Reloj Is a Digital Clock Made with 7400 ICs and Notes From the Past
Alvaro Prieto designed his 7400 Series Digital Clock using his dad’s notes from a project he created back in the late ’70s.
Old school digital clocks made using ICs are great projects, especially if you have an influx of integrated circuits just sitting in a box gathering dust. There's a myriad of different digital clocks that integrate ICs into their designs in one form or another, but not many that have been built using notes that are decades old. Alvaro Prieto’s Reloj 7400 Series Digital Clock is one of those builds that were created via notes from the late 1970s, but not from some unknown professor’s dusty textbook, but his dad’s old university digital project.
Prieto found his father’s detailed write-ups and schematics, and decided to try and recreate it. He explains, “The design is fairly close to the original, but with the addition of a crystal-backed alternate clock source. The 555 timer clock source works but drifts like crazy, so if you want to use it as an actual clock, you still can with the alt source.” Beyond the 555 timer clock, Prieto used a 74LS90 IC as a counter IC to generate a 10kHz clock, which has two clock inputs, four counter outputs, and four reset inputs.
He then implemented a series of 74LS90s to drop the 10kHz down to 1Hz, which is used for second, minute, hour counters, as well as display drivers and seven-segmented displays. Several other ICs, a 74LS47 BCD-to-7-segment decoder/driver IC, and some creative coding were employed to fire up the digital clock and get it functioning.
Prieto has uploaded a build log for Reloj on Twitter, along with links to the files, schematics, and code for those interested in recreating it.