24 Hr Digital Clock Only With Basic CMOS Chips




Introduction: 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 Digikey.com 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

Step 1: 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 DIgikey.com are included in the block diagram

Step 2: PCB Layout & Explanation

The PCB is a 5x8 copper clad. Refer to the images below to understand the layout of the board. The red lines represent jumper wires. The next step will include the PCB art for the actual etching.

Caution: The 4543 BCD to decimal decoders are placed upside compared to all the other chips

Step 3: PCB Art

The actual PCB art for etching. It should come out as 5x8. I always like to place actual ICs on the paper to make sure everything lines up incase the page setup size needs to be increased or decreased slightly.

Etching the board:

1) Lightly sand the copper clad board. This seems to make the pattern stick better.
2) Set your laser printer on max toner density
3) Print out the PCB art on magazine paper. It may jam a few times, but it gives great results.
4) Iron the PCB art onto the board at your iron's maximum temperature. Apply alot of pressure for 15 minutes.
5) Soak in water to dissolve paper. Remove any excess paper.
6) Fix any broken traces with a sharpee.
7) Etch in ferric chloride which you can get at Radio Shack.

Step 4: Drilling Holes & Example Images

I drilled my holes with a testors hobby hand drill. It worked out pretty well, except for a few poorly drilled places where I wasnt paying much attention.

Step 5: Populate the Board

Populate the board with components. Be careful with the 4513s as they are upside down in orientation compared to the other chips on the board. Also, the crystal oscillator is a SOIC chip. I got mine on by pre-soldering the pads and having the chip melt into the pads. I am not sure if this is the best approach, but Iv never dealt with SOIC chips before.



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95 Discussions


1 year ago

Resetting minutes to 00 after 59, or resetting hours to 00 after 23:59 would be a problem that way


1 year ago

4543 drivers has internal current limiting, no need for resistors.

Are the capacitors electrolytic or ceramic?... and what are the voltage of the capacitors?

Forgot to mention, the display (and your eyes) will not be fast enough to see miliseconds displayed.

You need to copy the "seconds" portion as the miliseconds. Then cancel the AND gate for 59 to 00, and use a different clock signal (100 Hz) as this one uses 1 Hz.


2 years ago

how the drop down resistors for each 7 segment display will be implemented ?
will each resistor be implemented at the output and input of the 4543 ?

1 reply

Each resistor will be placed at the every output of the 4543 IC, connecting with its respective input at the 7-segment display. Use a 500Ω resistor for a bright display, or a 1000Ω resistor for a dimmer display. Not using any resistor will burn the display.


2 years ago

Good Day, is there any alternative for 4543 BCD to decimal Coonverter and 4020 Divide by 16384 counter??? any suggestion?? for other IC??? please reply asap thanks or email me on sohaymeenpanontongan@yahoo.com


2 years ago

Made the circuit, works great! Can anyone please tell me how to avoid the push buttons mechanical chatter, so when they're depressed to set hour/minute, it will only increment 1 hour or 1 minute at a time? The problem is that for each button press, it is now incrementing more than 1 hour or minute...It's the only "problem" that I'm having. Thanks!

2 replies

A small cap, maybe 0.1 microfarads in parallel with the switch should do the trick, mostly. Can you post a picture of the completed clock? I'd love to see :)

Thanks for your advice, but unfortunately the solution that you've kindly made don't seems to work. I've done a google search about that problem (how to avoid mechanical chatter or contact bounce) and I can't fix the problem with none of those I've tried. This picture is from the 1st clock that I've made, and the second one, that is not finished yet, has the same problem.
Thanks again!
Best wishes,

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?

1 reply

Yeah. Of course you can. But you have to use also 4060 and JK or D FF to divide pulse to standard 1Hz for your circuit.

You can use CLK pulse from pin 9 of 2nd 4020 IC, through 1 or 2 Transistor connected series with LED. That'll make LED blink as you want.

I think 1N4148 is fine for this circuit. Trust me.

I think 1N4148 is fine for this circuit. Trust me.

So I ordered the CMOS 32.768 KHz Crystal Oscillator, how do I wire this thing up? There are no pins on it and it is very small.