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.
At the heart of the clock is a traditional timebase derived from a 32.768kHz crystal oscillator. This frequency is widely used in timekeeping because it can be divided cleanly to exactly 1Hz. A CMOS frequency divider, such as the CD4060 and CD4013, performs this task, producing a precise one-second pulse that serves as the master clock for the entire system. From this point onward, time is counted purely in hardware, one pulse at a time, just as early digital clocks did.
The one-second pulses are fed into a cascade of CMOS BCD counters. The first pair of counters handles the seconds, counting from 00 to 59. Once the count reaches 60, additional logic resets the seconds counters back to zero and generates a carry pulse for the minutes section. This same structure repeats for minutes, which again count from 00 to 59 before rolling over and advancing the hours counters. For the hours, the logic is slightly different, as the clock operates in a 24-hour format. Dedicated reset logic ensures that the count wraps correctly from 23 back to 00.
Each digit of the clock is displayed using a seven-segment LED module driven by a BCD-to-seven-segment decoder. These decoder ICs take the four-bit BCD output from the counters and translate it into the appropriate segment pattern. This greatly simplifies the design, as there is no need to generate segment patterns manually using discrete logic. Current-limiting resistors are placed on each segment line to protect the LEDs and keep the brightness consistent across all digits.
User interaction is also handled entirely in hardware. Push buttons allow manual setting of minutes and hours. When pressed, these buttons temporarily override the normal 1Hz clock signal and inject faster pulses into the relevant counters, allowing the time to be adjusted quickly. A stop/run control lets the clock be paused during setting, preventing accidental increments while adjustments are being made. All switch inputs are conditioned with simple RC networks and Schmitt-trigger inverters to ensure clean, bounce-free transitions. The clock is designed to allow time adjustment while running, and when setting minutes, the seconds are automatically reset to zero.
Powering the clock is straightforward. The entire circuit runs from a regulated 5V supply, with local decoupling capacitors placed near every IC to maintain signal integrity and suppress noise. Despite the number of chips involved, the total current consumption stays around 200mA, well within the capability of a 1A power adapter. This makes the clock reliable for continuous operation without excessive heat or stress on components.
One of the most enjoyable aspects of this project is how it was physically constructed. Instead of using a printed circuit board, the entire design was built on a perfboard using point-to-point wiring. While this approach requires patience and careful planning, it offers tremendous flexibility and a very satisfying hands-on experience. Once completed, the finished circuit was mounted inside a homemade picture frame built from molded PVC picture frame bars, giving the clock a unique, personal enclosure that blends electronics with DIY fabrication.
Using discrete CMOS logic for a clock like this comes with clear advantages. The design is fully transparent - every function can be understood by following the signals from one IC to the next. There is no firmware, no debugging tools required, and no dependency on proprietary development environments. The components are inexpensive, widely available, and electrically robust. From an educational perspective, this approach provides deep insight into digital design fundamentals such as counting, decoding, gating, and timing.
At the same time, this style of design has its limitations. Compared to a microcontroller-based clock, the circuit is physically large and component-heavy. Adding features like alarms, date display, or power-saving modes would significantly increase complexity. Power consumption is also higher than that of a modern low-power MCU solution, especially due to the continuously driven LED displays. Any functional change requires hardware modification rather than a simple firmware update.
Despite these trade-offs, this CMOS digital desk clock succeeds exactly at what it set out to do. It demonstrates that reliable, accurate timekeeping is possible using nothing more than classic logic ICs and careful design. Beyond the technical aspects, the main motivation behind this project is its retro-style design and construction technique. I primarily designed it as a fully handmade piece to decorate my home workshop and to celebrate traditional, hands-on electronics.
Schematic of this project is available in https://github.com/dilshan/cmos-clock. This project is released as an open hardware design under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. You are free to use, modify, and share the design with proper attribution.
