I would like to have an array of eight 7- and 8-segment LED numerical displays on my Adafruit Trinket. I know that there are some prebuilt arrays of 4 LED's. So 2 in array would do just fine. However, my Adafruit Trinket doesn't have enough ports for that. I have heard something about connecting it to a small piece of controlling hardware that makes me able to have way less ports in need. What is the appropriate way of connecting a lot of those LED's on only a few ports on a micro controller?

My ideal output would be as follow: 88 88:88:88 or maybe 88 88.88.88 So ideally there would be a small space between the first group of 2 digits and the rest. Basically, the last 6 digits will represent a time in minutes, seconds and hundreds of seconds.

  • Do you have to use LED's? An LCD with a serial interface would be a better match for that board. You can solve the problem with an I/O expander, but consider simply switching to something built with a full size ATmega328p chip, like one of the mini or micro boards - which would get you both sufficient I/Os and more program resources. May 16, 2014 at 14:35
  • The screen should be visible while the sun is blasting on it. That is why I opted for those. Is there even a better solution for outdoor displays? May 16, 2014 at 14:39
  • Bright sunlight readability is a tough enough requirement that you should pick your specific display first, and then pick a board to drive it. May 16, 2014 at 14:49
  • Do you have a suggestion for a display? I don't have enough knowledge (in fact very little) to be able to pick the correct one. May 16, 2014 at 15:01

1 Answer 1


For LED displays, I advise using a MAX7219.

This circuit:

  • can manage up to 8 digits (7-segments + 1 LED for DP or colon separator) or up to 64 LEDs
  • needs only 3 pins to manage display
  • can be daisy-chained if you need to interface with more than 64 LEDs
  • requires only 1 resistor to be fully functional
  • is easily supported on Arduino with this library

I have used it in several circuits and happy with it.

Note that there also other libraries that support this circuit but I haven't used them.

This circuit has only one downside: its price (about $10 each); but the price is counter-balanced by its simplicity and the fact you don't need any additional component (e.g. transistors) besides a simple resistor.

  • Wow thanks for this answer! I will look into it this weekend. Looks perfect for what I need! May 16, 2014 at 23:26

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