Fork me on GitHub

Project Notes

#041 Shifty

Drive 8 LEDs with 3 pins using a 74HC595 shift register.


This is a common circuit - appears in most Arduino tutorial series.

I/O pins are finite resource, hence the many techniques to multiplex output. This approach uses a shift register to provide an 8-bit wide output. Shift registers can be chained to provide even more bandwidth.

The 74HC595 is driven using only 3 digital pins:

  • latch pin (ST_CP)
  • clock pin (SH_CP)
  • Data in (DS)

The latch pin is taken low while a new value is loaded. The new value is presented to the shift register output pins when the latch is taken high.

Data is loaded with the clock and data pins. When the clockPin goes from low to high, the shift register reads the state of the data pin. So to shift out 8 bits we need 8 cycles. We could do this bit by bit, but in this case we can use the shiftOut function to shift out a whole byte in one command.


The Breadboard

The Schematic

The Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.