60 seconds counter.

Discussion in 'The Projects Forum' started by shawnteo93, Nov 4, 2011.

  1. shawnteo93

    Thread Starter New Member

    Nov 4, 2011
    5
    0
    HI all

    I am given an assignment by my lecture. We are using two 74LS93N(4 bit binary counter) ICs to build a 60 seconds stopwatch. I have no problem building MOD-60 counter from 00 - 59 and it will reset to 00 when it hits 60. I have connect the these two 74LS93N to two 7 segment LED common Anode. I use MOD-6 and MOD-10 to come up with the MOD-60 counter.

    However, I do not know how to add in stop/start and reset buttons. The reset button switch will clear or reset the mod-60 counter to 00 when activated. ie, 00 until it is started again through the use of the start/stop switch.

    If the mod-60 value is running , activating the start/stop switch will stop the counter at its current count value, ie, 40, which indicates a time lapse of 40 seconds from the start. Reactivating this same switch will cause the mod-60 counter to resume from this previously stopped value.

    We are given one 74LS00 and one 74LS32 as well.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Do you have to use those chips in particular?

    Basically stop and start is done at the clock, as in the 1 second clock source. The reset function is just a matter of figuring out the logic from there.
     
    shawnteo93 likes this.
  3. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,357
    Start/Stop requires an S/R flip-flop followed by a gate to enable/disable the input clock. The flip-flop can be created using two gates. Two of the four gates of 74LS00 can be used to create the S/R flip-flop. A single gate from the 74LS32 or the two remaining gates of the 74LS00 can be used to create the clock AND gate (check out DeMorgan's equivalents).
     
    shawnteo93 likes this.
  4. shawnteo93

    Thread Starter New Member

    Nov 4, 2011
    5
    0
    Nope. I can use any other relevant ICs too. For my START/STOP I had used a AND gate. Where the 2-inputs are my clock and my start/stop button. Then the output goes to the clock of my MOD-10.
     
  5. shawnteo93

    Thread Starter New Member

    Nov 4, 2011
    5
    0
    Thankyou for this. But I don't quite understand this. So what inputs do I put into the IC.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Check out a 4026 sometime. It is complete, and is actually designed to do mod 60 countering (though I had to search to find the logic for it). If the power supply is low enough the chip will drive a LED common cathode display directly without resistors.

    Digital clocks
     
  7. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Welcome to AAC.

    To reset to 00, refer to page 3 of the 7493 datasheet. R0 and R1 are used to reset. Keep one of them pulled high (connected to Vcc) and connect the other to a N.O. momentary switch whose second contact is connected to Vcc. Put a pull-down resistor on the pin connected to the switch so the IC will count until the switch is pressed.

    As far as pausing, I like MrChips solution of using the parts you have on-hand. If you're not limited to those parts however, you can use a 555 or a 7474 to toggle the clock output through a transistor. Look up 555 (power) toggle circuit or 7474 T-flip flop circuit.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,357
    Usually, we would prefer students to come up with their own solutions. However, I realize that you are new to digital circuitry and it would be difficult for you to conceive this on your own. So here is the solution:


    [​IMG]

    The two NAND gates of U1, 74LS00 constitute an S/R flip-flop. Read up on this.
    The output of the flip-flop is used to enable/disable the input clock using an AND gate with active-LOW inputs and outputs (read DeMorgan's Theorem).
     
    shawnteo93 likes this.
  9. shawnteo93

    Thread Starter New Member

    Nov 4, 2011
    5
    0
    [​IMG]

    This is what I had done so far.
     
  10. shawnteo93

    Thread Starter New Member

    Nov 4, 2011
    5
    0
    I really appreciated that alot. Yes, I have studied for only one semester on Digital Electronics1.

    And the S/R flip flop you are talking about is it similar to NAND latch ?
     
  11. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    Well, actually this is an SR latch, as you said, not an SR FF. An SR FF would have a clock input too, but it is very similar in operation and circuitry to the latch.
     
  12. Aresden

    New Member

    Nov 11, 2011
    3
    0
    having tis project too ...SP xD
     
  13. Gunbardo

    New Member

    Nov 4, 2012
    1
    0
    Hmm... However, after I applied the above solution to my 60 sec binary counter, the counting process seems to stop after it has finish cycling through 60 sec, how can I implement it such that the counter can continue to count infinitely?
     
Loading...