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#196 7-Segment CD4026 Drive

Driving a 7-segment LED display with CD4026 Counter

The Build


Here’s yet another way to drive a 7-segment single-digit display unit - using a CD4026.

The CD4026 is a 5-stage Johnson decade counter with decoded 7-segment display outputs and display enable. With RESET and CLOCK INHIBIT low, and DISPLAY ENABLE IN high, the 7-segment display outputs progress through the 0-9 sequence on the rising edge of the CLOCK pulse.

The chip has a CARRY OUT that triggers every 10 clock input cycles, so it can be used to chain the units for multi-LED displays. However, it has no explicit support for controlling the decimal-point if present in the 7-segment display unit.

CD4026Drive_chip - ST HCF4026BE

I can’t find much in the way of history of the chip (except that it appears to have originated at TI), and of course it is now quite obsolete anywhere near a microcontroller.

It’s an interesting alternative to using shift register for driving a 7-segment LED (as in the ShiftDrive project). While a latched shift register provides random addressing and clean transitions to any digit, it requires the 7-segment display outputs to be decoded externally (like in code). On the other hand, the CD4026 takes care of the decoding, and external circuits just need to send a counter pulse.

The 7-segment single-digit display unit I’m using is a common cathode unit, similar to the SC56-11. A common anode (or common cathode unit with different pinouts) just requires minor modification to the circuit.

Sample Code

The Arduino is nothing more than a pulse generator in this circuit. I could have used a 555 timer or any other source of a trigger pulse.

But the Arduino is handy in that it can also power the circuit - total current in this configuration peaks at ~11mA.

CD4026Drive.ino is a simple program that clears the counter on startup, then sends a pulse per second to the CD4026 - so a simple monotonic second counter.



The Schematic

The Build

Credits and References

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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.