# Digital Clock Circuit Modification Question

Discussion in 'Electronic Basics' started by Zach Zaborny, Sep 27, 2004.

1. ### Zach ZabornyGuest

Hello all. I am fairly new to the world of digital electronics, and to get
me started, I am building a digital clock out of CMOS 4000 series logic ICs.
The schematic for the clock can be seen at the following URL:
http://elektronik.kai-uwe-schmidt.de/digital_uhr_en.php. Now, that clock
uses 24 hour time, I want there to be a switch that will let me enable 12
hour time. By using some other gates, I have come up with an addon to the
circuit. It can be seen here:
http://random.nts-technologies.org/images/Hosted/clockmod.jpg. When the the
first digit of the hour is "1" (0001) and the second at "3" (0011), the quad
input AND (made of 3 dual-input ANDs) is triggered and it resets both and
sets the hour's second digit to a "1" by pulsing the "CLK" pin.

Will this work, though? I have my doubts.

-Zach

2. ### CFoley1064Guest

Subject: Digital Clock Circuit Modification Question
Hi, Zach. I'm not sure what's going on with the second link - it seems to be
broken or missing. You're describing the right concept on a 12/24 switch,
though. If you get together another 3-input AND gate (requiring another IC,
I'm afraid) and switch between the AND output on the schematic and the one
you're proposing, you should be spot on. All you really need are the three 1s
- you don't have to add any 0s to the decoding for the alternate reset signal.

A couple of points. First, using the 555 to generate a 1.000 Hz timing signal
is weak. You'll get drift, mostly from changes in capacitance with temp. If
you're in for a penny, you're in for a pound. If you're going to go to all the
trouble of doing this much counter logic with 4000-series CMOS, add another
divide-by-50 or divide-by-60 counter to get 1.000 Hz from your power supply
transformer. It may drift slightly from hour to hour, but our friends at the
power station are kind enough to bump the power frequency during the night to
keep the daily total remarkably steady at 5,184,000 cycles per day (60 Hz).

The second thing is, this circuit has been basically a wire wrap or perfboard
wiring exercise for at least a decade. It would be a lot easier with a PIC.

Good luck
Chris

3. ### Zach ZabornyGuest

Okay, at first I had no idea how to use the mains line for a clock, but this
circuit has helped, it is for a binary output clock:
http://www.hanssummers.com/electronics/clocks/binary/circuitm.htm. One
problem though: his site is not based in USA, but rather in Europe. The
power in hi area oscillates at 50hz, while mine at 60hz. I used his
modification for 60hz and edited his design a bit for it:
http://random.nts-technologies.org/images/Hosted/1hz.jpg. It looks like it
will work. Except, can I use a CMOS 4000 series dual-input AND for the
oscillator instead of the 74LS08? I plan to run mine all on 9V, so it would
be more appropriate.

As for the 12/24 switch, I didn't notice my mistake . But, will it reset
the hours to "01"? As the AND triggers, it outputs a pulse to the RESET and
CLK pins on the 4510 for 2nd-digit hours, as seen here:
http://nts-technologies.org/random/images/Hosted/clockmod.jpg.

As for it being easier on the PIC; I think that's cheating. I would rather
play with discrete logic circuits and parts than have one microcontroller do
all the work I do understand it's ease of use being handy though. Call me
old fashioned.

-Zach

4. ### OlafGuest

you might want to check http://www.mcamafia.de/nixie/ncp_en/ncp.htm ,
under the header 'Getting started with it: the proper timing' you can find
another CMOS-based way of generating 1Hz. It uses a crystal and is
therefor independent of the frequency of the mains line. (The circuit
works, I've build it from this site and my clock is running on time for a
few months now.)

bye, Olaf

5. ### Zach ZabornyGuest

Thanks, I'll probably use that for it's simplicty and ease of us . The
Nixie clocks are very nice, and may be my next project, since it looks very
interesting.

Thanks once again,
Zach

6. ### OlafGuest

if you're interested in nixie's: you can find a lot of homebrewn
nixieclocks at
http://homepage.ntlworld.com/electricstuff/nixiegallery.html and at
http://home.wanadoo.nl/olafdonk/nixie/ you can see some pictures of my
nixieclock (just ignore the Dutch). I've used Peter Wendt's circuit for
this clock. At http://home.wanadoo.nl/olafdonk/display/ there are some
pictures of the big 7segment display's I'm using for my current
clock-project. I still needed a (non-pic-oriented) circuit for that, so
the link from your original post was most welcome ;-)

bye, olaf