Now that the LEDs are all mounted on their PCBs, leave them on the grid for now, and check them for any solder bridges.

They'll be fairly obvious. You can clean those away by dragging a soldering iron, or using copper braid to absorb some of the solder.

Breadboard Testing

Next, build out a small test circuit using your Pro Trinket and a breadboard. I have an Arduino Uno permanently attached to a breadboard for testing, so I'll be using that to test the code and circuit.

Attach your leads to the breadboard, and insert the pogo pins into position. My breadboard wasn't quite wide enough at first, so I used a pair of pliers to hold the pins by the bottom (below the thin line), and then inserted them with a little pressure. Holding above the line results in the pin bending.

Attach the potentiometer into the breadboard, and upload the code to your testing device. You can alter the pins in the code to your preference.

Once the code has been uploaded, turn the potentiometer to full brightness.

I put on a pair of sunglasses after testing the first LED, because they are insanely bright, and I had trouble aligning the rest for testing.

Holding the matrix of PCBs in your hand, align the thru-holes of the first PCB with the pogo pins (being very, very careful that your polarity orientation is correct), and push down until the LED lights up.

If it's crazy bright, it works! Next try the other 9 LEDs on the grid.

Once one color is done, test the other color grid.

Try turning the potentiometer while an LED is depressed against the pogo pins to test the brightness adjustment.