15 second digital timer

Discussion in 'The Projects Forum' started by mrgthreepwood, Aug 22, 2007.

  1. mrgthreepwood

    Thread Starter New Member

    Aug 22, 2007
    4
    0
    I am trying to complete a project and am quickly nearing the deadline. The project is as follows:

    "The circuit will display an elapsed time, 0 to 15 seconds, repeating continuously. A two digit display will indicate 0 to 15. A green LED will be lit from 0 to 5 seconds, a red LED from 6 to 10 seconds, and a buzzer will sound from 11 to 15 seconds. A 555 timer and 741 op-amp must be incorporated into the project."

    I have been working with the following:
    5V Vcc
    two 7490 decade counters
    two 7447 BCD (7-segment decoder)
    one 741 Op-amp (required)
    one 555 timer (required)
    two seven segment displays
    various resistors and capacitors

    I have attached my current schematic. I have torn down and rebuilt this circuit many times. I currently cannot even activate the attached 7-segment LED. I will need gates (NOR, I think) for the yet-to-be-added LEDs and buzzer. Also, the op-amp is not yet incorporated in the schematic.

    Thanks for any help all of you can provide!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You look to be somewhat on the right track with the 555 and the first 7490, although I can't make out pin numbers..

    What is the 741 supposed to do, though? Drive the buzzer?
     
  3. mrgthreepwood

    Thread Starter New Member

    Aug 22, 2007
    4
    0
    To be honest, I'm not completely sure. I initially had the Op Amp "feeding" the 555 timer circuit (555 is set in Astable config.). Op Amp output to Thresh. and Trig of 555. I have a circuit built on a breadboard (sorry, no photo) that counts to 99, but I need to configure it to reset after 15 (count 0 - 15 and restart continuously) and I have not been able to successfully add the op amp.

    This thing is making me nuts...:confused:
     
  4. mrgthreepwood

    Thread Starter New Member

    Aug 22, 2007
    4
    0
    I calculated approximately 1Hz output on the 555, which I understand is a good freq to work with on the 7490s. Any idea how to set up the 7490s to restart the count after 15 seconds?

    Also, my LEDs are not functioning. I'm sure I missed something...

    I believe my NOR gates would be connected between the 7490s and the 7447s. Is this right?

    I'm still not sure what to do with the op amp...

    Here's the current version (not much has changed, just started over again):
    [​IMG]
     
  5. mrgthreepwood

    Thread Starter New Member

    Aug 22, 2007
    4
    0
    Here is the semi-almost-not-quite-there-yet circuit. The LEDs (red and green) flash at different intervals and the buzzer sounds after a few seconds... not the desired outcome, but it is doing something.

    Latest schem.:
    http://farm2.static.flickr.com/1108/1216718448_091ac584ca_b.jpg

    More help or other ideas to make this thing work properly? (project description above)

    Thanks!
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    For the reset to zero, use external gates to generate a clear to the counters when the counts = 15 and the next clock occurs. The same external logic can be acitve during the timing periods for driving the LED's.
     
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