#196 7-Segment CD4026 Drive

Driving a 7-segment LED display with CD4026 Counter

The Build

Notes

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.

Construction

Breadboard

The Schematic