Digital Clock Project Part I: The Brain

Digital Clock Project Part I: The Brain


I somehow developed an obsession with clocks over the last few years. I don't know if the idea for this project started it, or if this project was just an early manifestation of it. I guess the seed may have been planted when I started using the name "Clockbroke" over a decade ago, even though it's inception had noting to do with clocks. I love any project that tells time, and nixie tubes make me drool. This is a project that I tinker with from time to time, then put away for months. I'll finish it before I die... probably.

It all started when I decided to do a digital logic project, and wanted to use only what I had on hand (at least for starters). I had lots of counters, so a clock was the choice. My favorite part of any digital project is working through the logic via a sort of 0's and 1's based pseduo-code.

The image attached to this post is the "brain" of the project. It does the heavy lifting as far as counting goes. The next steps are the clock source (I'm leaning towards a combination of 60Hz AC pulse with a crystal backup for power outages), and then the display. Right now, it's just arrays of LED's showing the time in binary. I'm hoping to design something both unique and user friendly.

The brain is made with "banks" of chips that make up "count to 60" circuits. Each bank handles a different granularity of time (seconds, minutes, hours). Each bank provides a cascading trigger to increment the next bank. Every time the first bank counts 60 cycles of AC, it increments the seconds bank, which increments the minutes bank after 60 seconds, and so forth. The hours bank counts to 12, and each time it passes 12 it toggles an AM/PM LED.

The counters are 74163 4 bit Synchronous Binary Counters (2 per bank). They are apparently obsolete now. Any 4-bit synchronous counter should work for this particular design, though the pinouts will likely change. I'll post a schematic once it is more finalized. The counters are decoded by a pair of 7400 Quad NAND Gates. There are 5 groups of LED's. The first 2 groups are the 1's and 10's place for the seconds bank, the second 2 groups are the same for the minutes bank. The hours bank needs only one LED group and counter, as it only counts to 12. I use a 7404 Hex Inverter chip with a pair of matching RC circuits to provide a VERY ROUGH 60Hz pulse for testing purposes. This chip/RC comb is in the lower right of the image. To the left of that is the "Set" circuit. It is merely a toggle switch de-bounced via a pair of NAND gates in a 7400 chip. It is also a temporary solution for testing purposes.

So, there is the brain. I'll post again when I have the timing worked out, and again when the display is going. This project is obviously just for fun. These days you can buy a single chip to perform accurate timing out of the box rather than use multiple, obsolete chips. A $20 digital wristwatch is 100 times smarter and 100 times smaller. Call me a geek, but this is a million times the fun, and 100*100 falls way short of that. Of course, you'll never find me wearing this circuit on my wrist, and definitely NOT in an airport.



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