Discrete CMOS Digital Desk Clock

This project began with a simple idea: to build a fully functional digital desk clock using only classic CMOS logic ICs that are still readily available in local electronics markets. No microcontrollers, no firmware, no programmable logic - just counters, flip-flops, logic gates, and seven-segment decoders doing exactly what they were designed to do. The result is a 24-hour digital clock powered from a 5V, 1A supply, drawing roughly 200mA, and assembled entirely by hand on a perfboard using point-to-point wiring.

Schematic files for this project are hosted in the GitHub repository. Additionally, a PDF version is available for download within this article.

1. C1, C2 – 27pF – 2 pcs
2. C3, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15, C16, C17, C18, C19, C20 – 0.1µF – 17 pcs
3. C4 – 1µF – 1 pc
4. C21, C22, C23, C24 – 10µF – 4 pcs
5. D1, D2, D3, D4 – 5mm Red LED – 4 pcs
6. J1 – 5.5mm × 2.5mm DC Barrel Jack – 1 pc
7. Q1 – 2SC945 (TO-92) – 1 pc
8. R1–R42 – 330Ω – 42 pcs
9. R43 – 10MΩ – 1 pc
10. R44 – 100KΩ – 1 pc
11. R45, R46, R50 – 10KΩ – 3 pcs
12. R47, R49, R51, R52, R53 – 1KΩ – 5 pcs
13. R48 – 22KΩ – 1 pc
14. SW1 – DPDT 8mm On/Off Self-Locking Switch – 1 pc
15. SW2, SW3 – SKRGAED010 Tactile Switch – 2 pcs
16. U1, U3, U11 – CD4518 – 3 pcs (datasheet)
17. U2, U5, U6, U8, U12, U13 – CD4511 – 6 pcs (datasheet)
18. U4, U7, U9, U10, U14, U15 – 5161AH / D198K 7-segment display (0.56″) – 6 pcs (datasheet)
19. U16 – CD4060 – 1 pc (datasheet)
20. U17 – CD4013 – 1 pc (datasheet)
21. U18 – CD4081 – 1 pc (datasheet)
22. U19 – CD4071 – 1 pc (datasheet)
23. U20 – CD40106 – 1 pc (datasheet)
24. Y1 – 32.768kHz Crystal (2mm × 6mm) – 1 pc
25. DIP-16 IC Socket – DIP-16 IC Socket – 10 pcs
26. DIP-14 IC Socket – DIP-14 IC Socket – 4 pcs
27. 2.54mm Female SIP Socket (40-pin) – Round Female SIP Socket – 2 pcs
28. Prototype Board – 18 × 30cm FR-1 Single-Sided Board – 1 pc
29. Hook-up Wire – 30AWG Wrap Wire (8 colors) – 1 set

Notes:

1. All resistors are 5% tolerance, carbon film unless otherwise specified.
2. Capacitors (Ceramic/Mylar):Rated for 25V or 50V operation.
3. Capacitors (Electrolytic): Minimum voltage rating of 16V.
4. High-quality solder wire and acid-free solder paste are required.
5. For best results use a 900M-T-SK or similar chisel-style tip for optimal heat transfer.

The clock is based on a crystal-controlled time base. A 32.768kHz crystal oscillator generates a stable frequency, which is divided down to exactly 1Hz using CMOS divider and flip-flop ICs.

This 1Hz signal drives a cascade of BCD counters that count seconds, minutes, and hours. Additional logic resets the counters at 60 seconds, 60 minutes, and 24 hours to maintain correct time in a 24-hour format.

Each BCD output is decoded and displayed using seven-segment LED displays.

The design uses commonly available CMOS ICs such as:

1. CD4060 for crystal oscillator and frequency division
2. CD4013 flip-flops for signal conditioning
3. CD4518 BCD counters for time counting
4. CD4511 BCD-to-seven-segment decoders
5. CMOS logic gates (AND, OR, Schmitt triggers) for reset and control logic

All components were selected because they are easy to source from local electronics markets.

The entire circuit operates from a regulated 5 V DC supply. The clock consumes approximately 200 mA during normal operation.

Decoupling capacitors are placed close to each IC to reduce noise and ensure stable operation. A standard 5 V, 1 A power adapter is more than sufficient for continuous use.

Instead of using a PCB, the entire circuit was built on perfboard using point-to-point wiring. This approach allows flexibility and encourages careful signal routing and layout planning.

I recommend building the circuit in stages:

1. Power distribution and decoupling
2. Crystal oscillator and 1Hz time base
3. Seconds counter and display
4. Minutes counter and display
5. Hours counter and display
6. Control and reset logic

Testing each section before moving to the next makes fault-finding much easier.

Seven-segment LED displays are driven through BCD-to-seven-segment decoder ICs with current-limiting resistors on each segment.

When wiring the displays:

1. Keep segment wiring neat and consistent
2. Double-check common-anode or common-cathode orientation
3. Test each digit individually before final assembly

Good wiring discipline is especially important with point-to-point construction.

Push buttons are used to set minutes and hours. These switches inject faster clock pulses into the counters when pressed.

The design allows time adjustment while the clock is running, and setting the minutes automatically resets the seconds to zero. All switch inputs are debounced using RC networks and Schmitt-trigger inverters for reliable operation.

After completing the wiring:

1. Verify the 1Hz signal using an LED or oscilloscope
2. Check counter rollovers at 60 seconds, 60 minutes, and 24 hours
3. Confirm proper reset behavior
4. Test time-setting buttons

Most issues in this type of build are related to wiring errors, so careful inspection goes a long way.

Once the circuit was fully tested, it was mounted inside a homemade picture frame built from molded PVC picture-frame bars.

This enclosure gives the clock a unique look and reinforces the "fully handmade" nature of the project, combining electronics with simple mechanical fabrication.