I was digging through a box of parts from a previous project, and I found a bag of 2N3904 NPN transistors, perfect for building up a 1 off test driver for my first iteration of display boards.

The perfboard on the left side is the NPN driver board (14 NPNs wires as common emitter), the board on the right is one of my ATSAMD21E18 protoboards and the board in the middle is the display. After some futzing around to get bit I/O assignments correct and clean up my character generator table, the display looks pretty good. The multitude of wires makes me glad that the PCB for the SPI interfaced version is out being fabricated. Seven of the LEDs are wired as single LEDs on that circuit and the others have 2 LEDs in series on a single circuit. The single LED circuits get a 3.3K current limiting resistor and the double LED circuits get a 1.8K current limiting resistor. The LEDs are running in a 5V circuit. The brightness between the single and double circuits matches pretty well with this approach. The SPI interfaces version implements the same approach, just all on one board.

It's funny, I have gone away from perfboard projects in the last few years. Most of what I build is surface mount and when I mess up a connection or two on the pcb, it is easier to put in a wire or two than to build the whole thing. This driver board was sort of an exception because it was all through hole parts, I don't think I will ever need more than this one, and I didn't want to wait the turn around time on the PCB fabrication.

Anyway, the display looks good in operation.