555 Timer ... Retrigger with brief reset

Discussion in 'General Electronics Chat' started by lath, Mar 16, 2013.

  1. lath

    Thread Starter New Member

    Mar 16, 2013
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    I'm an amateur having fun with electronics but this has me a little flustered. I'm looking for a pointer...

    I'm been experimenting with 555 in monostable mode. It works...when triggered the output at Pin 3 goes high.

    I'm trying to search for a way to cause this to happen:
    1) Trigger it and pin 3 goes high
    2) Retrigger and a new cycle begins. But pin 3 goes low briefly (like 0.5 seconds or so) and then pin 3 goes high for the duration of the configured cycle.

    How would I cause #2?
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Use the reset pin (4). As long as pin 4 is held low, pin 3 will be low.
     
  3. elec_mech

    Senior Member

    Nov 12, 2008
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    Could you post a schematic of your setup? How are you triggering the 555?

    I'm just a bit confused on what is happening. You trigger the 555, pin 3 goes high for x time then goes low, correct?

    After pin 3 goes low, you should be able to trigger the 555 again with no problems.

    Now, if you're attempting to trigger the 555 while pin 3 is still high, you will run into problems, but we can help overcome them. Is this what you are trying to do?

    If so, the simplest way I can think of at the moment is following Tracecom's advice - add a button to the reset pin (be sure to use a pull-up resistor) and press the button first to reset the 555. This will force the pin 3 to go low and you can trigger the 555 again.

    I've found this site to have a wealth of knowledge on the 555.
     
  4. lath

    Thread Starter New Member

    Mar 16, 2013
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    Yes, elec_mech I'll make a diagram.

    Following tracecom's I've been able to realize the reset in a test by connecting pin 4 to ground.

    Yes, elec_mech I want to trigger the 555 again when pin 3 is still high. From the many online sites I've read about making the 555 retriggerable. This site made it clear to me that "retriggerable" means "extending the output pulse". What I want to do is make that "extended pulse" go low for a brief period just after second trigger.

    But, let me make a diagram to clarify what I'm doing. Thanks!
     
  5. elec_mech

    Senior Member

    Nov 12, 2008
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    Interesting, I learn something new all the time here. I didn't realize there was a way to extend a timing cycle while on - cool.

    Could you list out, step-by-step, what is you want to happen?

    Example:

    1. Press trigger button
    2. Output goes high for x time
    3. Once output goes low again, repeat step 1 for normal operation
    4. If trigger button is pressed while output is high, then:
    5. Turn output low for x time, then:
    6. ???
     
  6. timescope

    Member

    Dec 14, 2011
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    To extend the timing cycle without the output going low, just add an external discharge transistor to pin 7 to discharge the capacitor before it reaches the threshold.

    Timescope
     
  7. lath

    Thread Starter New Member

    Mar 16, 2013
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    This is my first diagram with LTSpice. I thought I would try it out rather than freehand something...

    This scenario is working:
    1) Trigger the piezo
    2) LED goes on for the time period defined by the R5 and C2 (t = 1.1 RC).
    3) The LED goes out
    4) Repeat from #1

    I've been wanting to make this scenario work...basically it shows that the piezo has been retriggered:
    1) Trigger the piezo
    2) LED goes on the time Period, t = 1.1 RC.
    3) Trigger the piezo again within the time period, t=1.1 RC
    4) The LED goes off quickly and back on for the timer period, t = 1.1 RC.

    I've seen how I can do the "reset" of the first time period by connecting pin 4 to ground.

    I've been wondering all of these:
    1) Maybe the 555 can do this in the "retriggerable monostable" mode found here and somehow first ground pin 4 (maybe a second 555 with a transistor switch).
    2) Maybe there is some combination logic gate with a second 555 that could be used to instead turn off the relay I'm considering to add. (But is this getting more complicated than it needs to?)
    3) Or, maybe there is a better "retriggerable monostable" than the 555 that has the functionality I'm looking for.

    Thanks in advance!!!
     
  8. elec_mech

    Senior Member

    Nov 12, 2008
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    I'll have to give this some thought. It gets tricky if you want the LED to turn off for a duration. For instance, you could reset the 555, but it will likely happen so fast that you won't see the LED go off. To ensure this, you'll probably need another 555 as you mentioned. Again, I need time to ponder this. Perhaps a smarter cookie here will have a solution.
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I'm far from a smarter cookie, but ponder this:
    A normally-on (with pullup) npn transistor on the high side of the LED connected to the trigger signal through a diode/cap/resistor The low pulse turns off the tr for a short period. Just haven't fleshed it out yet.
    (It's Sunday) :rolleyes:
     
  10. lath

    Thread Starter New Member

    Mar 16, 2013
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    Thanks elec_mech... Any suggestion appreciated.
     
  11. lath

    Thread Starter New Member

    Mar 16, 2013
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    I think I know what you are suggesting tubeguy...interesting...I'll give that a quick try.
     
  12. lath

    Thread Starter New Member

    Mar 16, 2013
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    I've been working on this for a few weeks and I got this to actually work in this scenario:

    1) Trigger the piezo
    2) LED goes on the time Period, t = 1.1 RC.
    3) Trigger the piezo again within the time period, t=1.1 RC
    4) The LED goes off quickly and back on for the timer period, t = 1.1 RC.

    I left out the piezo trigger in this image to simplify...

    It works, but does this look ok? I was sort of wondering why I had to "decouple" the split signal with the op amps (this was something I picked up watching this http://www.youtube.com/watch?v=ZmYMKASTSO8 at about 14:00 into the vid).

    [​IMG]
     
  13. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
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    Eww . . op-amps. Analog black magic. Sorry, you are now talking well outside my level. :rolleyes: But hey, if it works, it works.

    I liked Tubeguy's suggestion, but in my limited understanding, I was afraid the LED would have to lag all the time behind the 555 output very time the output went high which includes the first time the trigger is tripped. I also wasn't sure how to account for multiple rapid triggers, the fear being the RC circuit may not have enough time to charge/discharge between rapid triggers.

    As an alternative, I think I may have a solution using two 555's, shown below.

    Most importantly, look at the timing diagram and let us know if this is what you are trying to accomplish. The hardest hurdle for me has been "seeing" what it is you want to do.

    R1 and C1 work as an edge-trigger preventing the output from staying on indefinitely in the event the trigger signal stays low.

    When a trigger signal is sent, U1 output goes high and the LED comes on. During this time, the reset pin to U2 is tied to the output of U1 and is now enabled but still off.

    If a trigger signal is sent while the output from U1 is still high, the timing for U1 is reset thanks to Q1. Because U2 is enabled, this second trigger signal trips U2 and its output goes high, turning off the LED through the PNP transistor Q3.

    If a third trigger signal is sent while U2 is high, U2 timing is reset thanks to Q2 and the LED is off for longer. If this is not desired, Q2 and R4 can be removed.

    I have not tested this, but I think it will work . . . :D
     
  14. lath

    Thread Starter New Member

    Mar 16, 2013
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    Thanks elec_mech...I'll experiment with that too! I'll be back...
     
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