Hackaday Clock A Day Entry: CMOS Logic Clock

This is my entry into the informal Hackaday "Clock a day" project. A 7 Segment LED CMOS clock.

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I spent some time doing things properly and like a good engineer would I sat down in front of the schematic editor and I have produced a set of reasonable quality schematics for this 7 segment LED CMOS logic clock. The clock is simple: it displays the time in hours, minutes and seconds and features two buttons to set the time.

I'm using a crystal of 32.768Khz to provide a time base and in classic style, I then divide down that signal into hours minutes and seconds with CMOS counters and a little glue logic. The logic amounts to a few AND gates and INVERTERS that compare the seconds, minutes and hours and appropriately reset the LED counter drivers.

This is a schematic to breadboard to PCB project. I am documenting the build process with videos and short explanatory texts. I know you guys can read the schematic, so I'll try not to bore you.

The counters drive the 7 segment LEDs. There are 6 LEDs in all for the hours, minutes and seconds that the clock displays. Below you can see a short video of the the 7 segment decade counters driving a couple of LEDs.

The jumper cable mess on the top board is impossible to follow, plus you can barely see the chips underneath all those connections. (There is a slightly clearer picture of the board in the project log section below). It is a lot quicker to use jumpers rather than spending hours measuring, cutting and stripping hook-up wire. Plus, hook-up wire is expensive, ya know!

Some of you may recognize the circuit on the bottom board. If you have watched Ben Eater's YouTube channel it is the same clock signal generator that he uses for the computer he has built. The 555 timer is setup to pulse at around 5 Hz and there is a manual pulse button for slowly clocking the circuit. The whole thing is powered by a 5 Volt, 1 Amp power adapter.

With the LEDs and counter chips tested I can move on to bread-boarding the 1 Hz signal generator and testing that.


I love the deadlines. I like the sound they make as they go wooshing by.

BOM_Retro CMOS Clock (20 ICs)_2020-12-01_17-32-32.csv

Bill of materials in CSV format. I use about $12 worth of components.

ms-excel - 3.48 kB - 12/01/2020 at 17:33


  • 2 × 74HC04N Logic ICs / Gates and Inverters
  • 5 × CD4060BE Logic ICs / Counters
  • 6 × CD4026BE Logic ICs / Counters
  • 2 × 555 timer
  • 6 × 7 Segment LED

View all 9 components

  • Project delayed for approx 30 days

    Mario Gianotaa day ago 0 comments

    I am using the wrong kind of ripple binary counter (the CD4060BE in the schematic). I spent a frustrating day trying to find the fault with the breadboard hookup. Anyway, I eventually found the problem and have found a chip that will do what I expect it to do (I think it will, but you know how it is.). The supplier is in China so I will have to wait about 30 days for delivery. As soon as it arrives I will be able to quickly test the counter reset logic for hours, minutes and seconds. The new binary counter chip is a CD4020B. Despite this problem I am confident that the project is proceeding nicely.

  • LED Blinkers

    Mario Gianota01/09/2021 at 20:06 0 comments

    The LED blinkers sit between the 7 segment LED displays and blink on and off at 1 Hertz. The circuit in the video tests them. They appear to be working fine (as far as I can tell). They are driven by a 74HC365 buffer chip that I purchased from Ebay at an extortionate cost. The top board provides the 1 Hertz clock signal to the buffer chip.

  • Component shysters

    Mario Gianota01/05/2021 at 02:49 1 comment

    Discovered I need to buffer the LED blinkers with a non-inverting hex buffer. Searched everywhere on the Internet for a CD4050E, found one on Ebay and was charged an unbelievable £3.99 for *one* chip. I ordered it and sent the guy a rude message :D

  • Testing The 1Hz Signal Generator

    Mario Gianota01/04/2021 at 00:39 0 comments

    It took a while (several hours actually), but I finally got the 1Hz signal generator working on the breadboard. There was only one minor error in the schematic (see links) which I have corrected. I still don't have a oscilloscope to test the signal frequency, but the LED appears to blink in time with a digital clock that I have so I am fairly confident. I guess I will have to wait for the clock to be fully assembled and then see how well it keeps time.

  • Project resumed

    Mario Gianota01/03/2021 at 18:37 0 comments

    I have built the 1Hz clock signal generator on breadboards, but I suspect that the value of the resistor used isn't accurate enough. I am using a 470K resistor instead of a 330K resistor. I have ordered some 330K resistors from Ebay and I will resume work in a five days, or so. It took me hours to discover this problem. *sheesh*

  • Project Resumed

    Mario Gianota01/03/2021 at 14:25 0 comments

    Okay. I know I said earlier that I was going to abandon the project, but I have reviewed the schematics and have decided that I have put too much work into their production to abandon the project. I need to build all modules on breadboards and fully test them to verify the schematic. This may take some time. I will test the 1 Hertz Clock first. Stay with me guys if you are following: I will get a PCB out at some point in the near future.

  • 7 Segment Counters Circuit Tested

    Mario Gianota12/01/2020 at 14:15 0 comments

     I built a small test circuit for the CD4026 decade counters and 7 segment LEDs and connected it up to Ben Eater's clock, which I built a while ago. All good, everything worked.

    Next up: build the 1Hz crystal oscillator test circuit. After that, the counter logic reset circuitry needs to be tested and then I will be ready to move to PCB.

  • Waiting For Components

    Mario Gianota11/27/2020 at 10:07 0 comments

    I have ordered a bunch of resistors from Ali Express and am waiting for them to be delivered. Some components are coming from Ebay and I will be able to build a few test circuits on breadboard to verify the schematic. I am reasonably happy with the schematic, but of course I have no idea if it works until I build the test circuits.

View all 8 project logs

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