Electronic Timer

Discussion in 'The Projects Forum' started by rsgino7, Dec 24, 2008.

  1. rsgino7

    Thread Starter New Member

    Dec 22, 2008
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    Hi there, I started building a 7 segment LED timer following this circuit diagram: http://www.aaroncake.net/circuits/counter.asp . Picture 0020.JPG

    Picture 003.jpg

    Picture 004.jpg
    However, after putting everything together, it doesn't work. I have taken pictures of my breadboard. If anyone can please help me out it would be great.
    Question: What do you put into Pin14 of the First IC (U1). It says count in...? what does that mean
     
    Last edited: Dec 24, 2008
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Is there a ground line present?

    A schematic is much more informative than breadboard photos - especially when the IC's are hard to identify.

    A COUNT input wants to have some digital pulse applied to make the counter count. Get the data sheet for both of the IC's you have. Just put the device number into Google and download the PDF.
     
  3. rsgino7

    Thread Starter New Member

    Dec 22, 2008
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    Is there a switch required for pin 14 that give a shock to run this circuit?

    Oh after further reading, i found this: The circuit accepts any TTL compatible logic signal.

    Now, how do you generate a TTL compatible logic signal?
     
    Last edited: Dec 24, 2008
  4. nop

    New Member

    Nov 26, 2008
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    Yes a switch or clock from a 5V source will need to be fed into pin 14. TTL are the voltage levels the chips use to define a logic level '1' or a '0'. You don't have to worry about this unless you want to use different kinds of chips that are not all TTL. And as beenthere said earlier, I don't see a ground (negative terminal) either. Pins 2,3,6,7 and 10 of the first chip should be hooked up to ground and pins 8 of the second chip should also be hooked up to ground.
     
  5. rsgino7

    Thread Starter New Member

    Dec 22, 2008
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    Okay, I have fixed those issues with the pins. Now I get the 7 segment LED to display 0, but it doesn't count. So, I do need drops in voltages....

    nop what exactly did you mean by a switch or clock being fed into pin 14. What kind of switch.
     
  6. rsgino7

    Thread Starter New Member

    Dec 22, 2008
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    Hey thank you guys. I got it running. Without All About Circuits I would have never succeeded. One more thing, if i want to have two digits do I have to buy all the components again like another 74LS90 and 74LS47? And would this count to 99, or would both displays just simply repeat 0 through 9 like the single one does right now.
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Yes. And you will need another 7-segment display too.

    The second 7490 will not count when the first 7490 is counting, unless the count is already "9" on the first 7490. Another count would place the first 7490 back into "0" but also generate an "carry" output. This carry output signal is what you would connect to the second 7490 clock to make it counts.

    So the count sequence will be 00, 01...09, 10,11...19,20,..98,99, 00.
     
  8. rsgino7

    Thread Starter New Member

    Dec 22, 2008
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    0
    Thank you very much
     
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