So what is multiplexing:
It is basically a way to split information in to little peaces and send it one by one.
this way you can save a lot of pins on the Arduino and keep your program quite simple.
In our case we split the image that we want to display to 10 peaces (10 rows), We want to scan the rows of the matrix( light up one row at a time) and send info from the Arduino to the columns.
All the columns are positives of the LEDs and the rows are negatives so if the first row is connected to ground and we send information to the columns we will only light the first row.
To get a good display we need to scan the rows very fast, so fast the the human eye thinks that all of the rows are connected at the same time.
So why the 4017:
For this LED matrix I wanted to use this useful IC.
Heres a good site to learn the basics of this IC : http://www.doctronics.co.uk/4017.htm
The 4017 decade counter is used to allow multiplexing.
This IC basically scans the rows of the matrix( lights up one row at a time).
In our case we want to connect the rows to ground but the 4017 doesn't build to sink current, so to solve this little problem we need to use a transistor with a resistor.
The 4017 has 10 output pins so we need 10 resistors and 10 transistors, we connect the 1K resistors to the outputs of the 4017 and the base of the transistor to the other end of the resistor.
Then we connect the collectors of the transistor to the rows and the emitter to the ground.
Heres the data sheet of the transistor we need to use : http://www.fairchildsemi.com/ds/2N/2N3904.pdf
The shift register:
This little IC is a very useful one it allows you to control lots of outputs with the use of onlt 3 pins from the micro-controller. By connecting more IC's you can increase the number of outputs with losing more micro-controller pins.
You can read more about them and how to use them with arduino in this link: