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!

12 comments:

Glenn Kun said...

hey there buddy. where should I get the "hacked quartz clock o.? I need to do this project before october 5 T_T and my only idea is to use a 555 timer and 192 ics. I can already build a 4 digit count down/count up timer. however, unfortunately, I do not know how to set the clock. please help me.

Christian said...

hi, i build the schematic that you have given but unfortunately, it doesn't work. the 7 segment only lights up and nothing happen they didn't change. what seems to be the problem?? please help me.

nathan said...

If the number isnt changing then it looks like the pulses aren't being sent to the decade counters

Christopher Handwerker said...

@Glenn Kun, you should be able to salvage one out of any dollar store analog Quartz clock. (See the link in the blog post). Alternatively you could use a crystal oscillator, but those usually come in such high frequencies you would need a lot of counter ICs to divide it down to a one second time base (board space was an issue for me). Using a 555 timer would be my last choice because RC circuits are not that accurate and susceptible to changes in temperature. While a 555 timer might keep time for a little while, even the slightest frequency change adds up when you have a clock running 24/7.

@Christian, use a logic probe to get a better idea of whats going on in your circuit, and whats going wrong. If you don't have one you really should get one!

lemoneyes86 said...

hi~ I got a question about hacked quartz clock. I tested my hacked clock.I connected a LED with one of the hacked clock output.but, not same as explained, it blinks almost 10 times per sec( maybe 10hz) and when connected with 2 output, it doesnt blink. is it different according to the clock?
(I hacked mecury battery clock and it is not tik tok motion style clock , continuative second hand stle)

tonot said...

did you use a 555?????
i think is one next to the quartz

Harsha Alva said...

what are the values of the resistors used?

Miracle of The World said...

what are the type of diode used???

cyril said...
This comment has been removed by the author.
cyril said...

can you tell me how to make a mod 24 counter? or email me: cyrilcuthbert@yahoo.com

thanks! :D

Lavender said...

Can you tell me, how were the resistance values from the cd411 decoder to the 7 segment display calculated?

Really need to know, as we are planning to implement a milli- second counter as well.

Thanks in advance.

Sands Wendt said...

Hi, I think the the Hours 0-9/0-4 CkA should be tied pin 8, aka QC of Minutes Mod-6. I got this schematic to work perfectly that way. Thanks!