LED Cube With Arduino and Custom PCB

This instructable details the design and building process for a 5 x 5 x 5 LED cube, controlled with an Arduino, which resides on a custom printed circuit board. 

Additional information, photos, and videos can be found on my website.

The finished product is shown in the video below:

Step 1: Design Concept and Materials

I have seen many designs for LED Cubes, and they all share the same problem:  How to control so many LEDs with so few pins.  Many designers choose to utilize shift registers, which uses a serial load with parallel output.  I was not to fond of this idea, primarily because of the time needed to shift all bits and the possible resulting trailing effect, so I started from my own drawing board, see attached.   

My design uses 5 x 3-8 line decoders (also known as DEMUX) to convert a 5-bit parallel binary output to a one-hot 25-bit parallel output, which drives the columns of LEDs.  "One-Hot" means that only one of the 25 output pins will be "hot" at any given moment.  If the five output pins of the arduino are: 01010, this is the number 10 in binary.  The decoders take interpret this signal and in turn power on output pin number 10 of the 25 columns (numbered 0-24).  See the attached design for illustration.

As many other LED cubes do, my design also uses NPN transistors to switch the cathodes of each plane of the cube.

My design also includes a custom designed Printed Circuit Board, to eliminate the many unsightly wires that would otherwise be needed.    

Materials:

Part No.          Description                          Vendor             Quantity     Each        Total
74HC238       3-8 Decoder                        eBay                  5                $0.70        $3.50
LED                5mm Diffused Blue LED   eBay                 125            $0.09         $11.25
RES               150 Ohm Resistor                                         30              $0.05        $1.50
2N4401         NPN Transistor                    RadioShack    5                $0.20        $1.00
POT               10k Ohm Trim-Pot                RadioShack   1                 $1.49        $1.49
Arduino         Arduino                                   SparkFun        1                 $30.00      $30.00
PCB               PCB + Shipping                    AdvancedCircuits 1          $51.42      $51.42

Grand Total: $100.16

Second Thoughts:  Be sure to get one-HOT decoders, many will have every output high, but the one selected output LOW.  This is the opposite of what we want here.  Be CERTAIN to check the truth table in the  data sheet of whatever chip you purchase. 

Step 2: Cube Construction

The first step is to construct the cube of LEDs.  My LEDs were very cheap and the leads were incredibly short.  This is why I used additional wire, which worked out very nicely.

Firstly, make yourself a wooden jig to hold the LEDs when you solder.  I spaced mine 1" apart.  Solder all the CATHODES (-) together in the plane. 

Once you have all 5 planes done, its time to go up!  Use a 1" spacer between the planes as you solder.  Be sure you run enough wire vertically for all 5 planes, and then some extra.  

Done? Congratulations! You just made about 300 solder joints!

Second Thoughts:   Upon completion,  it appears that the Cube leans, and thats because it does.  When attaching the planes to the vertical wires, I attached to the same side of the LED lead each time.  To avoid this, alternate which side of the lead you attach to the vertical wire. 

Step 3: Prototype Circuit

Once you have the cube constructed, solder test leads to each anode and cathode set.  I used cat5 cable, purely because it is cheap and available.  If you don't have any spare wire, go to your local telephone system installation company and ask to buy their scrap, you can probably get it for less than 30 cents a foot.  

Assemble the circuit (as described in the pdf) on a bread board.  Choose a corner of the cube to be the origin, and connect that anode to output 0 from the decoders.  The next anode will be in the alleged X direction, and then you go to the next row down (output number 5) you will have moved in the alleged Y direction.  Don't forget resistors!  I used 150 Ohm resistors in series with each column (between the decoder and the column)

To wire up the cathodes, use the NPN transistors and switches to ground.  You need to use a resistor between the base pin and the output from the arduino.  If you have never used transistors before, they are fairly straightforward: for an NPN type, the two outer leads are like the two terminals of a simple switch.  The center lead is the signal that, when HIGH, completes the circuit.  Thus connect pin 1 to ground, pin 2 to arduino ouput, and pin 3 to the cathode plane of the cube.  

Step 4: Programming With Arduino

Once the Cube is wired up to your prototype circuit, write some test code!

One clever function I developed to efficiently realize the output to the decoders is documented below, and also found in the code attached.  This is where some bitwise black magic obviously occurs.

/**
 * Displays the anode column with the given number value; [0, 24].
 */
void displayNum(int num){
  //constrain the argument to be between 0 and 24 inclusive.
  num = constrain(num, 0, 24);

  /*
   * AND: selects the bit, the bit at weight will be 1 if the pin is to be high
   * >>: shifts the selected bit to the end of the word, making the value a 0 or 1
   * first result is lsb
   * digitalWrite: write the approptiate result (HIGH or LOW)
* to the appropriate decoder pin
*/
for(int weight=1, pin=0; pin < DECODER_BITS; weight*=2, pin++)
digitalWrite(decoderPins[pin] ,(num & weight) >> pin);

//delay, this is the absoloute minimum time the light will be displayed.
//ensures adequate delay for decoders as well.
delayMicroseconds(MICRO);
} 


The rest of the code I used is attached here as well.  This is broken into 4 main pieces. 

LEDs.h:  
     Contains all pins definitions, and arrays containing pins for swift iterations. 
DisplayBasics.pde:  
     Contains a few basic "shapes" in the cube, for use in patterns. 
Patterns.pde:
     Contains patterns which the cube can display.  Each is documented in the code, and can be seen in the video in the intro step of this instructable. 
LEDCubePCB.pde:
     This is the final version of my code, and its setup() and loop() functions.  you will notice that I choose which pattern to display at reset based on the position of the potentiometer (discussed next step).  I would encourage a better way to change patterns, also discussed in the next step. 

Step 5: Add-ons

To make the LED Cube more functional as a standalone piece of decor, settings need to be adjustable on the fly, and not by re uploading code each time you want to change the pattern.

One add-on I used was a potentiometer, whose analog reading was directly related to the delay time of the animation, as seen in the video.

Second Thoughts:

Another intent I had was to use jumpers to select which pattern to display--this can be seen on the PCB design.  however, I never tested this concept, and forgot that a pin reading is unstable without a reference voltage.  If you try this, you will probably need a pull up resistor configuration.  Whatever you do, don't do what is shown on the PCB design, and do test it. 

Also, as cheap as they are, potentiometers are versatile, and a second one could easily be used as a pattern selector. 

Step 6: PCB Design

I designed this circuit and PCB in Eagle, which is free PCB design software, available at www.cadsoftusa.com .  I have attached the eagle files for your reference or reuse, but as stated previously, some re-working my be desired.  

If you are new to PCB design, it is easy and fun!  One good tutorial can be found at the instructable titled "Turn your EAGLE schematic into a PCB" .

Second Thoughts:
One thing to watch out for when designing a PCB is the size of the drill holes.  Most parts in the library are good, but be sure to check wire connections, like those of the anode columns. 

Also, if you like the project, but not the cost of the professionally fabricated PCB, you could also easily create this on a perfboard , or even use a toner transfer do make your own PCB while still using Eagle .

EDIT:  The attached eagle files have been fixed for the jumper issue, they now jump to GND instead of VCC

Step 7: PCB Manufacture and Assembly

To send a design out for manufacture, you will first need to create a drill file and gerber files.  This area is not my specialty, but the instructable "Professional PCBs almost cheaper than making them at home" nails it, follow the instructions to a T and you will have no problems. 

EDIT: Due to a few requests, I have attached the gerber files needed by the manufacturer to this page.  Please notice that the issue with the jumpers has been fixed in this version. (they now jump to GND, which will work by using the arduino internal pull-up resistors.  

Finding a manufacturer:
I got my PCB fabricated at Advanced Circuits , and here's why:
1.  US Based:  Don't get me wrong, I order TONS of components Hong Kong direct, the postman is used to seeing Chinese stamped padded envelopes in my box, but matters here is that when I am antsy for my board to arrive, I DON't have to wait three weeks for shipping!

2. $33 each. yup, just $33 per board for standard spec orders.  Whats the catch? Minimum order of 3, see below.

3. STUDENT DISCOUNT!  Advanced Circuits will let you order only one of their $33 each special for students!

4. Free DFM check, basically it makes sure your drill and gerber files are correct before you give them any money.

5.  Free surprise with each order, its popin good ;)

Assembly:
Begin soldering components with the lowest profile, meaning height.  Start with resistors, then the IC sockets, ... , and finish with the cube itself last. 

I used all through hole components, so assembly is pretty straightforward.  If you've never soldered on a PCB before, here are the keys:

1.  Use a good, clean iron.  Clean your tip with tip tinner and a wet sponge.
2.  Heat the PART*.
3.  Apply solder to the BASE OF THE HOLE.
4.  Allow solder to seep into hole before removing heat. (about a half second)

*Be cautious not to overheat components containing semiconductors, as they are easily damaged by heat.  I used sockets for all DIP chips (decoders)

Step 8: Done!

Once your custom board is assembled, test it out!  Upload the code and snap on your arduino. 

For more pictures, descriptive videos, and more projects, check out my website at:
https://sites.google.com/site/andrewmontag/personal-projects/led-cube

Comments? Questions? Post-em!

Second Thoughts:
If the lights don't act as you anticipate, first check that your decoder pins are placed properly, as indicated in the PDF of step 1.  Swapping bits will mess it up big time.