Thursday, April 1, 2010

Digital Clock - 7490 Decade Counters and a Hacked Quartz Clock



No microcontrollers needed here! I started playing with some 7490 decade counters and decided to build 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 (in the US) down to 1 Hz or you can build a crystal oscillator and divide that frequency down with decade counters.

I decided to take a more hacky approach and took a 1 Hz oscillator circuit out of an analog Micky Mouse clock. 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 1/2 Hz), connecting them directly together worked just fine. I also powered it off the 5 volt supply by using a current limiting LED across its power input, the alternative is to have a separate battery for the clock. Every quartz clock circuit is different so its something you have to experiment with on the breadboard before building!


digital_clock_schematic.pdf

The time is kept by six 7490 decade counters. The 1 Hz clock is pulsed into a 7490 wired up as a mod 10 counter (for seconds 0-9). The output of the mod 10 counter is pulsed 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. Good luck!