What is this?

This is a digital clock only made with logic chips. It features the RTC-72421A real time clock chip to count the time. The clock features a 2032 battery holder to continue counting the time, even when primary power is disconnected.

It's a very simple design, only using 7 chips and only making use of trough hole components. The total cost of this kis is at time of writing (April 2026) only 21 EUR! So it's a very affordable kit for everyone to assemble.

This kit is aimed at beginners, who want to learn basic electronics and soldering. I've also made a rich documentation for the kit, to explain how it works, how is can be assembled and troubleshooted.

The clock (seen in the above picture) was originally designed in Summer 2022 and was made on a self etched copper PCB. Now in 2026, I have revamped the schematics, which I drew into my personal journal, into kicad to make it more visually appealing and to explain the design, directly in the schematic.

The only negative of this clock, is that is uses the RTC-72421A real time clock chip. In 2022, when originally designing the clock, this chip was realily avaiable, but as of 2026 has become out of stock almost everywhere. You still get the chip on Ebay for a pretty low price, but as they are not concidered "old stock", I don't know if they're avaiable long term.