Help! with 74LS93N Counter

Discussion in 'The Projects Forum' started by idmond, Oct 11, 2010.

  1. idmond

    Thread Starter Member

    Oct 5, 2010
    Our Professor Gave us this problem to design a 7-Segment display decoder from scratch, just using AND OR NOT gates.
    After i finished doing all the truth tables and drawing the logic diagram,
    i decided to simulate it first before getting into any hardware stuff.

    so i used my decoder to display numbers from 0 to 9 in decimal and for this i needed a 4-bit binary counter
    (the one that came in my mind was the 74ls93n, since I already had bought one from our local store), but i should test the counter first
    before implementing it in any circuitry to make sure that whenever i encounter errors in the (0 to 9 counter), i should know then that
    they are not from my counter.

    i used Multisim simulator and looked in the 74ls93n datasheet from Texas Instruments for the appropriate pins configurations
    to make it count from (0 to 9).

    Now, after i found every thing i need, i used Multisim and started connecting the outputs of 74ls93n
    to drive 4 LEDs instead of 7-Segment , just to test it first, and ran my design.
    well, every thing just functioned the way i thought and the LEDs started counting from (0000)[0] to (1001)[9].


    the simulation file is in the attachments..

    Now, i took my counter and decided to test it for real this time on my breadboard and connected every thing just the same way i did it in the picture above with the simulator.

    i checked my connections again to make sure that i connected everything right and pressed the switch ON and OFF to simulate the clock pulses for the counter (since i don't have timer or pulser) just the same way you see it in the design above, but the LEDs just went crazy and started counting in a sequence that i couldn't fully understand :confused:
    and had nothing to do with any sequence in any number system.
    sometimes it just repeats the same number and i don't know what exactly went wrong, i checked my connections again and again and again and they were the same ones i did in the simulator and worked there just fine !!!

    so, anyone has any idea why did this happened or why it acted that way (just for the record, i used another counter from different manufacturer and it acted the same way)
  2. SgtWookie


    Jul 17, 2007
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  3. retched

    AAC Fanatic!

    Dec 5, 2009
    Yes, I have to agree. It sounds like switch or contact bouncing.

    Also, I dont see any decoupling capacitors on the IC. That can lead to odd behavior also.
    idmond likes this.
  4. Wendy


    Mar 24, 2008
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  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    The 7400 series of ICs really do need a bypass cap from the VCC to ground as close to the IC as possible. Anything between 0.1 to 0.01 uF is usually fine.
    idmond likes this.
  6. SgtWookie


    Jul 17, 2007
    Along with what Marshallf3 said (0.1uF/100nF poly metal film or ceramic preferred)
    you must also have bypass capacitors as close as possible on the 7805 regulator from IN to GND and OUT to GND; 0.33uF on IN and 0.1uF on OUT, or oscillations are likely to occur.
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    By the way, "switch bounce" is not always "dreaded".
    I used it as a random number generator for gaming.
    idmond likes this.