The Interference of NE555 and CD4017

Discussion in 'The Projects Forum' started by mapleman555, Apr 17, 2015.

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  1. mapleman555

    Thread Starter Member

    Dec 15, 2014
    Yes another 555/4017 question. I'm teaching myself electronics with these two (so far) chips and lots of LEDs. As you know if you've been reading my threads here that I'm on a flashing LED kick and have had a couple of glitches along the way, like making the 555 and 4017 builds work right. I have discovered some thing along the way and would like to get some feedback on it if I can.
    OK, this is what I've heard while researching flashing LEDs with a 555 chip. Most sources say you don't need to connect pin #5 of the 555 to any thing, not even ground. Then recently I heard that you might want to connect it to ground with a very low value capacitor to stabilize it, to protect the chip from outside interference. I tried that and it works, it solved a problem I was having with the 555 builds.
    And now that I am connecting the 555 to a 4017 I was having problems, mainly a very random flash sequence. I just tried the same capacitor I used on the 555 build, a 107.2nf ceramic, on one of my two 555/4017 builds. I connected it to the #12 pin and it took most of the randomness out of it. And, because I wasn't satisfied, I just hooked one, a 23.33nf, up to the #5 pin of the 555 chip of this 555/4017 build and it completely took the randomness out of the flashing LEDs and there is a repeating pattern.
    So to make a short story long, is there some good info on the web or personal experiences here about this problem and what to do about it? I think this is a major discovery that will help me with the rest of my builds/projects.

    Scott Wang : The title "Interference" is too narrow to figure out what's the problem, so I modified it to -- The Interference of NE555 and CD4017.
  2. ScottWang


    Aug 23, 2012
    You didn't show the circuit, even in the another thread is the same.
  3. Papabravo


    Feb 24, 2006
    A 107.2nf capacitor is not likely to exist. That's two too many significant digits.
  4. mapleman555

    Thread Starter Member

    Dec 15, 2014
    I'm going by what my handy little component tested said.

    Didn't have enough time for that today, hopefully in the next couple of days.

    I was just thinking, I have the two 555/4017 projects setting on the desk next to my computer, monitor and broadband modem where I built them. Would they be interfering with the chips?
    Last edited: Apr 17, 2015
  5. ScottWang


    Aug 23, 2012
    Since when I bought the first kit to play with ne555 and cd4017 that it has been over thirty years, and other also play a lot that they designed by myself, even the stepping motor tester, I never have found any problems for that.

    absf likes this.
  6. Reloadron

    Active Member

    Jan 15, 2015
    Yes, the ageless 555 in many of its flavors is a cool chip. I view the 555 timer chip much like a view a few select rifle cartridges that have been around a long, long time. When a certain rifle cartridge has been around and produced over 100 years you can figure it is a good cartridge. When a chip like the 555:

    The 555 went into commercial production around 1972 and here we are in 2015 about 43 years later and they still make the 555. It's like a living dinosaur.

    If you are going to use the 555 for experimentation, which it is great for, you really should get a good data sheet like this one from Texas Instruments which explains the chip in detail and affords some excellent pictorials of the inner workings of the chip. Pin 5 of the 555 is by name the Control Voltage pin:
    Controls the threshold and trigger levels. It determines the pulse width of the output
    5 Voltage I waveform. An external voltage applied to this pin can also be used to modulate the output
    When not used this pin is generally tied to ground through a .01uF capacitor. Depending on the version of the 555 this may or may not be required for operation. Another good practice is the use of a decoupling capacitor between pin 8 Power and pin 1 Ground. Overall it wouldn't hurt to become real familiar with the 555 timer's operation if you plan to use the chip in experimenter circuits.

    The 4017 CMOS Decade Counter Divider is another old classic chip. Again, the Texas Instruments data sheet is very good with a pictorial showing the operation of the chip. The CMOS versions of the 4017 are very susceptible to noise and again good decoupling is a must when using this chip. Strange things begin to happen sometimes, especially when using the chip with LEDs where random counts and errors can easily be seen. Pin #12 which you mention is the Carry Out and I can't see where placing a cap from pin 12 to Ground should do much of anything. Get to know and understand the chip, especially its weaknesses.

    I think you will discover beyond the excellent help the AAC members afford you will discover the data sheet is your new best friend when working with chips like this. :)

  7. Søren

    Senior Member

    Sep 2, 2006
    Decoupling of pin 5 is, by some, used without even questioning why and it depends a lot on the application at hand, whether it's needed - this pin is fairly low impedance (around 3k3), so in many situations you don't need to decouple it.

    When you draw close to max. output current, however, the current draw from the supply line will result in a voltage dip, which will be at pin 5 as well (although a bit less).

    In your application, driving a digital circuit, which have a very short rise time, this will dip the voltage at every clock cycle. When the dips happen, the trigger- and threshold values will dip and the results may be multiple pulses each time the 555 clocks. in a few instances, it will just take off and "never" stop.

    In such cases the decoupling cap is needed (even 1..10nF may sometimes do).
    Reloadron likes this.
  8. MrChips


    Oct 2, 2009
    NE555 is a power hog and creates havoc to the supply lines. I suggest you switch to the CMOS variety such as LMC555 or TLC555.

    In any case, make sure you have a 0.1μF ceramic capacitor and a 10μF electrolytic capacitor across the power rails at the 555 chip.
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    #12 likes this.
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