555, resistor and capacitor

Discussion in 'The Projects Forum' started by nerdegutta, Jan 13, 2010.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
    2,515
    785
    Hi.

    This may be off topic, for that I'm sorry.

    I read quite a bit about the 555 timer. When it comes to monostable mode, what will be best. High value resistor and low value capacitor, or low value resistor and high value capacitor?

    This site says high resistor....
    http://home.cogeco.ca/~rpaisley4/LM555.html

    This site says low resistor...
    http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/index.htm

    Or is it just me, not understanding...?
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Depends what your goal is, accuracy or battery life. That is what they were talking about (I think). If you need low current then you want a big resistor, but then the capacitor drift makes more difference. You want accuracy then a smaller resistance will be your thing.

    Personally I favor large resistors and good capacitors.

    When you are designing something your first goal is to set targets.

    So, what are you trying to do?
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    Generally I use 47k to 1M & 100pF to 100uF, also successfully used 1M & 10,000uF together to see how long of a timing cycle could be achived-2.5h, but do not expect to find many high val caps with verry high leakage resistance.
     
  4. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
    2,515
    785
    Hi. Thanks for reply.

    First of all I'm trying to understand.

    It is for my 320 pcs LED circuit. My circuit does not have to be excatly on the hundreds of a second. Less than 10 minutes.

    But now it makes more sense to me. Thanks again.

    Jens
     
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