# 24 hour digital clock logic circuit

#### arian78

Joined Nov 22, 2021
37
Hello!
I'm a bit of a newbie in making circuits, and i need help! I have to make a 24 hour clock, and i do have 6 anode common seven segments, 2 7493 ic. 4 4026 ic. 2 7447 ic, 7492 ic, 7400 ic, 7432 ic, 7483 ic, 7408 ic, and a 555 ic, now here's the problem... i believe i did make a great mistake in buying 4026 ic and i had to buy 4033 ic, but do i need anything else? I have already made the 555 circuit with 1 hz, so no problem in that, only for the rest of the circuit i got confused and professor told me that i can't use 4026, so... can i get some help with this? cause i'm confused as hell now! And if i have to buy any other ics, please do tell me!

#### ericgibbs

Joined Jan 29, 2010
17,427
hi arian,
Welcome to AAC.
Do you have a draft sketch of a proposed clock circuit that you could post.?
E

#### arian78

Joined Nov 22, 2021
37
hi arian,
Welcome to AAC.
Do you have a draft sketch of a proposed clock circuit that you could post.?
E
Hi! Thanks for your answer, unfortunately no, professor used to give us the sketch for the previous circuits, but for this one i have to design it all from zero, which made me even more nervous and confused actually.

#### dl324

Joined Mar 30, 2015
15,536
Could you post the complete text of the problem so we can see what you're being asked to do?
i do have 6 anode common seven segments, 2 7493 ic. 4 4026 ic. 2 7447 ic, 7492 ic, 7400 ic, 7432 ic, 7483 ic, 7408 ic, and a 555 ic
Why would you use 7493+7447 and CD4026/33? Have you learned about the pitfalls of mixing CD4xxx with TTL?
i believe i did make a great mistake in buying 4026 ic and i had to buy 4033 ic
I think using either of those counters would be a big mistake. I don't know of anyone who's figured out how to use the 'ungated C' feature of CD4026 to implement divide by 60 and 12.

#### arian78

Joined Nov 22, 2021
37
Hi again! Thank you so much everyone, professor just updated me, said i can't use 40 ics and 45. After a bit more searchs, i saw this one:

Do you guys think it can work as a 24 hours clock?
I'm
gonna use 7447 instead of 7448 tho.

Last edited by a moderator:

#### dl324

Joined Mar 30, 2015
15,536
professor just updated me, said i can't use 40 ics and 45
Could you post the complete text for the problem so we know what you're expected to do?
After a bit more searchs, i saw this one
Wouldn't it be more educational for your teacher to require you to design one on your own?

#### arian78

Joined Nov 22, 2021
37
Could you post the complete text for the problem so we know what you're expected to do?
Wouldn't it be more educational for your teacher to require you to design one on your own?
Yes. I do have the components i said in my first post, and if needed i can take more of them. Now i need to design a digital clock with said components, it should be a 24 hours clock, and it should show seconds, minutes and hours, and when reach to 23:59:59 it should be reset to zero and start again.
Now he doesn't care who designed it as they didn't actually taught us how to design a circuit from scratch. So... that's the problem, i honestly got confused over all of this.

#### dl324

Joined Mar 30, 2015
15,536
Now he doesn't care who designed it as they didn't actually taught us how to design a circuit from scratch. So... that's the problem, i honestly got confused over all of this.
Study the circuit for seconds. When the count gets to 59, the seconds counters should be reset to 0. What does the circuit you found do?

Where did you find that circuit? My advice is to avoid schematics from authors who don't know how to draw them well. We usually want the flow in a schematic to be primarily left to right and top to bottom. It makes sense for a clock to deviate from that because a right to left flow makes more sense.

People who leave pins floating generally don't know what they're doing.

#### arian78

Joined Nov 22, 2021
37
Study the circuit for seconds. When the count gets to 59, the seconds counters should be reset to 0. What does the circuit you found do?

Where did you find that circuit? My advice is to avoid schematics from authors who don't know how to draw them well. We usually want the flow in a schematic to be primarily left to right and top to bottom. It makes sense for a clock to deviate from that because a right to left flow makes more sense.

People who leave pins floating generally don't know what they're doing.
Nice, thank you, i'll search for the seconds circuits!

#### dl324

Joined Mar 30, 2015
15,536
i'll search for the seconds circuits!
There's no need to. Just analyze or breadboard the seconds circuit you found. If it works, you're good to go. If it doesn't, you can try to determine what it's doing wrong and correct it.

Do you see any advantage using 7492 for the 10 second counter instead of using 7490 for both.?

#### arian78

Joined Nov 22, 2021
37
There's no need to. Just analyze or breadboard the seconds circuit you found. If it works, you're good to go. If it doesn't, you can try to determine what it's doing wrong and correct it.

Do you see any advantage using 7492 for the 10 second counter instead of using 7490 for both.?
The problem with analyzing is I don't have any 7490 at the moment, so... is there any simulation program that i can use to simulate it and see if it works before ordering components?

#### dl324

Joined Mar 30, 2015
15,536
The problem with analyzing is I don't have any 7490 at the moment

Look at the circuitry for seconds. What is it supposed to do when the count gets to 59? What does this circuit do?

is there any simulation program that i can use to simulate it and see if it works before ordering components?
I'm partial to Digital Works. It's free, but it doesn't have usable commercial parts. The author chose to use the IC packages instead of the usual symbols found in datasheets. It also has a lot of warts, but I've used it long enough and created enough components for commercial parts that I can look past the imperfections.

I don't recommend that people start using simulators before they understand how to design for themselves. It's akin to using a calculator before you've learned how to do arithmetic long hand. If you do that, you'll get dependent on the calculator and not be able to do it well without one.

#### arian78

Joined Nov 22, 2021
37

Look at the circuitry for seconds. What is it supposed to do when the count gets to 59? What does this circuit do?
View attachment 256196
I'm partial to Digital Works. It's free, but it doesn't have usable commercial parts. The author chose to use the IC packages instead of the usual symbols found in datasheets. It also has a lot of warts, but I've used it long enough and created enough components for commercial parts that I can look past the imperfections.

I don't recommend that people start using simulators before they understand how to design for themselves. It's akin to using a calculator before you've learned how to do arithmetic long hand. If you do that, you'll get dependent on the calculator and not be able to do it well without one.
This circuit is supposed to reset to zero the moment it reaches to 59 seconds, and then it should send a signal to minutes so the minutes unit show us the first minute, once the minutes reach 59 seconds, they are supposed to send the signal to the hours unit, i totally get that, and when i look at the wirings, i believe it's doing what it should do... but that in my noob mind, and on paper, i'm not sure that it really does that or not.

#### dl324

Joined Mar 30, 2015
15,536
but that in my noob mind, and on paper, i'm not sure that it really does that or not
Look at the 10 second counter and the resets on the seconds counters. Reset comes from somewhere else so the seconds counter can't reset itself to 0 when it reaches 59. The seconds counter is supposed to clock the minutes counter when it rolls over.

In the circuit you found, at what count does the seconds counter clock the minutes counter? Keep in mind that the clocks trigger on the falling edge. Also note that reset is asynchronous. You can determine that from the datasheet. It's easiest to just look at the logic diagram if you can read them.

From a TI databook:

#### dl324

Joined Mar 30, 2015
15,536
This circuit is supposed to reset to zero the moment it reaches to 59 seconds
You need to be careful with the boundary conditions. When the seconds counter reaches 59, you don't want it to reset until that second has elapsed. So you don't reset the counter until it's about to reach 60.

#### arian78

Joined Nov 22, 2021
37
Look at the 10 second counter and the resets on the seconds counters. Reset comes from somewhere else so the seconds counter can't reset itself to 0 when it reaches 59. The seconds counter is supposed to clock the minutes counter when it rolls over.

In the circuit you found, at what count does the seconds counter clock the minutes counter? Keep in mind that the clocks trigger on the falling edge. Also note that reset is asynchronous. You can determine that from the datasheet. It's easiest to just look at the logic diagram if you can read them.

From a TI databook:
View attachment 256197
View attachment 256198
I'm looking at them, the reset pins in all the counter has been connected to a OR gate, if i'm not mistaken.
In the circuit i found it was supposedly, as the maker of the circuit said, the seconds will reset at 59 and they'll clock a minute and it goes on, but i think i still can't understand something which i'm so sorry about.
I can read the logic diagram i think, i know the gates and everything there.

#### dl324

Joined Mar 30, 2015
15,536
In the circuit i found it was supposedly, as the maker of the circuit said, the seconds will reset at 59 and they'll clock a minute and it goes on,
You can't believe everything you see on the internet. I can tell you that the seconds circuit doesn't work.
but i think i still can't understand something which i'm so sorry about.
Break it down into smaller blocks.

Look at the 1 second counter. What type of counter is 7490? What type of counter is 7492? Also note that 7490 and 74LS90 have the same functionality, but different electrical characteristics. 74LS is lower power than standard 74xx:

without giving up any speed:

#### atferrari

Joined Jan 6, 2004
4,688
I believe in flow diagrams as a must to start programming and in block diagrams as a must to start designing circuits of certain complexity.

OP arian could draw his own and move from there. So hard is it?

#### arian78

Joined Nov 22, 2021
37
You can't believe everything you see on the internet. I can tell you that the seconds circuit doesn't work.
Break it down into smaller blocks.

Look at the 1 second counter. What type of counter is 7490? What type of counter is 7492? Also note that 7490 and 74LS90 have the same functionality, but different electrical characteristics. 74LS is lower power than standard 74xx:
View attachment 256200
without giving up any speed:
View attachment 256201
View attachment 256202
Now i'm understanding things! For seconds, minutes and hours we'll use 7490 because it has 10 states, means it can count from 0 to 9 then reset, and for the tens we're using 7492 because it has a 12 mod and a 6 mod, which makes sense for my work, cause in the tens of the minutes and hours, it should only go up to 5 and reset while for the tens of the hours, it goes up to 24... if that makes sense, my english is a bit rusty some times.
Now i know how they function... kinda... just need to figure out the connection from 555 timer to them... right?