I was playing around with some 7490 decade counters and ended up building a digital clock.

The first trick was getting an accurate 1 second time base. There are a few options here, you can divide the 60 Hz AC line frequency down to 1 Hz or you can build a crystal oscillator and divide that frequency down with more counters. With limited space on this breadboard I ended up taking a 1 Hz oscillator circuit out of an analog quartz clock I found in the junk box.

There are a few different ways you can wire these quratz clock circuits. It took some experimentation to find a configuration that worked. With my clock I didn't have to run the outputs through diodes or transistors. Each output is ½ Hz and simply wiring them together worked just fine. I powered this oscillator from the 5 volt supply by wiring a current limiting diode, in this case an LED, across the power connections. This added an unexpected "feature" where the LED blinks off every second.

The time is kept by six 7490 decade counters. The 1 Hz clock goes into a 7490 wired up as a mod 10 counter (for seconds 0-9). The output of the mod 10 counter then goes into a 7490 wired as a mod 6 counter (for seconds 0-5). That circuit is then duplicated for the minutes (0-59). The hours are then counted by two 7490s wired up as a mod 24 counter (0-23).

The tens of seconds are displayed by 3 LEDs in binary. The 7 segment displays are driven by four 4511 BCD-to-7 segment decoders. I needed thirty-three 100 Ohm resistors in total for all the displays and LEDs.