Help Please. 555 accuracy

Discussion in 'The Projects Forum' started by Tim Saunders, Apr 29, 2009.

  1. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    Hello first time poster here and I'd appreciate any help.

    The Situation -
    I've built a basic servo controller with these components:
    -Regulated 5v power
    -A capacitor before and after the regulator (electrolytic and ceramic)
    -One NE555 timer
    -A timing capacitor (ceramic)
    -A diode (for draining timing capacitor)
    -I couple resistors and a pot to select pulse width
    I have a scope so I was able to set up the pulse range from 1ms to 2ms which is what is needed.

    The Problem -
    Jitter is the problem. The pulse width varies just enough to make the servo jitter, so I need to have a more accurate pulse width. Any idea's on improving the accuracy of the pulse width?

    NOTE: The jittering seems to oscillate, this is reflected in the scope reading as well. Also the servo and controller are connected to the same 6v DC source.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Welcome to AAC!!:)

    How are you controlling any stray capacitance in parallel with your timing capacitor? Also, what is the value of your timing capacitor?
     
  3. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    Thanks for the warm welcome. The lessons on this site taught me most of what I know about electronics, it is very usefull.

    The timing capacitor 22nf
    Ok, stray capacitance. That is when you have capacitance between two wires in your circuits right? I don't have a method of eliminating it right now, how would this be done? This would explain why connecting the scope minimizes the oscillating.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    22nF should be big enough for stable operation. Still, if this is on a breadboard I might try going with a bigger timing cap and a smaller timing resistor. Stray capacitance would have less effect then, being a smaller percent of the total timing capacitance.

    Try minimizing the parallel run distance of any wires or traces connecting to the timing capacitor. You might also try beefing up your ground pathways.
     
  5. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    I'll try your suggestions. Thank you so much!
     
  6. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    I tried your suggestions, they worked! I used a 1μf electrolytic capacitor instead of that .022μf ceramic and it worked like a charm! No more jitter. Since I had to adjust my resistors I realized the range of the servo is actually from .4ms to 2.1ms (cheap servo), which gives me about 80° more motion. Thanks again.
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Try powering the 555 circuit and the servo off of two separate power supplies/battery packs, just to see if that could be the problem. Unless you have a "low dropout" 5V regulator, it won't regulate from a 6V source.

    Ken
     
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