RODALCO

I have seen various threads with TTL requests regarding how to and build a TTL clock or counter with discrete components.

As I have build around 7 of these clocks in the early 1990's and these TTL, LED clocks are still going strong I decided to have a look in my archives and post the basic diagramme for it.

I have learned building them also by trial and error and from various sources like TTL Cookbook (Don Lancaster) and Digitale klokschakelingen (Herbert Bernstein) written in Dutch origin Germany.

As i like the old TTL technology and with carefull and accurate ratsnest wiring a very accurate clock can be build at very low cost but labour intensive but that doesn't matter for a hobby.

Important is the decoupling of every 74xx IC with a 100 nF disc capacitor.
I omitted these in my earlier clocks and later addition of these was painful but not impossible.

The numbers 6 and 9, can be made with tails by adding an extra 7408 AND gate to drive a BC 547 to sink the A and D segment when these have to light up.
A 74247 IC has those functions built in but i have never seen one of those.
IC's.

Attached is the basic schema which works well and i'm more than happy to give more info to potential TTL clock builders. Also i'm happy to post more photo's if required.

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6502Dude

Nice diagram!

The first TTL clock I built I used 74393 counters (because I had a bunch of them) 74141 chips and Nixie tubes for display.

After nearly 20 years, it is still operating fine.

TTL logic is still great to work with, reliable, and "oops" tolerant.

Top Cap

Hi Folks, I recently knocked up a timebase generator using parts from the junk box. An adjustable 20MHz Xtal and a few 74390's, lovely chips that contain two equivalent 7490 decade counters so you reduce the component count quite a lot. I deliberately clocked them the way I have so that each frequency output is a symetrical square wave. I added the LED as a last minute thing just to let me know that the count chain is working. If you can find an adjustable Xtal like I did, the whole thing can be tuned to a frequency standard.
Les G4CNH

Attached Images

clock.JPG (81.8 KB, 198 views)

raiz

hello !
Can anyone help me build a frequency generator(60 hz) for my digital clock .i m using 7490 & 7447.but i can't generate 60 hz frquency time base.

andales

raiz, you should have started a new thread.Anyways, hope the link could help you.

http://www.doctronics.co.uk/555.htm

Mike, K8LH

I recall purchasing several $5 (USD) kits back in the 70's when several "single chip" clock ICs started appearing on the market. I still have one of these $5 kits laying around here somewhere. They included a little 8-pin DIP oscillator/divider IC, circuit board, 3.579545 crystal, trimmer cap', and a couple discrete components (resistor and cap'?) and they output a 1-Hz TTL level "heartbeat" signal.

Anything like that still floating around now days?

Mike

Hero999

A 555 timer is no way near accurate enough for a digital clock, build a crystal oscillator with the frequency of a multiple of 60Hz then use a divider to divide it down to 60Hz.

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6502Dude

Using a CD4521 and a 4.194304 crystal will provide a very nice 1hz time base for a clock.

Datasheet - http://www.datasheetarchive.com/data...rticle=3639562

RODALCO

raiz If you look at the schema with TTL clock at the beginning of this post, you have already two 60 counters in there with 7490 IC's.

One is used for the seconds and one is used for the minutes.

If you wire the two 7490's up as in the schema, after the 7413, and use two gates from a 7400 you can use that to decode the 60 Hz from the mains to one Hz for your clock input.

edited for typo's RF.

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Electricity, Electric clocks, Meters and Trains are great.

raiz

hi guys! I m back but with another problem now I have made the clock it works well but it is a 24 hour clock can any body tell me how to make a 12 hour clock using 7490 ics

Hero999

Get the hour counter to reset after 12?

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RODALCO

Hi raiz
Attached is an image from the 12 counter.
I posted this in an other thread at this forum too.
Glad that you got it working all right.
I prefer the 24 hour set up for clocks anytime but it is easy to convert to 12 if you like.
I hope that you decoupled all the IC's with a 100 nF capacitor across the 5 volts IC supply points.

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Electricity, Electric clocks, Meters and Trains are great.

To be confirmed

Hi there, another newbie here.

I am building something like this and I found the article very interesting, but I don't know why the 7490 ICs are decoupled with the capacitor. Could anyone explain why this is done and how the power supply is decoupled with a capacitor across it?

Very interesting site btw, I'll be back to check out more.

ericgibbs

hi,
As the individual transistors in the TTL devices switch ON/OFF they draw a short current spike from the +Vsupply rail.

This spike can pull the Vrail down a few tens of millivolts, this voltage spike then can be detected
by the other TTL gates and cause a mis-operation.

If a decoupling capacitor is connected across the TTL power rails, when this switching current occurs, the capacitor will supply the current and effectively remove the voltage spike.

Also capacitors generally have an impedance [resistance] that decreases with frequency,
so any fast spike on the power lines 'sees' a low resistance path and is suppressed.

This is a simple explanation, you will find as you study the subject of 'noise' its fairly complex.

Its good engineering practice to fit a small capacitor across the power pins of all the TTL ic's.
A low value of about 0.1uf will get the job done, also a 10uF capacitor on the pcb is recommended.

Ask if this is not clear enough.

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Eric
"Good enough is Perfect"

PIC tutorials:
Nigel's: www.winpicprog.co.uk/
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

To be confirmed

Aah that makes a lot of sense now. Thanks a million for your post, which was very clear

I only had a 10 nF capacitor to hand, but I can't get the 7490 to produce a working output yet.

I tried, with a 10K resistor to input a signal into input A (with the crude method of touching it briefly against the +5V wire, but it's stuck on High at QC. Plugging in and out the wire between QA and Input B does vary the output on QB, QC and QD but it is completely random. Sometimes one or two of them flicker and sometimes one or two stay on. QA never lights up and placing a voltage or grounding Input A has no effect. Mabye manually applying a clock is causing problems. I might build a 555 clock later just to see if that works.