/ Electronics

Digital Clock - 7490 Decade Counters and a Hacked Quartz Clock

No microcontrollers needed here! I was playing with some 7490 decade counters and decided to build a digital clock.

Schematic PDF.

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 (in the US) down to 1 Hz or you can build a crystal oscillator and divide that frequency down with decade counters.

I wanted to make due with what I had laying around the lab. I took a 1 Hz oscillator circuit out of an analog novelty Mickey Mouse clock I had laying around.

There are a few different ways you can wire these up explained very nicely here. With my clock I didn't have to run the outputs through diodes or transistors, each output is ½ Hz, connecting them directly together worked just fine.

I powered the oscillator off the 5 volt supply by using a current limiting LED across its power input. This had an unexpected "feature" that caused the LED to blink off every second... sure I'll take it.

Every quartz clock circuit is a little different so it's something you have to experiment with on the breadboard before building.

The time is kept by six 7490 decade counters. The 1 Hz clock signal is fed into a 7490 wired up as a mod 10 counter (for seconds 0-9). The output of the mod 10 counter is fed into a 7490 wired as a mod 6 counter (for seconds 0-5). That circuit is then repeated 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.