Input to the 7490 needs to be on the negative edge of a pulse. Trying to manually apply a signal can cause many problems. In effect, you are dealing with extreme switch bounce. Build up the 555 just to see how much better things go.

__________________
All those who believe in psycho kinesis, raise my hand.

 

After a bit of figuring out why the device wouldn't work even with a fully functioning 555 circuit, I realised that touching the negative wire changed the state of the QB-QD LEDs so after wondering if I needed NPN or PNP transistors, I just got an ordinary NPN transistor between the input and ground, and applied the signal to the base. That part worked perfectly. So I connected this to Input A and linked QA and Input B together and I got the binary numbers flashing up as they should.

I knew of the negative edge trigger but I thought that it would be on the negative edge of a positive signal into the input. I didn't realise that it was positive already and that applying a positive signal did nothing whatsoever. Output A was at zero potential some of the time I believe, which is why touching the QA and input B wires together produced an output.

Thanks for the help!

 

Thanks ericgibbs for your explanation re the decoupling capacitors.

To be confirmedCheck that the TTL reset gates go somewhere.
if you have a single digit counter, put both to ground, or plus 5 volts.

The inpulse needs to be debounced via a schmitt trigger 7413, otherwise you will get random readings.

I try to put a clearer schema on this site. haven't got software to draw schema's but give it a try in XL.

Regards, Raymond

__________________
There are more ways to get to Rome.

Electricity, Electric clocks, Meters and Trains are great.

 

I disagree, If one uses a cmos version and a good quality film capacitor the
the frequency can be adjusted to exactly 60Hz and it will be very stable. I say a cmos version as it can have much higher value of timing resistors to bring the frequency down to 60Hz.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

It's still going to be highly temperature dependant, no reason not to use a crystal, they're easy to derive a clock from and very accurate even over wide temperature ranges.

__________________

 

Sorry, it would be rubbish! - a crystal is hundreds of times more accurate and stable (if not thousands?) than an RC oscillator.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

As far as temperature stability goes, RC oscillators are horribly unstable unless you use matched NTC and PTC resistors and caps for the circuit, at which point why not just use a crystal...

__________________

 

And even using the matched temperature coefficient components it's still a LONG way short of a crystals performance! - it makes no sense NOT to use a crystal.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

hello..
I'm Turkish.my english is very bad.but I must to write the mail.
I want to a oscillator circuit with 7400.Do you understand me.please help me.
have a good time...

 

berem, What do you want to drive with it ?

What frequency do you require ?

The 50 or 60 Hertz mains frequency is very accurate long term.
Some short term variations may happen, but over a 24 hour period it is very accurate.

A 7400 is a NAND gate, in TTL it is most often use to divide 12, 24 or 60 counters.

Regards, Raymond

__________________
There are more ways to get to Rome.

Electricity, Electric clocks, Meters and Trains are great.

 

You can use an oscillator. They are as easy to use as a 7400 IC. Just provide 5V and the output oscillates at TTL levels, or CMOS if you want.

You can now buy oscillators that contain the dividers so you can obtain them at low frequecies like 50 or 60 Hz.

 

Hi Barem, if you want to make your own basic crystal oscillator then this site may help you:-
http://www.z80.info/uexosc.htm

I tend to agree with the other posts on here though, you can by a ready made crystal oscillator in a can, same size as a 14 pin chip, some even have a trimmer for accurate setting.
Les

 

Hi all, This is my version of the TTL clock, it displays 0:00:00 to 23:59:59. [REVISED]
Make sure you read the datasheet to the TTL IC's on the +5V and GND connection pin.

Schematic:
http://www.geocities.com/iyung_w/files/24hrs_clock.pdf

Datasheet:
http://www.ee.washington.edu/stores/DataSheets/74ls/

Good Luck!

 

Thanks for posting that schema.

Should't it be 23:59:59 ?

Cheers, Raymond

__________________
There are more ways to get to Rome.

Electricity, Electric clocks, Meters and Trains are great.

 

hi to all.....i m new member......i have to make a digital clock using TTL IC's 7492,7447,74160,7490 etc..............i have read all the threads............but i was very astonished that there was not pin diagrams of these IC's ........any one who can tell me the whole schematic diagram please tell me as soon as possible.............thaks