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The [http://www.nxp.com/acrobat_download/datasheets/74HC_HCT595_4.pdf 74HC595] is a really handy, very cheap 8-bit latching shift register chip. I have used it for directly and indirectly driving LEDs and indirectly driving Nixie Tubes. Multiple '''74HC595's''' can be chained together, allowing for an almost limitless number of outputs. And all the while, only four microprocessor pins (or three if you tie pin 13 (output enable) to ground) to control one or more '''74HC595s'''.
 
Here I demonstrate how to interface it with the [http://www.buspirate.com/ Bus Pirate]. The Bus Pirate is really handy for learning how to interface an unfamiliar chip without having to write a line of microprocessor code. Or if you simply haven't built that bit of the project that interfaces with the '''74HC595''' just yet.

[[Image:BusPirate.jpg|thumb|Bus Pirate v2go with DIY probe cable.]]

Hardware setup:

# Connect Bus Pirate '''MOSI''' to pin 14 (serial data in) of the '''74HC595'''
# Connect Bus Pirate '''CLK''' to pin 11 (clock) of the '''74HC595'''
# Connect Bus Pirate '''CS''' to pin 12 (latch enable) of the '''74HC595'''
# Connect Bus Pirate '''AUX''' to pin 13 (output enable) of the '''74HC595'''

From the terminal, enter '''RAW3WIRE''' (Fast Speed, Normal outputs) mode:

 M <enter>
 8 <enter>
 2 <enter>
 2 <enter>

Enable the '''74HC595's''' outputs by setting the Bus Pirate '''AUX''' pin low ('a'):

 a <enter>

(the '''74HC595's''' output enable pin is active low.)

To change the ''74HC595's''' 8 outputs, simply write one byte (''0b11111111'') and toggle the Bus Pirate '''CS''' pin (''{}''):

 0b11111111 {}  <enter>

(writing the byte fills the shift register and toggling the '''CS''' pin latches the data in the shift register.)