Precise Digital Clock

[thecritic]

I am working on a simple project of building a 7-segment display Digital Clock. I am using the counter CD4033 counter to count time. The clock pulse is provided by an 555 timer circuit that is set in astable mode to generate a pulse every minute.

The problem I am having is that my clock isn't precise enough. I tried larger capacitors in the timing circuit and it helped but that too wasn't enough. I think the timing is changing with room temperature and humidity. So, I am in search of clock pulses in alternative to the 555 timer. Any help will be greatly appreciated.

 

[Oznog]

It MUST be a crystal. Really the only way to do it, which is how clocks do it. Capacitors vary wildly from spec, and vary by quite a bit with temp or age. So do resistors.

Crystals run much faster than a 1-sec pulse, you'd need a 15-bit counter to reduce a 32.768khz clock xtal to 1-sec pulses. So typically you'd do this with a microcontroller.

 

[thecritic]

Thanks Oznog for the Reply. So, where do I get those CRYSTALS, and HOW do I use them. I am net proficient enough to use those microcontrollers but Instead I will be using separate Binary Counter ICs to extract 1-sec pulse from those 32.768 Khz signal.
I currently need help only in the part of generating precise 32.768 Khz signal.

 

[Mike - K8LH]

When some of the first hybrid Clock/Calendar ICs (with 7-segment drivers) started coming out in the 70's there were a few companies selling these little 1-Hertz crystal time base kits which included a little 1 inch by 1 inch circuit board, 3.579545 MHz TV color burst crystal, 8-pin oscillator/divider IC, capacitors, etc. for around $5.

I wonder if there's anything like that around now that you might use?

 

[House0Fwax]

Have a look at this idea;
What's up doc?

 

[QuietMan]

A CD4060 datasheet shows how to use a 32.767Khz crystal, and the crystals themselves are pretty common, since they are in almost all digital watches. If you look around they aren't that hard to find, even in an old watch laying around. You can probably buy a crystal where the CD4060 is sold.

 

[KMoffett]

If use the circuit in HouseOFWax's link (I have), and add the 4022/4017 divide-by-60 circuit here: **broken link removed**, you will get a very accurate, 1 pulse per minute clock.


Ken

 

[thecritic]

A CD4060 datasheet shows how to use a 32.767Khz crystal, and the crystals themselves are pretty common, since they are in almost all digital watches. If you look around they aren't that hard to find, even in an old watch laying around. You can probably buy a crystal where the CD4060 is sold.

Thanks, I got the IC and I got the crystal too (broke apart an old watch). But I don't know how do I wire them. Here is what the datasheet shows.

Should I use external Gates or Are they already internally present? Which components should I add?