# Need help in digital clock (hour, minute, and second) circuit

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by dj_ash1983, Oct 15, 2006.

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1. ### dj_ash1983New Member

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Hello to all. I'm new here, and I'm kinda noob on electronics stuff.

Recently, our Digital Circuits instructor is throwing yet another project on us to complete this week, and it's about making a digital clock made out of only pulse generator (analog circuit using LM555), decade counters (74LS90), BCD decoders (74LS47), AND Gate ICs (74LS08), and 7-segment LEDs. Our instructor intentionally gave us only the second-hand section of the diagram so that we have to use or brains to figure out the rest of the circuit (the minute-hand and the hour-hand). Yeah, it was a challenge to me coz I really got interested in these kind of projects, but I'm about to give it up already coz I'm all out of ideas. So far, I got only the "ones" part of the minute-hand section, and I had a really hard time to figure out the "tens" part. If you're getting confused about what I just said, let me elaborate you about this using an illustration:

This is what should look like in a clock, right?

But this is what I got so far.

As for the circuit diagram, you can take a look at it here:
http://i13.photobucket.com/albums/a251/dj_ash1983/circuit.jpg

I really need help from you guys so I can pass it on time. Thanks in advance, I really appreciate it.

2. ### on1aagNew Member

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Clock circuit.

Hi dj ash1983,

The circuit looks OK for the first two 74LS90's, forget the third one.
If you copy this part of the circuit twice you've got two divide by 60
counters. The clock of the third 74LS90 should be connected to the
output of the nand gate of the second 74LS90. Done that you've got
seconds "ones" and "tens" and minutes "ones" and "tens". The last part
is a divide by 24 counter. The clock of the fifth 74LS90 should be
connected to the output of the nand gate of the fourth 74LS90.
And here's the schematic of the last two counting sections.

on1aag.

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3. ### dj_ash1983New Member

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I got the hour-hand section (it went pretty smooth coz i just have to tinker the output wires to get the 12-hour format), but I think I had a hard time with the minute-hand part.

You said that "If you copy this part of the circuit twice", what part you are referring about? Oh yeah, I did copied the whole second-hand part of the cicuit but what I get was that the "ones" part is the only functional and not the "tens" part which is always "0".

Also, about what your said "The clock of the third 74LS90 should be connected to theoutput of the nand gate of the second 74LS90", there are 4 outputs (QA, QB, QC, and QD), right? Which of them should I connect?

Sorry to bother you, but can you make the whole second-hand and minute-hand circuit? Don't worry about the hour-hand, I got it already.

Thanks so much for your help. I really appreciate it.

Last edited: Oct 16, 2006

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5. ### on1aagNew Member

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Clock Circuit.

Hi dj ash1983,

Here's the rest of the circuit, it isn't that difficult.
If I do your homework, do I also get your degree ?
I always wanted a degree in chemistry.

on1aag.

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6. ### dj_ash1983New Member

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Lol You're making it sound kinda harsh y'know, hehehe

Like I said, I'm still a noob with this. I always rely on my group members when it comes to school projects at our while I'm the "financer". But I came to realize that I have to do these things on my own y'know. And I think electronics stuff is an interesting thing to have as a hobby.

And BTW, I didn't take Chemistry. I took Computer and Electronics Technology, but it's much more on PC troubleshooting/networking rather than electronics. Although I'm noob at electronics, I'm pretty good at PCs and IT-related technology.

Anyways, you really helped me a lot. Thanks so much.

Last edited: Oct 16, 2006
7. ### dj_ash1983New Member

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Oh yeah, I have to set it to 12-hour format aside from 24-hour.

Getting the 24-hour format is a cinch, but I had a hard time to analyze how to set the clock to 12-hour. During the time I was working with the minute-hand section (and got pissed off coz I still don't make it work right, that is before I showed up here in this board ), I noticed that instead of resetting back to "0" when it reaches "9", it skips to "1". If only I remember how did I just do it...

Anyways, sorry to bother you again, but I really need your help. Oh yeah, I always stick to the only required parts I mentioned above.

Last edited: Oct 17, 2006
8. ### renren1227New Member

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help please, i need full detail of a digital clock. please T_T

9. ### RODALCOWell-Known Member

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I will post a schema from a TTL clock in the next couple of days.
I have built 6 of these with no major problems over the last 15 years for family.
Very important is to decouple each IC with a 100nF ceramic capacitor.
Regards
Raymond

10. ### asamasterNew Member

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please i want a full schematic diagram of a 24-hour digital clock circuit using 7490 as main ic, i really need it badly....my email is jeffrey_torralba@yahoo.com...

11. ### dj_ash1983New Member

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Hey guys, it's been a long time since my last post here.

Seems like our li'l school project (which I passed with flying colors) took a bit of attention from some people here. How nice, hehehe

As for the schematic diagram, you may refer to the on1aag's attached pic (the 2nd pic). There's no more detailed that that anyway. And as for the "full" schematic diagram of the clock itself, the only part missing on on1aag's diagram is the clock pulse trigger diagram using LM555. Refer to my very diagram on my very first post.

If you guys REEEEEEEEEEEEALLY need a nice and neat schematic diagram, I can make one and post it here, but you guys have to wait since I have to find the circuitmaker pro from my backup cds first so I can verify it if it's working or not.

12. ### trashedguyNew Member

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can u show to me the diagram for hours my IC is 74LS192 that connected the up count to the minute output. i need it badly the output of my hours is 0-12 what should i do to make it 1-12 plsssss post a reply or show me the circuit plssssss

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since you are using the 7490 i advice you to search 'how digital clock works' on howstuffworks.com. they have shown a very ingenious way of making mod6 and mod10 oout of the 7490 with out using any gate circuitry. and i think i still have my old simulation of this clock back when i built it. i made in ewb. i will look for it and if you need it then tell me.

14. ### dj_ash1983New Member

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I don't think you can make a 12-hour format out of 74LS92 ICs. I've tried that before, but I'm stuck at 24-hour format. There are other ICs out there that can do 12-hour format, but doing that in 74LS is impossible. Correct me if I'm wrong.

Last edited: Sep 26, 2008
15. ### latinengineerNew Member

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hEY CAN ANYONE POST THE FINISHED DIAGRAM FOR THE CIRCUIT, I AM HAVING TROUBLE DOING THE MINUTES AND HOURS...
THANKS

16. ### rathin2jNew Member

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hello do u guys really want circuit of the digital clock using 7490?if yes than do tell i m hving schematic and pcb layout both,it was published in one of the magazines for electronics called "EFY" but i dont know that it works or not bcoz i hv not tried .so if u want than do tell me frankly.ya but i will b sorry for late reply.

17. ### jondamon360New Member

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i could use the schematic for the digital clock, i got it to count up to 59 seconds and reset but i cant get the the minute hand started

18. ### nncrumNew Member

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hey dj_ash1983 did you ever finish the schematics on your digital clock...if so can you help me out?

19. ### m3tasm_elzayatNew Member

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please i want also the diagram of digital clock counter with two switshes to set hours and minutes (its also my faculty graduation project)

20. ### blueroomelectronicsWell-Known Member

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Then shouldn't it be something of your own design?

21. ### sinnedNew Member

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hi..people i almost gor the full digital clock,..but i have one problem in hour part.i cant solve it.................,my prolem is to convert 12 hour to 1,instead of to go back in 0..i use this kind os I.C,74192,555,7447.