These little 4 digit LED displays sure aren't nearly as efficient or have as high of a resolution as modern displays but gosh darn-it I have to string a few of them together to build a serial display module.
To start off, thanks to JLCPCB for sponsoring this project as well as providing pcbs to get the ball rolling. JLCPCB Prototype for $2 (Any Color): https://jlcpcb.com
I'm certainly no stranger to vintage LED displays (... well vintage displays in general I guess), so when I found these adorable 4 digit 14 segment DL1414 LED bubble displays on aliexpress for roughly $0.70 a piece I had to buy a handful. I have no idea what I plan on using them for but I felt the urge to strap a good few of them together to build a larger display. I figured emulating the venerable commonly available 16x2 lcd would be a good starting point and would only require 8 of these chips.
Each display has 7 data inputs, 2 address inputs, and a write enable input pin. To write something on the display you need to set the ascii value for the character you want displayed (these are alphanumeric having 14 segments per digit after all), set the 2 bit address for which of the 4 digits you want written to, and finally strobe the write enable pin. The exact timing requirements and signalling order is described in the datasheet, but this gives a general sense in what is required from a microcontroller to write to a single chip. My issue though is I want to be able to write to 8 chips using as few pins as possible. The solution: SHIFT REGISTERS!!!
I opted to use three 74hc595's in series to give me 24 pins to toggle in whatever pattern I wish while only using 3 SPI control outputs from a micro (and an optional pulled down 4th output enable pin to disable all shift register outputs). The trick I used is many of the pins between the 8 chips can be tied together. For example the 7 data inputs can all be shared among the chips in a bus since only one chip will ever be updated at once. The same goes for the 2 address input pins. So far we have 9 pins driven by the total 24 pins on our shift registers. What differentiates the chips and allows my micro to write to specific chips is the write enable pins. Each chip's write enable gets its own signal from a different output from the shift registers. So now we are up to 9+8=17 out of 24 pins on our shift registers. This is where I stopped but we'd actually be able to add an additional 7 chips without having to add any more shift registers. I just ended up breaking out the remaining 7 pins to test pads. However, for me 16x2 was a nice round number that I was happy with.
So now that we can control 32 individual digits with a minimum of 3 SPI pins (4 if you want to dim them) lets throw together a nice pcb to mount them all and write some simple software to control the whole show.
And the final assembled display ...
Now the keen eyed among you may have noticed the many obvious cracks in the plastic. Well like an idiot I tried to clean the dusty chips with some IPA and apparently it reacted to produce these cracks. At least the displays were still functional and I learned an important lesson (and luckily I had the foresight to socket the chips and I've ordered another set of 20 of these chips so I'm happy). Additionally, I accidentally swapped the address pins when I designed the board so I had to compensate in the software. Oops, but in the end it all works so I guess no biggie lol!