Help on decade counter

Discussion in 'The Projects Forum' started by harisudharsan, May 21, 2014.

  1. harisudharsan

    Thread Starter New Member

    May 6, 2014
    16
    0
    Hi all,

    I am trying to create a decade counter by using the below schematic. When I switch on the circuit, by default it displays 5 or 7 (mostly 7) on the display. When I connect pin 14 of the counter to the -ve, I expect it to count the next number. But if it is 7, it changes to 5 and vice versa. But does not change after the first time.

    http://www.next.gr/uploads/82-b66141494b.png

    Currently I am giving the clock input manually (you can see a black wire hanging from pin 14). I connect this to the ground to give the clock input.

    I tested the chips individually and it looks to be working fine. I tested 7447 by giving the -ve input on the pin 3(i think) and the seven seg display showed 8 . Then I connected LEDs on the 4 output of the 7490. I looked for different leds to glow for every time i connected pin 14 to ground and that worked too.

    I checked the connections, it looks good. For your expert eyes, do you see any obvious mistakes i have done ?

    Note: I do not have 220 Ω resistor handy. So taking up the risk of burning the components, I am proceeding with building the circuit. I hope not using resistor cannot be the reason for the improper display.


    With Regards,
    S.Sudharsan
     
    Last edited: May 21, 2014
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Put in the resistors. They don't have to be exactly 220Ω.
    What values do you have?
    Anything from 220Ω to 1kΩ will work, with decreasing brightness as you increase the resistance value.

    You cannot clock the counter manually unless you debounce the input.

    Use a 555 timer circuit set to a low frequency such as 1Hz.
     
  3. harisudharsan

    Thread Starter New Member

    May 6, 2014
    16
    0
    I do not have any resistance with me handy. I got to purchase.. But not using the resistance is the reason why the 7 segment is not showing the numbers ?

    If so, why is it showing 5 or 7 alone ??
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the 7490 has a not really feature that is somewhat forgotten these days, if you pull an output up or down, it stays and counts from there. this was used in the days berore the presettable ttl counters were available to preset the count. if you pull too much current or pull it up too high, it stays till counting.
     
    harisudharsan likes this.
  5. harisudharsan

    Thread Starter New Member

    May 6, 2014
    16
    0
    if so, how to fix this problem ?
     
  6. DickCappels

    Moderator

    Aug 21, 2008
    2,662
    633
    You will probably fix the problem by first following MrChips' advice and put in the resistors.

    Its not only a matter of the danger of damaging components. The large current spikes that occur when switching voltage across the diodes without benefit of current limiting, can cause your power supply voltage to dip and that can cause the internal states of the chips to change.

    If you don't put in the resistors, then trying anything else is a waste of time.

    By the way, triggering the clock input by manually grounding it will probably get the counter to advance by more than one count sometimes because the contacts, whether in a switch, button, or just a wire touching another, will bounce of if I may, "stutter" giving you several low-to-high transition every time you close the circuit.

    Below is the Google search for button debounce circuits.
    http://bit.ly/1krdTSF
     
    Last edited: May 22, 2014
  7. harisudharsan

    Thread Starter New Member

    May 6, 2014
    16
    0
    I tried adding the resistors(220Ω).. Still no luck.. But now it gives some symptoms of working. But still it is not displaying a proper number. The display keeps changing for every key press. But it is not showing any readable characters..

    Attached is the circuit. Do you see any obvious errors in my circuit. ?

    (All three pix are same except that they are from different angles for better visibility.)

    Thanks,
    S.Sudharsan
     
  8. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    I simulated your circuit and it worked in proteus.

    Are all your segments connected correctly? What's the part number for your display? Instead of using a wire to send clock to the input did you try using a proper push button switch like in my schematic?

    Though TTL input has a default high, it would be better to put a 10K resistor to Vcc. Try also put a 0.1uF cap on the input to GND for debouncing.

    Allen
     
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  9. harisudharsan

    Thread Starter New Member

    May 6, 2014
    16
    0
    Guys.. Atlast the circuit showed symptoms of working !!!!! Butbit worked for only twice.. I think the culprit is circut debounce. I am working on resolving that..
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Not "I think the culprit is circuit bounce".
    I know it is circuit bounce. I told you in post #2.
     
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