Summary of 6 Digit LED Clock
Summary: A DIY large LED clock was built using six 2.3-inch red 7-segment SA23-12SRWA displays, a Microchip PIC18F26K20 microcontroller, and a Dallas DS32KHZ temperature-compensated 32.768 kHz crystal. The design uses simple multiplexing (one GPIO per digit anode and one per segment), with decimal points and colons tied together. Because the segment forward voltage is high (7.4–10 V), the anode drivers must provide voltage translation and enough current. The schematic is intentionally simple for a quick build, and similar displays remain available from major distributors.
Parts used in the Large LED Clock Project:
- Six SA23-12SRWA 2.3-inch red 7-segment displays (common anode)
- Microchip PIC18F26K20 microcontroller
- Dallas DS32KHZ temperature-compensated 32.768 kHz crystal
- Prototyping pad board
- Anode driver circuitry (voltage translation/current drivers for displays)
- Interconnect wiring and basic passive components as required by schematic (resistors, decoupling capacitors, etc.)
- Power supply suitable for display forward voltage and microcontroller
Recently, I discovered six 2.3-inch red 7-segment displays in my collection that I had purchased from Rapid Electronics during their clearance of non-RoHS stock. Since I didn’t have a clock at my workbench, I decided to turn them into a Large LED Clock Project using a prototyping pad board, a Microchip PIC18F26K20 microcontroller, and a Dallas DS32KHZ temperature-compensated 32.768kHz crystal.
For this Large LED Clock Project, I used SA23-12SRWA displays, which no longer seem to be available from Rapid, although the similar SA23-12EWA can still be found. Fortunately, Farnell and several other major distributors continue to stock these displays, making them easy to source for anyone building a similar clock.
I wanted this to be a fairly quick project to complete, so the schematic is fairly simplistic. The schematic diagram of the clock is shown below:
The 7 segment displays are displays with common anodes, featuring segments made up of four red LEDs in a row and decimal points made up of two red LEDs in a row. Different techniques, like multiplexing, shift registers, or dedicated display drivers, can be employed to decrease the amount of GPIO necessary to operate the display. I’ve opted for simple display multiplexing, needing one GPIO for each digit’s common anode and one GPIO for every segment. This results in 7 common anodes and 8 cathodes, with the decimal points and colons linked together. There are numerous techniques available to decrease the required number of GPIOs even more, however, the selected microcontroller has an ample supply of GPIO for this particular task.
The segments have a slightly elevated forward voltage range according to the datasheet, which is indicated to be between 7.4V and 10V. Due to this factor, the anode drivers for the displays need to carry out voltage translation and supply sufficient current for the displays.
FOr More Details: 6 Digit LED Clock
- What display type was used for this clock project?
Six SA23-12SRWA 2.3-inch red 7-segment common anode displays were used. - Which microcontroller was used in the project?
The Microchip PIC18F26K20 microcontroller was used. - How is timekeeping provided in the clock?
Timekeeping uses a Dallas DS32KHZ temperature-compensated 32.768 kHz crystal. - How are the displays driven to reduce GPIO usage?
The displays are driven by simple multiplexing: one GPIO per digit common anode and one GPIO per segment. - Are decimal points and colons handled separately?
Decimal points and colons are tied together and share the same cathode control lines. - Why do the anode drivers need voltage translation?
Because the segment forward voltage is high (7.4 to 10 V per datasheet), the anode drivers must provide voltage translation and sufficient current. - Can fewer GPIOs be used than this design uses?
Yes, the article notes many techniques such as shift registers or dedicated display drivers can further reduce GPIO requirements. - Where can these displays be sourced now?
Farnell and several other major distributors continue to stock similar SA23-series displays.
