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Project Notes

#177 7-Segment Shift Drive

Test control of a 7-segment LED display via a 74HC595 shift register.

Here’s a quick video of the circuit in action:



While you can wire up each segment pin to a digital out on the Arduino as in the LED7Segment/DirectDrive project, it soon becomes pretty wasteful of pins. This project uses a 74HC595 8-bit shift register to feed segment values, requiring only 3 pins.

Since the 74HC595 has latched outputs, it also means all segment changes are simulataneous.

Register-Pin Mapping

Here’s a summary of how the bits in the shift register are mapped to LED segments:

Bit Segment
0 A - top horizontal
1 B - top right vertical
2 C - bottom right vertical
3 D - bottom horizontal
4 E - bottom left vertical
5 F - top left vertical
6 G - center horizontal
7 dp - decimal point

However the code uses bytes to represent dp,G,F..A from MSB to LSB, so I use MSBFIRST transfer.



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.