Moving beyond project books; digital clock project?

Question

Hi again folks! Beginner here and worked my way through Platt's Make: Electronics I and a fair amount of book II. Also built all of the projects in the Mims Digital Logic Workbook 1 & 2. This has taken me the better part of several months as I have to keep going back to basics on much of it (see a previous post!). It is fun to build these but I would like to make something of my own from scratch at some point. Two projects I have considered are a 12 hour digital clock (not a microcontroller based one - right now anyway) and a very basic calculator. I don't have any idea where to start on the calculator as I don't know where to start with memory one but the digital clock seems within reach, maybe..lol. Two questions:

1) First - any other ideas for moving beyond these project books for something from scratch?

2) For the 12 hour digital clock I considered a 555 timer but internet search says 555 it is way too unstable to use for time keeping. So I spent a few hours today reading about crystal oscillators. (at this point I realize I am not clear what the waveform of a crystal oscillator is and I don't have one to test. ) but I think I still need some kind of square wave generator after that, so could I not feed it into a 555? Obviously I don't know if, whatever makes the 555 unstable, would still be unstable with a crystal oscillator in front of it...

Then I need a way to go from whatever frequency the crystal is, like 1MHz to 1Hz and started reading about frequency dividers ). So then I came across a 74S124 TTL VCO (mission creep here) which I think, if I understand correctly, has the oscillator and waveform generator in one package. At that point I need to go into a counter of some sort and feed the led displays at the same time. Am I on the right track here? I am trying not to look at existing builds or schematics already out there but at the same time I sort of don't know I need a hammer to drive nails if I never saw nails before..does that make sense? I will add i am doing this as an exercise and not trying to build a super duper accurate/WWV controlled clock. Is there a better way to go about this than my stumbling approach?

Regards,

Robert Opalko

shabaz · Answer

Hi Robert,

Both of these projects are very logic gate heavy unless the scope is reduced perhaps for more limited functionality from each. One idea that could be done in a reasonable time, with a good chance of success, could be to implement a clock using a couple of 'unipolar stepper motors' which can be very low-cost from (say) aliexpress. The aim could be to make one stepper motor fully rotate once every minute, and then adapt to once every hour. Then the other stepper motor could be added to the circuit and used to rotate once every 12 hours. Anyway it's just an idea.

I think there's nothing wrong with using a 555 for this, since large circuits are modular anyway, and the 555 could be swapped out one day with a different circuit for more accuracy. Besides, you will gain an appreciation of what resistors and capacitors may help provide more stability, and how the circuit could be trimmed to center closer to the desired frequency.

A calculator could be simplified to doing just addition and subtraction for say a small range of integers, but it might not have the same sort of user interface that off-the-shelf calculators have, if you wanted to do it within a reasonable amount of circuitry.

jc2048 · Answer

If you want to see how a single chip does the oscillator and the divide-down-to-1-second-intervals job for a clock, have a look at the datasheet for this part

https://www.ti.com/product/CD4045B

That's '4000 series' CMOS logic.

I can remember there being an application note showing the design for a complete clock, so if you were interested I could probably find that for you [or a link to it online]. Even if you didn't want to be influenced by it, you'd have a fall-back if you got into difficulties with your own design.

Watches use much lower frequency crystals than that part: 32.768kHz is common. That requires fewer 'divide by two' stages to get down to 1Hz, so would be more manageable if you wanted to do the divider chain yourself using individual flip-flops or smaller 4-bit or 8-bit counters.

Another possible project might be a frequency counter for measuring frequency or time intervals. It's very similar to a clock in many ways: oscillator, counters, and decoders driving displays.

renga_68 · Answer

Hi, If you want to produce 1 PPS, use CD4060BE CD4060BE IC. Download the data sheet and there you can find the circuit for this. Time will be accurate also. Try this one.

Andrew J · Answer

Like you Robert, I worked my way through those books and funnily enough thought about making a clock. In the end I took one of his projects - ‘rolling a dice with a random output” - and extended it to allow the rolling of two dice controlled by separate ‘players’. I then added a button for each player to indicate ‘snap’ (I.e. that the die show the same value) and added extra logic to detect who pressed first to lock the other out and to compare the die values to determine if they were right or wrong. This could then increment an LED display (again, something played with in those books) to keep score. It requires extra logic chips and LEDs but can be worked out in increments and modularly.

you could then take it from breadboard(s) to an actual PCB by installing Kicad and playing with that. This would not need any special considerations for the PCB and would be easier than you may first think. Then it could be put in a project box for a complete end-to-end project.

A Clock would be more useful though

Moving beyond project books; digital clock project?

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