Step 13 Choose your LEDs

There are many things to consider when choosing LEDs.

1)
You want the LED cube to be equally visible from all sides. Therefore we strongly recommend using diffused LEDs. A clear LED will shoot the majority of it's light out the top of the LED. A diffused LED will be more or less equally bright from all sides. Clear LEDs also create another problem. If your cube is made up of clear LEDs. The LEDs will also partially illuminate the LEDs above them, since most of the light is directed upwards. This creates some unwanted ghosting effects.

We actually ordered diffused LEDs from eBay, but got 1000 clear LEDs instead. Shipping them back to China to receive a replacement would have taken too much time, so we decided to used the clear LEDs instead. It works fine, but the cube is a lot brighter when viewed from the top as opposed to the sides.

The LEDs we ordered from eBay were actually described as "Defused LEDs". Maybe we should have taken the hint ;) Defusing is something you do to a bomb when you want to prevent it from blowing up, hehe.

2)
Larger LEDs gives you a bigger and brighter pixel, but since since the cube is 8 layers deep, you want enough room to see all the way through to the furthest level. We went with 3mm LEDs because we wanted the cube to be as "transparent" as possible. Our recommendation is to use 3mm diffused LEDs.

3) 
You can buy very cheap lots of 1000 LEDs on eBay. But keep in mind that the quality of the product may be reflected in it's price. We think that there is less chance of LED malfunction if you buy better quality/more expensive LEDs.

4)
Square LEDs would probably look cool to, but then you need to make a soldering template that can accommodate square LEDs. With 3mm round LEDs, all you need is a 3mm drill bit.

5)
Since the cube relies on multiplexing and persistence of vision to create images, each layer is only turned on for 1/8 of the time. This is called a 1/8 duty cycle. To compensate for this, the LEDs have to be bright enough to produce the wanted brightness level at 1/8 duty cycle.

6)
Leg length. The cube design in this instructable uses the legs of the LEDs themselves as the skeleton for the cube. The leg length of the LEDs must be equal to or greater than the distance you want between each LED.

Step 14Choose your resistors

There are three things to consider when choosing the value of your resistors, the LEDs, the 74HC574 that drive the LEDs, and the transistors used to switch the layers on and off.

1) 
If your LEDs came with a data sheet, there should be some ampere ratings in there. Usually, there are two ratings, one mA for continuous load, and mA for burst loads. The LEDs will be running at 1/8 duty cycle, so you can refer to the burst rating.

2) 
The 74HC574 also has some maximum ratings. If all the LEDs on one anode column are on, this chip will supply current 8/8 of the time. You have to keep within the specified maximum mA rating for the output pins. If you look in the data sheet, You will find this line: DC Output Source or Sink Current per Output Pin, IO: 25 mA. Also there is a VCC or GND current maximum rating of 50mA. In order not to exceed this, your LEDs can only run at 50/8 mA since the 74HC574 has 8 outputs. This gives you 6.25 mA to work with.

3) 
The transistors have to switch on and off 64 x the mA of your LEDs. If your LEDs draw 20mA each, that would mean that you have to switch on and off 1.28 Ampere.
The only transistors we had available had a maximum rating of 400mA.

We ended up using resistors of 100 ohms.

While you are waiting for your LED cube parts to arrive in the mail, you can build the guy in the picture below: http://www.instructables.com/id/Resistor-man/