SDG #023 Build A 6 Digit 2.3" LED Clock

This is a full build log for a large format digital clock made from six Kingbright SA23-12SRWA 2.3 inch common anode seven segment displays, driven by a PIC18F26K20 microcontroller on hand wired prototyping board. It is a good case study in the practical problems that come with big LED digits: the taller character height means multiple LED dies in series per segment, so the forward voltage climbs well above the usual 2V and the supply rail and driver choice have to be planned around it rather than assumed.

The interesting engineering content sits in the multiplexing strategy. With six digits sharing a segment bus, each digit only gets a fraction of the frame time, so peak segment current has to be pushed up to keep average brightness acceptable while staying inside the display’s pulsed current rating. That in turn drives the choice of segment resistors, digit drive transistors and refresh rate, and it dictates how much of the microcontroller’s time budget disappears into the display refresh interrupt.

Timekeeping is the other half of the project. A crystal referenced timer is only as good as the oscillator behind it, and the build shows the value of measuring the actual clock frequency with a counter rather than trusting the marked value, then trimming in software to reduce long term drift.

For anyone considering a similar build, the takeaway is that the display module itself is the easy part. The real work is in current budgeting, wiring discipline on stripboard or perfboard, and firmware that keeps the refresh rock steady so the digits do not visibly flicker or ghost. Schematic and source code are published on the SDG Electronics website.