LEDs… in two dimensions!

A while back I made a simple modular display based on the 74HC595 shift register, and ever since then I’ve been wondering how possible it would be to translate the effectively one-dimensional display circuit to an LED matrix I’ve seen around M5 ever since I was a freshman. So, I got to work and tried to recreate the circuit I had before.

This circuit is based on the 74HC595 shift register, taking input from a Pi Pico and feeding the parallel output to a ULN2003 Darlington transistor array which is capable of handing higher current. The transistor array handles the low-side switching of the circuit (effectively, which columns are grounded) and the Pi Pico’s GPIO handles the high-side switching (which rows have power). This way, each column can be illuminated in quick succession to give the illusion of an image. The GPIO pins supply 5v, so the LEDs each have a 150 ohm resistor to limit the current.

*Of note, the Pi Pico’s GPIO is not rated for the current that these LEDs can draw, so be careful if you decide to do this!

For the programming side, I took the program that I had made which handles translating numbers into signals for the shift register, and then added a function to illuminate dots on the column. Using persistence of vision, this can lead to the illusion of a complete image across the entire matrix!