555 Circuits re: variable duty cycle

Discussion in 'General Electronics Chat' started by djsfantasi, Apr 11, 2015.

  1. djsfantasi

    Thread Starter AAC Fanatic!

    Apr 11, 2010
    General question. I have an astable multivibrator made from a 555, that pulses once every n seconds (2-3). I wasn't this to trigger a monostable 555 circuit. Where do I connect the interconnection?

    OR, what equation applies when calculating the duty cycle for an astable 555 circuit? Or what link tells me For example, I want the 555 to output a high signal 20% of the time and low 80% of the time.
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    connect pin 3 of the Astable to pin 2 of the monostable,via a capacitor and resistor integrator.
    djsfantasi likes this.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    High time for a monostable = 1.1 x (RC)

    You will have to calculate the duty cycle off-line based on the period of the astable and the high time of the monostable because a monostable is only a high time, not low.
  4. dl324

    Distinguished Member

    Mar 30, 2015
    Cap charges through Ra+Rb and discharges through Rb. Adjust the ratio of Ra and Rb to get the high/low combination you want.
  5. djsfantasi

    Thread Starter AAC Fanatic!

    Apr 11, 2010
    Thanks everyon. I think beween the three answers, I have what I need. If I'm wrong, I'll be Bach...