I am building my first project, a LED matrix. The matrix is controlled by a pic18F4550 and the rows are driven using a TLC5940 led driver (chose this chip for RGB later down the road). Right now I have a single line of LEDs working with each LED connected to ground. For a matrix would the best way to connect each column to ground be by using a shift register to drive transistors connected to each column?? If so what kind of transistor should I use and how do I tell that is the right transistor for future reference.

 

Take a look a the STPIC6C595, which is a Power Logic 8-Bit Shift Register.
Datasheet:
http://www.st.com/stonline/products/literature/ds/7726.pdf

If you use something like that, you won't need transistors/MOSFETs for your low side drivers, as they are right in the IC itself. Cool, huh?

However, you say you are using a TLC5940, which is also a current sink type driver.

It would help a great deal if you posted an image of your schematic, even if it's just a rough concept. Schematics are the "language" of electronics; the viewer can get nearly all of their questions answered in short order without wordy explanations that can result in confusion.
.PNG format is preferred, use the [Go Advanced] and [Manage Attachments] buttons to upload the image from your computer.

 

Sorry SgtWookie I don't have a schematic and I did mean sourcing (Still new to this stuff). I was thinking that I should multiplex my source and possibly use a shift register to hold the multiplexer so that I am not using all the pins on the uC and for better scallability if I decided to expand the array.

Do you think that would be the best possible solution for me to take.

 

Well, you'll need something that has open-collector outputs.

You could use a 595 shift register with 07 open-collector buffers on the outputs to switch PNP transistor on the high side. You'll need pull-up resistors from the PNP's base to +V.

The pull-up resistors should be about 4 to 10 times the resistance of the base resistors.

The base resistors for the PNP transistors should be calculated as:
Rbase <= (V+-Vbe)/(Ic/10)
where:
V+ = your high-side supply voltage.
Vbe = 0.7V (typical for small signal /switching transistors at light to medium load).
Ic = your desired maximum collector current.
Note that Ic/10 should not exceed 10mA, or you risk burning out the transistor or the '07 buffer.

 

If you have enough pins on your µC, I suggest discrete transistors or a 74**07 type chip. The 74**07 has 6 open collector or open drain (depending on whether you get the TTL or CMOS versions) outputs. The advantage with discrete transistors like an n-MOSFET is that you can have 30mA+ bright LEDs (you could even go to power LEDs if you wanted), while the 74HCT07/etc are generally limited in current to about 20mA, with overall limits for the power supply pins...

Since you have a PIC18F4550, you don't seem to be too limited in pins, as you have 40 of them, but if you are driving a large array it could be tough and you could end up needing a shift register or other suitable chip.

 

Thin film transistor will be a suitable one..

<SNIP>