Video Display

The video display was built by Bret Victor for a completely different project, and was modified for various purposes over the years, eventually ending up as the Coke display. It is a 30x7 red LED matrix display, multiplexed across the columns. No schematic currently exists, but I'm sure how it works is somewhat obvious. :)

Chip List

U1: 74F244 - buffer to source the current for a column, driven by the 374
U2: PAL22V10 - clocks the 374 and latches the column number: leds.pds
U3: 74LS374 - latches the current pattern (which rows are on)
U4-7: 74F138 - decoders to sink the current for a column
RP1: 220 ohm resister pack to limit the current for each row


To set the column number, write to address 0. To set row data, write to address 1. The PAL source refers to "enabling" and "disabling" the display, but this feature has been removed -- the display is always enabled now.


This isn't an especially good way of implementing an LED display. If I had known better at the time, I would have used ULN2003's to sink the current and whatever the PNP version is to source the current. As is, I'm pulling less current than I'd like to and it's still more than the specs say it should be. Also, the connections are wire-wrapped and the sockets are cheap - one of these is responsible for parts of the display occassionally going dim until you wiggle stuff around.