4 Digit Carrier Boards are Back

A project log for Homebuilt LED Displays

This is an attempt to re-create a long obsolete Hex LED display.

BharbourBharbour 10/28/2018 at 00:170 Comments

The carrier boards that allow up to 4 digits to be plugged in and run in a single SPI chain have come back from OSHPark. They are simple boards and they work fine. I built up the last two of the bare digit boards to test the carriers. The picture below shows the carrier board with 3 digits populated.

I wrote a quick and dirty test program to drive the board using an Aardvark USB to SPI adapter, and it looks good to the human eye. Unfortunately, photos with the digits lit turn into a big red blob. I need to try a contrast filter for that.

Building up the other two digit boards was semi-painful. The 74HC595 shift register chips I used are in QFN packages with a thermal pad in the middle. Due to the small size of the LEDs (0402), I got a solder paste stencil to make it easier to build. The thermal pads had WAY too much solder paste on them and the parts floated up when I reflowed them. TI has an app note (thanks Jarrett) on making footprints for QFN packages that suggested putting a grid pattern on the thermal pad with about 60% exposed for paste. That is going to take some effort to implement, so I just made a paste mask pad about 60% of the pad size. I will see how that works out on the next board rev that was submitted today.

My curiosity on whether displays could be built and how they would look is pretty much satisfied at this point. Now I want to see what is required to solve the solder paste mask issue.