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555 and 4017B

Discussion in 'Electronic Basics' started by Adrian, Jan 27, 2004.

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  1. Adrian

    Adrian Guest

    Hey, for my school work i am using a 555 timer in astable operation to
    provide teh pulse for a 4017B decade counter. I have it all working
    properly however when i move my hand whithin about 15cm of the circuit
    it pulses much faster - its not really a huge problem for my project
    but im just curious to know what casues it. so if anyone knows i would
    be interestd....thanks
     
  2. Is the circuit powered by battery or by an AC powered supply? This
    can have an effect on how much AC hum is riding on your circuit.

    I am suspicious that you may have left some input on the 4017
    unconnected to anything, which allows that input to pick up any
    electric field as a signal, altering the operation. Go over your
    circuit and account for each pin on the chip that is an input. 555
    also sometimes produce a glitchy output ( a very fast pulse preceding
    the normal logic output. This glitch can be right on the threshold of
    what it takes to clock the next chip, so that subtle influences can
    make a difference. Put a 10 k resistor between the 555 output and the
    4017 clock input to filter any glitch out, using the capacitance of
    the input to complete the low pass filter.
     
  3. AC input for the 555??? Hmmm! John, you don't randomly shout nonsense
    from my reading your many posts, so I know you have something in mind.
    I am very curious what kind of circuits you might think would couple AC
    to a 555.

    Regarding the poster's question...

    When I first started learning about the 555, my first IC actually, I
    learned that some resistor/cap combinations on the 555 would prove
    unreliable and I could get strange behaviors including changing
    frequency, etc. It has been a while since I studied these in detail,
    but I recall this happening when I chose very low resistance for
    setting the timing. I believe both resistors had to be into the
    thousands of Ohms and then I calculated the required cap based on
    the equations in the spec sheets from the manufacturer. I am therefore
    curious what frequency and what resistance/cap values you used to
    get that frequency.

    Dominic-Luc Webb
     
  4. One other thing, I have also had such problems when trying to drive
    induction type devices with 555/4017 circuits without diodes between.
    I don't suppose you are trying to run a motor, etc?

    Dominic
     
  5. (snip)

    I said, "AC powered supply". That is a power supply plugged into an
    AC outlet. This could be a DC wall wart or a bench supply. But if it
    is one that has no ground pin, it is possible for there to be quite a
    bit if capacitively coupled AC riding on both DC rails. If you don't
    believe me, plug in any DC output wall wart that has a line frequency
    transformer and measure the AC voltage between either of its output
    rails and the ground socket of the receptacle. Even if the supply has
    the DC grounded on one side, the circuit has a current path for
    capacitive current from other external line powered fields, especially
    from fluorescent lights. Circuits powered from isolated batteries
    have less problem with such capacitive currents because they have no
    place to go.
     
  6. Adrian

    Adrian Guest

    hey, yea im using a DC power supply (9V) and the resistors im using are

    R1 = 56 ohm
    R2 = 56 ohm

    and the capasitor is 2.2 uf
     
  7. Those low resistor values are working the grounding switch pretty
    hard, but they are way to low in impedance to be directly affected by
    the electric field changes caused by waving at the circuit. Have you
    looked at the output of the 555 to see if it is changing frequency or
    only at the 4017. At this point I still think a floating input on the
    4017 is the most likely culprit and the 555 output glitch is the
    second most likely.
     
  8. Si Ballenger

    Si Ballenger Guest

    You probably need to tie the clock line to ground on the chip
    with a 10k or 22k resistor to keep it from floating. I made
    something similar (below) and it would start switching when the
    cat got within a couple of inches until I added the resistor.

    http://www.geocities.com/zoomkat/RSswitcher.htm
     
  9. JeffM

    JeffM Guest

    using a 555...to provide [the] pulse for a 4017B
    I was thinking the Control/Modulation pin of the 555.
    Got it bypassed with a capacitor?
     

  10. I for sure had problems with some of my 555 circuits with
    somewhat higher resistances and these problems were resolved
    once I switched to values in the thousands of ohms. Also, as I
    recall, is it not so that these two resistors should never have
    same value? I think I saw this in an application sheet. There
    is also the famous "555 Timer Cookbook" by Walter Jung one
    could consult. I recall trying to keep them about 2 fold different
    values or more. I am now curious what frequency the poster is trying
    to get. Perhaps there are some preferable values that could be used?
    Perhaps the equation can be recalculated for resistances of at least
    a couple thousand to yield a new cap value giving same frequency?

    I think there was even a post here some years back in which this
    problem was even discussed by someone who seemed to know the
    internals of these timers pretty well.

    Dominic-Luc Webb
     
  11. Dominic-Luc Webb wrote:
    (snip)
    (snip)

    There is no such requirement. The resistors have to be low enough to
    provide bias current for the differential amplifiers that measure the
    capacitor voltage and low enough to not overload the grounding
    (discharge) switch. That is all.
     
  12. Adrian,

    Are you trying to get a frequency of f = 1.44 / [(Ra + 2Rb)*C] = 3896 Hz?

    Calculating a little, I think for such frequency I have used
    (successfully):

    R1 = 2 KOhm
    R2 = 18 KOhm
    C = 10 nF (note this is nanoF)

    One of my R's was a 25 turtrim potentiometer, maybe a 47 KOhm.

    Dominic-Luc Webb
     
  13. Balaji

    Balaji Guest

    I think there is something wrong with that notion. You see, the values
    of these resistances determine:

    a. the time taken to charge the capacitor up to the threshold and back
    to the lower limit. The time period of a 555 timer ckt connected as an
    astable is T = ln2 * (R1 + 2*R2)*C. In this case, we get a frequency
    of 3.9KHz. When you design a ckt with a 555 make sure you choose R's
    and C's such that your frequency does not exceed 100KHz. (Try to keep
    within 80KHz) If you need higher frequency, use the pin compatible
    7555 IC. (ln2 = 0.693 approx)

    b. the amount of current flowing into the pin 7 when the output goes
    low (or pin 7 starts sinking huge currents) This should be kept
    generally to a minimum. A value of about 100 - 150mA should be about
    the maximum allowed (check datasheets here ... i don't remember that)
    since the transistor inside then goes into saturation. How to find if
    it is limited in a design - (Vcc - 0.2)/R1, where R1 is the resistor
    connecting the Vcc terminal to pin 7. Check up the datasheets first,
    and then see if you have exceeded that.

    c. differential input to the comparators consume some current. Check
    that from the datasheet. It should be around 50nA.
    Now, that factor of 2 is because, if you have a CRO and if you observe
    the output of the timer, it remains high for more time than it remains
    low. In fact, the time it remains high is given as TH = ln2 * (R1 +
    R2) * C, and goes low for TL = ln2 * R2 * C. Now if we have R2 ~ 2*R1,
    we get TH ~ 2.1 * R1 * C and TL ~ 1.4 * R1 * C.

    See that if this ratio between R1 and R2 is increase further, the duty
    cycle (fraction of time period for which) the output is high
    increases. We generally prefer 50% duty cycle, but here it does not
    matter, since no time bound operations are done in the rest of the
    ckt. during the stable states of the timer. Besides, since 4017B is a
    CMOS IC, it does not consume power during the time the input is in any
    stable state - it consumes power only during transitions in logic
    levels.
    That's partially correct and others have posted pretty good values of
    resistances and capacitances. Make sure the capacitor is a ceramic
    one. At any cost do not use a electrolytic one for this purpose.
    Tantalum may do fine - check that up - your own risk. (Big deal!?)
    The problem being faced here is mostly that of what is called EMI
    emission. Although the effect of EMI is not generally so strong, for
    circuits powered with such a high voltage, it may be powerful. It will
    surely be powerful if

    a. you have used an electrolytic capacitor somewhere in the circuit,
    where it should not be. An electrolytic capacitor is allowed at the
    power supply to regulate and get a clean source.

    b. you or some part of your body is touching or is very close to a
    mains socket. Even a power source of 120V 60Hz is enough to cause
    fluctuations due to EMI emissions.

    c. Finally, the layout and design of the PCB is poor. To learn
    designing good EMI emission protection in PCBs, look up google or get
    this free book from Cypress Microsystems - "Perfect Timing". This
    deals with many aspects of timing and board layout and EMI.

    In fact this principle of EMI emissions is what is used to detect the
    presence of a working socket without touching it or detecting a
    concealed wire inside a wall. Many such circuits are quite popularly
    known.

    -Balaji
     
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