Clock Project Journal Entry #1: What is it, and what’s the point?

One of the things my desk is missing is a clock. Yes, I have a computer, watch, and phone that can all tell time, but when I’m in a full-screen game, I don’t want to have to go looking for the time. I’d rather have a clock large enough that I can simply glance at. First world problems, I know, but in any case, I thought that this would make for an interesting electronics project, despite how simple it may sound.

I will be making two custom circuit boards for this project. One will be the control board and the other will be the actual LED clock display. The project will be controlled by at Atmega328p microcontroller and the time will be kept by the DS3231 real-time clock chip. The LEDs will be driven with the help of some 74HC595 shift registers since it should make programming easier as opposed to multiplexing… I always gravitate toward using shift registers, I guess.

One of the things I wanted to make sure of was the LED configuration. As it sits now, each segment will have 4 red LEDs. At first, I tried driving them at their “full” 20mA each (so 80mA per segment) but I felt that it was too bright. Bumping up the current-limiting resistor brought it to a more comfortable brightness. With the higher resistor, the current drawn for the segment was about 4mA, which is low enough for a pin on the shift register to handle directly. With that, I don’t need any transistors, which makes me happy…

I was planning to use a single resistor for the four LEDs on each segment, but it’s bad form and I don’t want to go that route for the whole project. I already feel “guilty” about using shift registers instead of just multiplexing. I will be putting a resistor for each LED. The main reason why I wanted to go with the one resistor per four LEDs is because it saves space without having to go with surface-mount components. I’ll see how the layout goes with the 1/4 watt resistors and hope that I don’t have to go and get some surface-mount resistors. I’d like to use whatever I already have.
ds3231rtc_sch_1Here’s the schematic for the control board. The board layout is done, or at least the first draft of it is. The only thing left to add is a switch to either cut the power to the project or at least turn of the display. I haven’t decided yet. I’m planning on using some mini-slide switches I bought a long time ago but I need to create an Eagle part for it which may take a little time. I’m waiting on some digital calipers from Amazon so that I can measure the pin thickness.

The layout for the display is coming along slowly. There’s only enough space for two digits per board, which is not a bad thing since I could use them for other projects. The problem that does arise from not making them specific for the project is that I have to make a separate circuit board for the colon and AM/PM indicator (I don’t really want a 24-hour formatted clock).

That’s all I have to share for now. I’ll be back with more when more details get finalized. Stay tuned!

PS. I started working on this project about a week or so before Ahmed Mohamed’s clock incident happened. I found that to be a funny coincidence.


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