Measuring vs calculated output, 555

Discussion in 'General Electronics Chat' started by drbenne, Sep 13, 2014.

  1. drbenne

    Thread Starter Member

    Jul 30, 2013
    Hi guys,

    I'm using a PC O-scope to measure the output of a 555 timer in astable mode. The 555 I'm using is the NE555P.

    Basically the discrepancy I'm having is calculated vs. measured. I know it's not going to be perfect but it seems what I'm observing may be a bit off.

    R1 = 2153 ohms
    R2 = 4658 ohms
    C1 = 1 uF
    C2 = .01 uF
    Vcc = 5V

    So I'm calculating a frequency of ~125 Hz, P = 8 ms

    What I'm observing is f = 96 Hz, P = 10.4 ms

    I'm not sure what I'm doing wrong or if this is normal and I'm just being picky. I measured the resistors and capacitors to make sure they're correct. I tried the output with load and no load and there was a difference in period of about .6 ms (according to my O-scope).

    I attached my circuit and O-scope display



    20140913_172132.jpg 555.png
    Last edited: Sep 13, 2014
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    What are the capacitor and resistor tolerances? Have you actually measured them? What are the accuracy of the 555 thresholds per the spec sheet? Do you believe any of these could affect the frequency/period of the circuit you are using?
  3. absf

    Senior Member

    Dec 29, 2010
    Since the calculated freq is higher than the actual, I'd say the 1uF electrolyte capacitor is the problem. E. capacitors normally have higher leakage and tolerence than the ceramic/mylar caps. If possible, try to use 2x 470nF mylar caps in parallel in place of the 1uF and see whether the frequency improves.

    The 2 resistors 2.2K and 4.7K are very close to the calculated so I think they are just fine. If after doing the the above and it still gives you unsatisfactory freq then the next step is to add pot or preset resistors to R1 or R2.

    I simualted your circuit in proteus and get the same result as you calculated.

    meas vs calculated.PNG

    Last edited: Sep 13, 2014
  4. MrChips


    Oct 2, 2009
    You are doing nothing wrong. Don't expect the observed times to match theoretical calculations perfectly.
  5. drbenne

    Thread Starter Member

    Jul 30, 2013
    I measured all of them and what I listed is what my meter read (Fluke 117). I haven't gone too in depth of the data sheet yet but I plan on doing so while I work out the basic functions.

    Unfortunately I don't have any 470 nF mylars on hand. The biggest I have are 100 nF. I did simulate in multisim and got pretty close to my calculated results also. At this point I'm not looking for a particular frequency, I'm just doing observations with the components I have to get a better understanding of what to expect when designing circuits.

    Yea, that's what I was expecting but wasn't sure on how much deviation would occur due to tolerances and to what accuracy my meter was telling me when measuring my components. I also did another test using the same resistors but using a .022 uF and got a bigger discrepancy. I calculated a frequency of 5.68 kHz and measured around 4 kHz.
  6. MrChips


    Oct 2, 2009
    Remember that simulation would be based on theory and would be expected to be close to your calculations. If your observations are within 20% you're good to go. Don't sweat it.
  7. drbenne

    Thread Starter Member

    Jul 30, 2013
    I'm not sweating it too much. I just thought it would be closer, no worries.

    I guess the reason I'm scrutinizing so much is that I'm using this PC O-scope on a loan from my boss and I finally got it working. I'd rather have a standalone to work with as that's what I learned on and this PC version isn't too great from what I can tell.
  8. Sensacell

    Senior Member

    Jun 19, 2012
    Knowing where to expect accuracy and where not to are very important parts of engineering.
    Some things are deliciously precise and predictable, while others vary by orders of magnitude.
    When I was a beginner, I was often terribly frustrated by my calculations not agreeing with reality, until I learned that reality is very murky subject.
    drbenne likes this.
  9. vk6zgo

    Active Member

    Jul 21, 2012
    If you want to check the timebase accuracy of your PC 'scope,just loop the earth clip & probe tip together & rest it on the outside of a power flex.----Don't probe the mains directly!!
    You have a 60Hz supply,so the period should be around 16.7 milliseconds.

    Another way if your DMM has a frequency measurement range,is to check the output frequency of the 555.

    From memory,555s have a "sweet spot" at mid audio frequencies where the given formula works.
    At the upper & lower extremities of their operating range the formula becomes less accurate.
    drbenne likes this.