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Project Notes

#001 7-Segment Direct Drive

Test direct control of a 7-segment common-cathode LED display, performing a countdown from 9 to 0.

The Build

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



The 7-segment display unit I have is stamped “SMA42056”. It appears to be fairly common, however I’ve yet to find a datasheet for it. From inspection hwoever, it’s clearly a common cathode unit, and similar to the the SC56-11

There are two common cathode pins, but since these are internally connected there is no need to connect both to ground. Lighting a segment simply means providing a driving voltage to the corresponding anode pin.

This circuit puts the resistors in series of the anode pins. This ensures a consistent brightness no matter how many segments are on. Note that it is possible to put the current-limiting resistor on the common cathode; this is a bit of an approximation and will lead to varying LED brightness but may be “good enough” depending on your application.

Pin-Segment Mapping

Here’s a summary of how the Arduino pins are mapped to LED segments:

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


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.