Fried crystals mmmm

Parallel to my power supply project are two others, both requiring an accurate timebase.

These are my nixie tube clock and my frequency counter (which I will probably combine with my function generator that I will also build).

The two, being time-based, are inherently related. The nixie clock happens to use the mains frequency as a timebase and I really do not like the idea of using an un-isolated mains if I can possibly avoid it. The frequency counter schematics I have all seem to use a crystal timebase which seems more logical. After all, if all of our clocks and watches rely on them, why can’t my nixie clock?

With the mind to use the same time base circuit for both projects, I thought I’d quickly whack it up on the breadboard and see what I get.

I first tried the timebase from Miguel Pedroso‘s cmos frequency counter project. I liked the look of this project, as it seemed simpler than the other one I was looking at which looks scanned from an old electronics magazine. Also, it uses the more modern 4000 series cmos chips rather than the 74HC ones (which I am sure are good too). Anyway, his timebase was deceptively simple, involving only a CD4521 24-stage divider, a 4.194304MHz crystal, a trimmer cap, and a 10pF cap. Great, I like simple.

So I build it up and immediately notice something wrong. It’s not oscillating. What’s more, the 7805 regulator powering the thing is heating up, which it most definitely should not. I tried fiddling with it, checking my wiring, swearing at it, nothing worked. It could have been a dodgy trimmer cap or some other mistake of mine, after all Miguel Pedroso seems to have got his working fine and he did warn me the thing was touchy.

Reaching a dead end, I figured I’d try another schematic I found in a forum, which seems to agree with many others I find floating around the internet. Posted here:

1Hz timebase

My sincerest apologies to the original author and the gentleman (or woman) who posted and cleaned it up. I have forgotten where I found it.

Anyway, I build this up and the bloody thing still doesn’t work! On a whim, I swap the crystal out for another one and behold! My ‘scope starts twitching hi and low every second – making a 0.5Hz square wave. Joy.

I tried the original crystal and discovered it is indeed dead. For all I know, it could have always been dead, or it could have been fried due to the absence of resistors connected to the crystal in Mr. Pedroso’s diagram. Either way I got it working!

Even more fun, I noticed the other output pins on the 4521 give you different divisions of time. It appears I can get 0.5Hz, 1Hz, 2Hz etc. Handy!

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