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555 timer

  1. Sep 30, 2008 #1
    I need to build a timer(using a 555) with a frequency of about 1KHz and a duty cycle of about 25% to provide my clock signal. So the 555 is obliviously in astable mode. From my engineering data sheet:
    where C is my capacitance value and the two R's my resistance values. This should be a strait forward calculation right?
    then from the above equations
    [tex]R_B=\frac{D 1.44}{C f}[/tex]
    and[tex]R_A=\frac{D 1.44}{C f}\left[\frac{1}{D}-2\right][/tex]
    I get [tex]R_B=36k\Omega[/tex] and [tex]R_A=72k\Omega[/tex]
    This is a capacitance value of [tex]0.01\mu F[/tex]
    Does this look correct?
  2. jcsd
  3. Sep 30, 2008 #2


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    Looks good.

    72 kΩ isn't a standard resistor value, but you could just use two 36k's in series.
  4. Sep 30, 2008 #3


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    Or a potentiometer. You would be able to adjust out the tolerances from the other resistor and capacitor.
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