The Art of Analog Logic

In the age of the smartphone, there is a certain lost art in the craft of clock making and analog electronics. As a challenge to himself, RF engineer Harit Shah set out to build a clock for his home office without needing to write any code.

He accomplished this task in his project, 7400 Series Logic Clock, which keeps time using only 7400 discrete logic integrated circuit chips.

Each of the three counters for seconds, minutes, and hours is comprised of four ICs: a dual 4-bit binary counter, quad two-input AND logic gate, and two BCD 7-segment decoder drivers. The dual 4-bit binary counter handles the ONES and TENS section of the seconds counter and minutes counter while the AND gates handle reseting the counters. The BCD 7-segment decoder drivers then drive the 7-segment displays with the resulting output values of the counters.

To tackle the requirement of the hour counter needing to be pre-settable and can load in a fixed starting point Shah chose the 74LS190, a presettable decade up/down counter IC. Then to set the clock, a two OR gate configuration handles applying the minute and hour time sets.

Outlined in his project write-up, Not Your Typical 1Hz Source, he generates his clock's 1Hz source using a Maxim Integrated DS3231M, which is an RTC chip but also has a dedicated 32.768 kHz output (versus the traditional choice of a TCXO IC). The DS3231M's 32.768 kHz output is divided down to 1Hz using a SN74HC4020 14 bit binary ripple counter.

Ultimately, he demonstrates the capability to deploy an RTC chip as a standalone 1Hz source (with a couple caveats).

The intuitiveness of this project is really impressive and is a great example of the creativity analog electronics can inspire.