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Topic: shift registers and transistors (Read 1 time) previous topic - next topic

bwoogie

Wait a sec. I'm new to electronics and arduino but I think I have a basic grasp on it. I thought. I'm building a led cube as a first project. I thought the point if a shift register was to replace transistors but i just read you need both. True or false?

johnwasser

The shift register is to expand the number of controllable output pins.

The transistor is to increase the current and/or voltage capability of an output pin.

You may need both.

Some LED drivers act as a shift register and have adjustable current outputs that can drive an LED.

In a matrix (or cube) there are pins that only have to drive one LED at a time.  These generally don't need transistors.  There are other pins that have to drive a whole line (or plane) of LEDs at one time.  These generally  need transistors to handle the current.
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bwoogie


CrossRoads

Its really a matter of how fast you want to multiplex.
If you are ensuring that only 1 LED at a time will  be driven, then you can get by without transistors.
If you want any kind of decent speed tho, you will update a whole layer/plane at a time, and you will need something that can source or sink the current for that whole layer/plane.
If you use a common anode layer, than a PNP transistor or P-channel MOSFET can source the current for the layer, while a power shift register like TPIC6B595 can sink 20mA for each of 8 columns for max brightness.
If you use a common cathode layer, than a NPN transistor or N-channel MOSFET can sick the current for the layer - or maybe even a power shift register, have to look at the specs for other TPIC6x595 shift registers, some will do 350mA/IO pin - but sourcing 20mA/LED is a little tougher, perhaps a High Side switch chip like this
http://www.digikey.com/product-detail/en/AMIS39101PNPB4RG/766-1004-1-ND/1914838
Many folks use 74HC595 as an 8-output source, but I would speculate that most are over stressed that way.
Adding a PNP array to the output of 74HC595 would be better
http://www.digikey.com/product-detail/en/A2982SLWTR-T/620-1127-1-ND/1090370
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