24 Hr Digital Clock only with basic CMOS Chips

Picture of 24 Hr Digital Clock only with basic CMOS Chips

PCB art is included in this instructable so you can easily make it!

A 24 hour based clock based on dual decade counters, AND gates, OR gates, BCD to Decimal Converters, and a crystal oscillator. No micro controllers here. This instructable contains PCB art so you can easily make your own.

Quantity is in parentheses. Item number at is in brackets.

(6) 74HC390 Dual Decade Counters [568-1442-5-ND]
(6) 4543 BCD to Decimal Converters [568-3138-5-ND]
(2) 4020 Divide by 16384 Counter [296-2039-5-ND]
(1) 74HC32 Quad OR Gate [568-1434-5-ND]
(1) 74HC08 Quad AND Gate [TC74HC08APF-ND]
(39) 500Ω or 1000Ω drop down resistors for 7 segment displays
(3) 10kΩ Resistors
(6) Common Anode 7-Segment Displays [160-1575-5-ND]
(4) 3 mm Green LEDs
(1) 100 uF Capacitor
(1) 0.1 uF Capacitor across Crystal Oscillator
(1) CMOS 32.768 KHz Crystal Oscillator [SER3618CT-ND]
(3) Momentary Switches [P8071SCT-ND]
(4) 3 mm Green LEDs
(1) Power Plug [CP-102A-ND]
(1) 5V Power Supply

Remove these adsRemove these ads by Signing Up

Step 1: The gist of how it works

Picture of The gist of how it works
A 32,768 Hz oscillator provides the clock pulse and this is divided by two CMOS 4020 Divider chips down to 1 Hz. The 1 Hz signal is fed into the first dual decade counter [IC3] that runs from 0-9. The binary coded decimal from IC3 is fed into a 4543 [IC4] that is converted for a seven segment display. The block diagram omits showing the drop down resistors, but they're really there. Use either 500Ω for a bright display or 1000Ω for a dimmer display.

IC3 triggers IC5 by feeding its IC3's q3 output into IC5's clock input. IC5 runs from 0-5 and resets via an AND gate when it hits 6. The AND gate also feeds into IC7 which is the minutes section. IC9 resets at 6 as well. This completes the minutes section. 

The AND gate from the minute section increments the hour section. Another AND gate in the hours section resets IC11 and IC13 simultaneously when  they collectively reach 24. 

This probably sounded all convoluted, dense, and confusing. Refer to the diagrams below for more clarity.

Notice part numbers from are included in the block diagram
1-40 of 49Next »
habdallah1 month ago

Switches don't work on Proteus, any body else?

bobo13133 months ago

Hi, Can i use a normal 32.768 KHz Crystal Oscillator instead of the CMOS one ?

i mean i want to buy and use a normal Crystal Oscillator NOT the CMOS Crystal Oscillator,and what would be the part number on digikey?

JamillahG5 months ago

Hi, I just want to ask if what is the purpose of the momentary switch you have? And if the circuit has an on-off switch? If yes, will the time still be accurate after it was turned off and then turned on again?

Where should I connect the 5volt input? Thank you.

EngHossen6 months ago
jd'teror9 months ago

how to add rtc module, its be?

rishichavda11 months ago

Roughly how much did this build cost altogother?

Mattosx (author)  rishichavda10 months ago

It depends. If you already have the base equipment then the materials only is maybe $25

mfreeman121 year ago
Hello, I am very keen on making this but i am a novice and are affraid of messing it up badly. i am also wondering, will it keep time for a long time or should i expect to lose time? tfeedback would be much appreciated (:

You could always just build it on a breadboard so you don't have to solder anything. It wont be as pretty but until you get it right would be the easiest way.

Mattosx (author)  mfreeman1211 months ago

I have had mine running now for nearly two years and it keeps time great. Maybe a few seconds every several months is lost if it all.

My advice is that if you take your time and make sure you do it "cleanly" it should work, that is, nice solder connections and everything is tidy. We could always trouble shoot if here because I know the circuit so well.

jodjinar1 year ago
Hello.. Is this a countdown clock display?
Mattosx (author)  jodjinar11 months ago

No, it counts up as a 24 hour clock.

desmondtheredx11 months ago

Where can you submit designed PCB's and get them printed?

And what kind of voltage source are you using? Wall outlet into a transformer or batteries?

Mattosx (author)  desmondtheredx11 months ago

The voltage supply is around 5V. A lot of old cell phone chargers work great because they're around 5-6V and the circuit works with fairly small current.

And for circuits boards, I never found a good place to get just one or a few printed economically. This board is 40 square inches (8x5) and some places $3-$4 per square inch! So I make my own.

vdo_av8r1 year ago
clock has been keeping perfect time for about for about 4 month now.. but recently has been resetting the hours every 20 other words the minutes are only counting to 19 instead of 60 as before. Any clue as to what could be happening?
Mattosx (author)  vdo_av8r1 year ago
Yes, I know why. The 74HC390 decade counter has outputs Q0, Q1, Q2, and Q3 outputs a six when Q1 and Q2 go high which feeds into the AND gate. Likely there is a poor electrical connection between Q2 and the AND gate. This is occurring on the IC in the middle row, third from the left.
vdo_av8r1 year ago
Thank you very much for sharing this great project. I had to give it a try and had a blast. very inspiring. Here's the one I made.
hannishah1 year ago
can any one tell me if I want to add the alarm in this clock then what will be its logic and how can i do that??????"plzzzz zzzz"
jsumon2 years ago
make a clock
trialex2 years ago
Excellent work!

Do you know what the average current draw is?
Mattosx (author)  trialex2 years ago
Yes, I seem to recall it's hovering around 150 mA. One of the nice thing about this clock is that when the power goes out which it did during a thunder storm two days ago, it keeps the time. The logic seems to run off the 100 uF smoothing cap for a while. The ICs appear to use next to nothing for current.
trialex Mattosx2 years ago
Cool, thanks for the reply. That current draw is way less than I was expecting.

Having never been formally taught electronics logic, I've been trying to teach myself a bit, and a CMOS/TTL clock is what I'm hoping to achieve next. I think my biggest stumbling block has been the 60sec/60minute reset, so I'm studying your layout. Can't say I understand it yet, so standby for more questions :-)

I've been planning on using 4026's ( for my clock - they seem to be a combination of the 390's and 4543's (as in decade counter + 7 seg driver) you use. I'm still comparing datasheets, but can you comment on any advantage/disadvantage of the two options? Seems like using 4026's would save one chip per digit - note that this may not be a good thing if you are going for geek cred :-). The 4026's do have a very small output current limit which might mean transistors are needed before the LEDs, which means you lose the advantage of a smaller part count.

Thanks again
Mattosx (author)  trialex2 years ago
You would have a boatload of transistors if you use the 4026. You need a transistor for each segment and 6x7 = 42 transistors = 126 PCB holes for the transistors. The six 4543s would have 6x16 = 96 PCB holes. And I bet the transistors used as switches would consume more current.

Regarding the logic, the decade counter does what it says, it counts from 0-9. Such a decade counter handles the right digit of something like 59. Now we want the left hand digit to only go up to 5 obviously. So when the left digit hits six, or 0110, we feed the 11 of 0110 into an AND gate and the AND resets the 6 back to 0. It happens so amazingly fast that you never see the 6 on the display.
According to this website it says many different 7 - segment displays can be powered from the 4026 when operating it on 9V. We used this IC at school and we powered 7 - segment displays from it just fine. To me it seems a little at that a 7 - segment display counter IC would exist if it wasn't suitable for connecting it straight to an output?
trialex Mattosx2 years ago
I think I was trying to take the reset signal off the lines between the BCD->7seg chip and the display, rather than straight after the decade counter. I couldn't get these to reliably trigger an AND gate, so reset never happened. I have no idea why I didn't try it at the decade counter output, when I saw yours it was an aha! moment.

Looking more at the 74HC390 chip, it's a DUAL decade ripple counter, with totally separate inputs, outputs and resets. It seems like you could use a single 390 to drive two 7seg displays (i.e. seconds and 10's of seconds using the same chip). Any thoughts on this? (Still waiting for my 390's to arrive to test it myself).
Mattosx (author)  trialex2 years ago
Yes, the 390 is a dual ripple counter. I just used one on each for simplicity. I designed the first PCB layout between the 390 and 4543 and just copied the design six times. The design could shave off three chips.

bholeman2 years ago
Awesome. I thought it was such an unique project, that I went through and made one myself.
Mattosx (author)  bholeman2 years ago
Awesome! I am glad it works! You made my day making this!

I really like your acrylic case. I made a second clock and it's sitting in a box. Ready to be displayed. Did you buy it or make it?

It seems you also used photosensitive PCB boards? It seems that the traces came out very clean. And you are more daring than me — soldering the ICs onto the board without sockets!
I saw your Instructables post and decided to follow the steps to make it. The traces were made by spraying a sheet of copper with black flat paint then cutting the mask using a 60 watt laser cutter. After the mask was done, I dunked it in a combination of muriatic acid and hydrogen peroxide. As for the ICs, I totally missed that you used them at all until I was halfway through soldering. I cut the acrylic on the same laser cutter as the PCB. Would you like one? We have some different color plastics: smoke, clear, blue, red, orange, etc.? I still have the files to cut it. I'd be happy to cut one for you!
I am trying to remake the PCB layout , and I found that diodes in it, which are not there in the component list. So please give me the component number.

And I cant find where I should connect  Input power pins in your Design, Please can you point out that particular point in your layout.

Akshay Jadhav
Mattosx (author)  akshay+jadhav2 years ago
Hi there, I basically just used left over diodes. I believe any small signal Zener diode will do. They are quite small.

Regarding the power -- they really can be conned Nyerere, but if you look at the PCB art there are two large squares on the left. These can be used as the power connectors. The top would be positive and bottom square negative.
Ohh thanks a lot brother !
Incase i need any help please help me further
In next 2-3 days I'll submit my project
And A Big Thank to you and your Project :)
Have a nice day ! :)
trialex2 years ago
Hey I finally finished mine!

I made a few changes:
- I used a 4060 to divide a 32.678 crystal down to 2Hz - Which was then divided by a 4024 to 1Hz
- Used both "sides" of the 390's to save three ICs
- Used some ex-military displays that I salvaged from an old work project. They had the binary to 7 seg portion built in.
- Used point to point wiring

Still need to sort out a case / way to display it and what power supply I want to use.

Anyways thanks again for your help, I don't ever think I would have got the 60/24 reset working.

Photo attached (I think)

DSC06204 (Large).JPGDSC06206 (Large).JPG
Mattosx (author)  trialex2 years ago
Cute design! You really kept the component count down with those embedded chips in the displays themselves. That really cuts down on a lot of wiring. great implementation.

I chose to only use one side of each 390 to reduce the trace complexity on my PCB. It made it more linear.

How goes the time keeping? I am happy after two months mine has kept excellent time
ramesh2 years ago
Lovely clock project, and would like to know what is the minimum voltage level (peak to peak) of the clock signal required to run properly, for the dual decade counter 74HC390 counter. What is the minimum power supply voltage that can be feed to the logic circuitry only, to keep the correct time, in case of power failure? will 3x1.5 V dry cells or 4x1.2 V rechargeable NiCd/NiMH betteries would run the logic circuit satisfactorily?
Mattosx (author)  ramesh2 years ago
I am unsure, but I suspect the digital logic power requirements would be quite minimal. I have had a power failure and the clock kept its time basically. I suspect the 100 uF smoothing capacitor I added was supplying it satisfactorily.
hi, does any one can explain how to put the crystal because i dont understand if is of two legs or four and also why are two square join and the other two alone.
Mattosx (author)  jmartinez pedrero2 years ago
I was originally going to build a crystal oscillator using a crystal itself, but it was being ... finicky.

My design instead uses a 32 Khz crystal oscillator CMOS chip. The chip is a tiny surface mount device that has four legs that solder onto the pads. Each leg of the chip solders onto one of the four squares.

Here is the data sheet:
Llamarama2 years ago
I love this, everything about this seems so neat and tidy. Thanks for sharing 5 stars :)
1-40 of 49Next »