A Board With Drivers on it

A project log for Homebuilt LED Displays

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

BharbourBharbour 09/29/2018 at 00:230 Comments

I sat down last weekend to see what I could put together to drive the LEDs on this display board. About half the segments have 2 LEDs in series, so it needs 5V power (with current limiting resistors). Neither of the boards that i wanted to use have 5V tolerant I/O. With Radio Shack gone, I couldn't run down and get one of the bags of 50 2N3904 NPN transistors.

Looking around on Mouser's web site, they have the 74HC595 shift registers in little leadless packages. With a little messing with, I got two of those parts, along with 14 current limiting resistors to fit on a PCB. Swapping the two 0.1" connectors for one 0.050" connector, I was able to narrow the PCB by about 0.150" as well.

Green: Top side copper  Red: Bottom side copper  Yellow: Mid Layer 1 and there is a ground plane that is switched off in this view for visibility. The board is about 0.450" x 0.780"

I sent the boards out to OSHPark this week. With the 0402 LEDs and two of the QFN packages on the top surface of the board, I went ahead and ordered a stencil for solder paste. These things are not going to be a lot of fun to build. Next rev, I will fix the silkscreen on the bottom layer (it's reversed) and whatever electrical issues I find after building one.

I ordered the parts to populate these boards, and while I was at it I ordered some 74HC125 drivers to play with the existing boards.