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CD4017 chip output latching problem

Discussion in 'Electronic Basics' started by [email protected], Dec 9, 2012.

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  1. Guest

    HI all,

    I've been having problems with a timer circuit I'm building. It uses a chain of seven CD4017BE decade counters. The first in the chain gets clock pulses from a 555 running at about 10Hz. I take the last output from this chip (puts out one pulse for every 10 input pulses) and feed it to the input of the next chip where the same thing is done and so on so the pulses get time-divided by 10 at each stage. All's fine up to decade 4, then something oddhappens. Instead of just pulsing, the output goes high and remains high until the next pulse comes along and toggles it back to low, so this stage's output is high for far too long.
    For this prototype I'm using rat's nest on PCB construction and believe I've paid proper attention to grounding and decoupling. Funny thing is, if I transfer the components over to proto-board, the problem disappears. Do these symptoms ring a bell with anyone? Is the 4017 particularly layout-sensitive?
    It's driving me nuts.

  2. Guest

    I should just mention that if you're one of the people that suggested tips on the previous thread in relation to this issue, I have tried them all to know avail. :(
  3. Guest

    Well, I did read that the 555 is a bit of a current hog during switching, so I've used a 100uF electrolytic across pins 1 & 8 of it and hoping that - whilst not optimal - will do. Tomorrow I'll make up another pcb with a different trace layout and see if that cures it.
  4. ehsjr

    ehsjr Guest

    Use 100 _nf_ not 100 _uf_

  5. Jasen Betts

    Jasen Betts Guest

    I second that motion!

    a 100nF polyester or ceramic capacitor will have a much lower resistance than an
    electrolytic capacitor, and that's important for decoupling power
  6. Guest

    Yes, not to mention much lower parasitic inductance to boot! Perhaps a 100uF electrolytic AND a 100nF ceramic in parallel would be the best solution?
    BTW, have now discovered that the inputs to each 4017 stage must have pull-down resistors. Didn't know that; could be the source of the problem. I'm going to carry out the necessary mods and try it out again.
    Thanks, all.
  7. Guest

    Sorry John. I do try to but I'm using the Google Groups interface and it's rubbish. I really shout re-install Agent when I get a minute!
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