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I am building a one-shot monostable oscillator using 555 Timer. Trigger pulse is controlled by a pushbutton, and thus can be long. I don't want output pulse to stay high for that long (simulated by V2 and voltage-controlled switch in schematic below). For that reason, I want to use reset pin to reset cycle after,say, 50ms. I simulated circuit but I am not sure how to execute in reality control of reset pin (this is simulated by V3 in schematic).

Image of Schematic:

Waveforms:

Thanks.

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  • \$\begingroup\$ What you need is a non-retriggerable 555 one shot circuit. You can do this fairly easily with the addition of a few logic gates. Does this appeal? \$\endgroup\$ – K H Oct 14 '18 at 0:08
  • \$\begingroup\$ @K H My next goal is to make something similar. Not non-retriggerable, but with time interval between triggers. For example, after it was triggered first time, you would need to wait a minute until you can trigger it again. \$\endgroup\$ – DenR Oct 14 '18 at 1:22
  • \$\begingroup\$ If you want a specific time delay you likely need a second timer, but that is certainly possible. So on an input pulse, you want the output pulse to go high for a certain duration, and not be triggerable for another longer duration. If the input stays high for the entire longer duration (button is held down), do you want it to trigger again or only if the input goes low before then? \$\endgroup\$ – K H Oct 14 '18 at 1:44
  • \$\begingroup\$ @K H, yes, that is what I am looking for. (Maybe I should make this a new question). Regarding your question: It should trigger again only if pushbutton is released first. \$\endgroup\$ – DenR Oct 14 '18 at 14:47
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If I'm understanding correctly (I have to answer, can't "comment" yet), I think what you're looking for would be an RC trigger for your V2 push button, which can also be used as a solution to get rid of switch contact bounce.

Here is an in-depth video describing how to set-up contact debouncing, which again, I think is what you're after, rather than triggering the reset as you have on V3.

https://youtu.be/Nj-Q8FQxHhU

And here is a calculator for helping to determine how long of a pulse you may need for V2. http://www.ladyada.net/library/rccalc.html

Hope this is what you're after.

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  • \$\begingroup\$ I think this is not it. (Though it is useful, I might need to implement it). This is what I am looking for: Pushbutton V2 pulls the trigger low. This pulls the output high. With reset constantly high: If a pushbutton is held for 1s, then the output will be high for 1s. I want the output to be high for 50ms no matter how long the trigger has been low. That is why I want to pull the reset pin low 50ms after trigger has been pulled low. \$\endgroup\$ – DenR Oct 13 '18 at 21:19
  • \$\begingroup\$ Right, if you send a "pulse" from the push button V2, created by the RC factor that is faster than 50ms, your output will be 50ms, no matter how long the push button is held down. \$\endgroup\$ – Jesse Rutherford Oct 13 '18 at 23:52
  • \$\begingroup\$ I understood it after @Phil G pointed it out. I have to mark one answer as solution, so I am choosing yours, since the method was correct, even though it would take much longer for me to see it without Phil G. \$\endgroup\$ – DenR Oct 14 '18 at 1:18
  • \$\begingroup\$ Sorry, I'm on my phone today, so I couldn't easily post a schematic and such. I realized that my answer was a little vague. \$\endgroup\$ – Jesse Rutherford Oct 14 '18 at 1:38
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Use the RC approach suggested above, keep the reset high, let the threshold reset the flip-flop.enter image description here

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