Large 7 seg display clock help!

Hey guys,

I'm looking to create a DIY large 7 segment display clock that consists of 4 digits. Each segment will have at least 3 LEDs. Since I have never done this before I would like some advice and input.

I currently have an Arduino Uno which I'm hoping to use for the project. I understand that I won't have enough current from the IO pins to drive the LEDs, so I'll need an external power source.

Should I be using an IC? If I should be, which one? How many will I need?

Should I wire the LEDs as common anode or common cathode? Which is easier to implement?

I'm hoping to get some experience creating PCBs so I'd like to have all the components and wiring figured out before I design a PCB.


The first advice would be to use a driver IC, such as a MAX7219. For this application, with 84 LEDs, you would require two of them, but there are other ICs.

Where you have the option to wire the LEDs in the segments however you like, this makes far more sense than wiring them in series and then having to use higher voltages to drive the series strings. You need only the basic 5 V supply. For four digits, you would drive two digits with each MAX7219 and have the option of four LEDs per segment instead of three or if using three, reduce the multiplex ratio to achieve a little more brightness.

The MAX7219 performs all the multiplexing for you and you require only three pins to control one, two or more as they chain together. Only one resistor and two capacitors for each MAX7219. :grinning:

I was looking at this video (Addressable Big 7 Segments Display PCB - YouTube). I like the idea here as it would give me some practice with MOSFETs as well but I think there are some mistakes on how the board is wired. For example, the mosfets are oriented in a way that would only open the channel when the output from the ws2811 is low. I dont think that makes a lot of sense, do you ?

With all those LEDs in parallel, a MOSFET would be a good choice.
A better choice would be a TPIC6B595 to replace the GND connections.
Shift in a 1, the output goes active (LOW), and lights the segment.
One shift register per digit, no messing around with multiplexing.

Maybe use TPIC6A595 or TPIC6B595 if more current drive is needed.

If you Have to drive it from the high side, then pair up a 74HC595 with MIC2981 for current source into the anodes for each digit.