# 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
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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 AAC Fanatic!

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

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3. ### GopherT AAC Fanatic!

Nov 23, 2012
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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
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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
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878
Thanks everyon. I think beween the three answers, I have what I need. If I'm wrong, I'll be Bach...