Digital Stopwatch Troubleshooting

Discussion in 'The Projects Forum' started by FerventPiranha, Aug 9, 2013.

  1. FerventPiranha

    Thread Starter New Member

    Apr 30, 2012
    6
    0
    For a final project for a class my partner and I decided on a digital stopwatch of my design. A 555 timer (LM555CN) is generating a clock signal that's fed into 6 cascaded decade counters (74LS390N dual decade IC) with AND gates (actually 74LS00Ns wired as such) truncating the tens digit to 5, and appropriate OR gates (actually 74LS02Ns wired as such) used with the reset inputs. Outputs from the decade counters are fed into BCD/drivers (74LS47N) then through appropriate resistors into seven-segment displays. Fairly normal.

    The issue I'm looking for insight into is the broken counting of the ones digit of seconds. Chips have been swapped around, and all 3 ICs are fine. Ones digit of minutes and hours work fine (tested by feeding clock signal directly into their clock A inputs), and the outputs are wired identically for all 3 ICs. When the clock signal is fed into the seconds ones digit it counts 1, 2, 6 & 7 quickly, 3, 4, 5, 6, 2, 3, 4, 5, 6, then the sequence 2, 3, 4, repeats forever. Can't figure out what's causing it. Pin outs all trace out fine (from what I can tell). The tens digits have not been tested yet, though obviously tens of seconds would never count with the current setup. The circuit works perfectly fine in Multisim.

    Attached is the Multisim file as well as a pictures of the circuit (physical and screenshots for those who need them) all in a zip file. Any help is appreciated. Thank you in advance.
     
  2. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    On the breadboard, all the pin 2 are connected to High. That would make the units S M and H in CLR state and stops counting.

    I simulated the unit and tenth second circuit and it works fine. I dont see any 0.1uF capacitors between the Vcc & Ground of the breadboard. Try to put 4 or 5 of them along the +5 and gnd strip. Also add one 10uF electrolytic cap on the entrance of the +5V supply.

    Attached is the simulation done on proteus.

    Allen
     
  3. FerventPiranha

    Thread Starter New Member

    Apr 30, 2012
    6
    0
    I'll definitely try some bypass capacitors. I'm hoping that'll fix the issue. The rail all the clear pins are connected to is actually a switched rail, through a SPDT on-(on) switch, not a VCC rail. Thank you for the input.
     
  4. FerventPiranha

    Thread Starter New Member

    Apr 30, 2012
    6
    0
    I added the bypass capacitors last night and everything seems like it's working perfectly. Thank you for the advice!
     
Loading...