555 timer accuracy

Discussion in 'The Projects Forum' started by citizencoolguy, Feb 7, 2011.

  1. citizencoolguy

    Thread Starter New Member

    Jan 17, 2011
    12
    0
    Hello,
    I'm having a problem with the accuracy of the 555 timer chip. page 3 of the datasheet gives the initial accuracy as 2.25% but that's with a 0.1uF capacitor whereas I'm using something around 5000pF. I've tested the chip with 1000pF and get results that are almost 12% off and then when I use the 5000pF its much more reasonable, something like 3%, so I figure that the initial accuracy is based heavily off of the chips internal capacitance, but the datasheet doesn't mention what that might be and none of the manufacturers seem to know since it's such an old part. I was just wondering if anyone had any idea as to how that accuracy changes over different capacitor values.

    Thanks
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,692
    903
    Do you know the accuracy of your resistors and capacitors?

    John
     
  3. citizencoolguy

    Thread Starter New Member

    Jan 17, 2011
    12
    0
    The capacitor is 1% and the resistors are 0.1%. I need the frequency to be about 57kHz +/- 5% and with those values in the worst case scenarios I should be within about 1.5%. so that gives 3.5% left to the initial accuracy of 555 chip and I want to make sure that the chip will be inside that range. I realize that a 555 timer probably isn't the best choice in this situation (I've also toyed with 4060 timers) but the circuit needs to be cheap so this is what I have to work with :/
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    1,222
    I remeber from working with RS232. That 57600 is a common bitrate for RS232. So if you use a 1.8432 MHz simple crystal oscillator made by some invertes. And divide this by 32. You will get 57600 Hz. I think that will be simpler and cheaper than using 1% caps and 0.1% resistors, amd also the price of a 4060 is about the same as 555 circuit. And by using a 4060 you can connect the crystal directly to the the 4060. Se data sheet for more info
     
    Last edited: Feb 7, 2011
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