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24hr digital clock, please help !

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2005-2006 » Archive through 15 February, 2006 » 24hr digital clock, please help ! « Previous Next »

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Dave_39
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Username: Dave_39

Post Number: 1
Registered: 02-2006

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Posted on Tuesday, 07 February, 2006 - 01:25 pm:   Edit Post Delete Post Print Post

This is my first post on a internet forum so please forgive any errors.
I am currently trying to run a science club at my childs school, the main project
will be to build a 24 hour digital clock using 7093 timers and 7447 decoders with a
couple of 7408 AND gates. I have got the seconds and minutes working fine but
am having trouble getting the hours to work correctly. The problem is with the reset,
the single hours needs to reset at 10, and at 4 when the tens of hours counter
is 2. Having two reset conditions seems to cause problems. I have attached the
relevant bit of the circuit that does'nt work, if anyone can surgest a fix I would
be very grateful. Thanks in anticipation.
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Epithumia
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Username: Epithumia

Post Number: 163
Registered: 07-2005

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Posted on Tuesday, 07 February, 2006 - 02:10 pm:   Edit Post Delete Post Print Post

Hi,

I think you need 2 AND gates and an OR gate. One AND gate is QD and QB of the units counter to decode 10. The other AND gate is QC of the units and QB of the tens to decode 24.

OR the outputs of the two AND gates and feed that into the reset.

You can save a chip by replacing all 3 gates with 74LS00 NAND gates (because an OR gate is the same as a NAND gate with its inputs inverted).

Alternatively, how about using the 74LS90 counter? That with reset at 10 automatically, and you only have to worry about resetting at 24.

Hope this helps.
'Bother,' said the Borg. 'We've assimilated Pooh!'
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Dave_39
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Posted on Tuesday, 07 February, 2006 - 02:56 pm:   Edit Post Delete Post Print Post

Thanks alot for the help & advice I'm a bit of a amateur myself but think i understand your answer.
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Dave_39
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Posted on Wednesday, 08 February, 2006 - 08:22 am:   Edit Post Delete Post Print Post

Epithumia. I studied your recomendation last night and can't work it out. Any chance you could give me pin conections for the 74LS00 NAND you spoke of ?
Thanks Dave.
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Epithumia
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Post Number: 164
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Posted on Wednesday, 08 February, 2006 - 09:12 am:   Edit Post Delete Post Print Post

Hi,

I got it slightly wrong. This should do it:

my circuit

Apologies for posting the image rotated, but the chatzone has a maximum width of 450 pixels.

I simulated the circuit to make sure it works.

(Message edited by epithumia on 08 February, 2006)
'Bother,' said the Borg. 'We've assimilated Pooh!'
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Epithumia
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Post Number: 165
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Posted on Wednesday, 08 February, 2006 - 09:25 am:   Edit Post Delete Post Print Post

If you used the 74LS90 instead of the 74LS93 then it's a lot easier:

my circuit
'Bother,' said the Borg. 'We've assimilated Pooh!'
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Dave_39
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Posted on Friday, 10 February, 2006 - 10:14 am:   Edit Post Delete Post Print Post

I have tried the OR gate solution and it worked, yipee !!
I will study the NAND solution and try that in the future. Thanks alot for all the help. Dave.
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Obiwan
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Username: Obiwan

Post Number: 102
Registered: 12-2005

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Posted on Saturday, 11 February, 2006 - 02:28 am:   Edit Post Delete Post Print Post

There is a 24 hour digital clock made from gates and counters in Lab Centers "Sample" schematics. You might take a look at that and see if it tells you anything.
May the Force be with (most of) you.....
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Oldandfeeble
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Username: Oldandfeeble

Post Number: 8
Registered: 10-2005

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Posted on Saturday, 11 February, 2006 - 05:31 am:   Edit Post Delete Post Print Post

If you are using seven segment displays
deriving a reset may take even fewer gates using those outputs.

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