Retriggerable 555 circuit

Discussion in 'The Projects Forum' started by KMoffett, Feb 1, 2010.

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  1. KMoffett

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    Dec 19, 2007
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    I was putzing with a project where I wanted to retrigger a 555 monostable. Searches almost invariable came up with "555's are not retriggable". I know about the 74123 and the CD4538, but had to try. So, I started thinking about what you could do to make one retriggerable. In the attached, the two circuits on the left are what I came up with that worked. After a little later googling I found:http://www.doctronics.co.uk/555.htm#retriggering. Their circuit is in my lower-right. Since I wanted a positive-transition trigger, as in my first two circuits, I modified their circuit to get the one in the upper-right.

    I don't know if I'm reinventing a wheel, but thought I'd post them.

    ken
     
  2. BMorse

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    Sep 26, 2009
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    I have done something similar for someone on here that needed a retrigerable monostable timer setup...... a lot simple than what you have done...

    here is my setup, with a debounce circuit for the input for re triggering...


    555 Trigger.png

    Monostable 555 timer 2.png
     
  3. KMoffett

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    Guess I may be a little thick this morning. I don't see how the circuit causes the 555 to be "retriggered", extending the monostable period, with each new input on the trigger pin. Once the trigger input is pulsed, it doesn't do anything until the monostable period ends.

    ken
     
  4. BMorse

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    sorry, I didn't notice that you are retrigerring before the cycle ends, and extending the cycle period, my circuit will just run through 1 cycle and then it can be re triggered....My bad......
     
  5. Wendy

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    Mar 24, 2008
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    Retriggerable designs can be used for missing pulse detection. I'm still trying to remember how Forrest Mimms did it.
     
  6. KMoffett

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    I think the monostable period would just be set to slightly longer than the pulse period, so it would time out if a pulse were missed.

    Ken
     
  7. Wendy

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    I knew that, I'm referring to his retriggerable 555 circuit. If I ever see it again I'll have it nailed.

    Of the 4 you showed I think I like the 2 diodes best. Looking at it I think you can get rid of D1 and R3 altogether though. This will leave D2 back biased when it is idle, effectively acting as a switch that is turned off. If I hadn't seen your design I wouldn't have thought of it.

    [​IMG]

    Cool, another one for my cookbook.

    The problem I have in general with transistors grounding the capacitors is it is possible the transistor can be switch in the linear mode (as opposed to saturation). If this happens the results will be erratic. The ideal solution is some way of keeping the 555 Schmitt Trigger input properties while doing the monostable, something I haven't figured out.
     
    Last edited: Feb 1, 2010
  8. KMoffett

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    Yup! I love it....very minimalist. Looks close to the doctronics one, but for positive triggers.

    I think the linear region issue would be an easily managed problem, and only a problem with fast signals.

    ken
     
    Last edited: Feb 1, 2010
  9. Wendy

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    I may need to return the formula back to T=1.1RC, but I think if you retrigger it the voltage on C1 will go down to 0.6V. I adjusted the formula to compensate.
     
  10. KMoffett

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    So, the monostable's retriggered extension period is slightly shorter than the monostable's triggered period.

    Ken
     
  11. Wendy

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    Thinking about it, why bother with Pin 7? This one I sketched up quite a while back. This is truly minimalist.

    [​IMG]

    A similar approach can turn off an oscillator while leaving pin 3 high, which is sometimes a requirement.
     
  12. KMoffett

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    Is there any problem with the trigger slowly drifting up with the voltage across C1, as opposed to the usual short High-low-high pulse?

    Ken

    On second thought, since its based on a 1/3Vcc comparator, it shouldn't.

    Now that's elegent!
     
    Last edited: Feb 1, 2010
  13. KMoffett

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    Bill,

    A 3AM thought:
    Without a retrigger pulse...The monostable cycle is initiated by the transistor shorting the cap and pulling the trigger low. The capacitor then charges from ~0V to 2/3Vcc and is then discharged by the internal transistor. If the trigger is tied directly to the capacitor, won't it initiate a new cycle as soon as the cap voltages drops down below 1/3Vcc?

    Ken

    Later... I bread boarded it. The cap isn't discharged at the end of the monostable, so it worked. Again Bill, it's elegant in its simplicity.
     
    Last edited: Feb 2, 2010
  14. Hop

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    Feb 14, 2013
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    Try this link to see Mim's solution:
    <snip>

    Edit: Link to copyrighted material removed by AAC moderator.
     
    Last edited by a moderator: Oct 1, 2014
  15. BMorse

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    I am sure that after seven years, Bill has this all figured out already, lol.
    And I am sure that Forest does not like his work scanned and re distributed without his consent.
     
  16. Wendy

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    Mar 24, 2008
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    Welcome to AAC.

    Congratulations, you have practiced the arcane art of necromancy, the revival of a long dead thread. Likely the OP (Original Poster) has solved his problem in the years that has passed, or thrown it away, or something.
     
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