors. Eventually, as the capacitor charges the voltage reaches 2/3 of the supply voltage. At this point in the process, the 'flip-flop' switch changes to zero volts and the reverse happens. The capacitor now discharges through our resistors until again the voltage reaches below 1/3 of the supply voltage - and 'flip', it switches to +9V again and the whole process repeats!

What About The Transistor?: This transistor is basically and amplifier. In this case, our transistor is an amplifier of current! Without it:

ALL the current should go into the capacitor to charge it up!
With the LED connected to the capacitor, the LED will take all the current.
Thus, there will be none left to charge the capacitor!

So, what one can do is connect a transistor. Transistors amplify current. We connect a transistor that takes very little current in but can give a 'big' current out and thus there is enough current to power both the capacitor AND the LED.

The last thing that needs explaining is why we have used 4 resistors of 330Ω in series! So time for some maths:

4 x 330Ω = 1.32kΩ

An LED produces a light intensity dependent on the current flowing through it. Unfortunately, our eyes can see this change but only up to a certain extent. Yes, the LED goes brighter with more current flowing through it, but our eyes do not 'see' the increase. If we take the mean value of the voltage on the capacitor =

1/2 Supply Voltage = +4.5V

The resistance we are using is 1320Ω. So,

Current = Voltage / Resistance

I = V / R

4.5(V) / 1320(Ω) =3.4(mA)

This current is very low but makes the LED reasonably bright! I have used high efficiency LEDs in build, however, the LEDs you use may have a different response. If you want to make your LED really bright: remove resistors, yet ALWAYS use at least 1 resistor! This is because most conventional LEDs have a maximum current of about 20mA and you cannot exceed the LEDs maximum current rating.

Conclusion: During 1 oscillation, voltage on the capacitor goes to 2/3 of +9V = 6V. And, Current (I) = 3.4mA! I sure hope that is clear enough! Thank you for reading this instructable and PLEASE VOTE for it if you feel it deserves it! I have worked very hard to produce this! Thanks!

- Sam Naughton B