One shot, latching, variable pulse

Discussion in 'General Electronics Chat' started by mcgski, May 22, 2009.

  1. mcgski

    Thread Starter Member

    May 6, 2009
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    OK, I tried searching the forum but came up empty. Still learning so forgive me if I'm asking a question that is straight forward. I've also done a bunch of reading on 555/556 timers and flip flops.

    I'm trying to take a digital input trigger (5v), create a one shot timed pulse that lasts say 1.5 seconds, and prevent the trigger from generating any other pulses until a manual reset occurs. The pulse will drive a relay coil. I'd also like to play around with the pulse timing to tune the range from .5-2 seconds. Once I figure it out I can use the same design to gate the trigger source for a period of time, say 10 seconds, to prevent it from sending the intial trigger.

    I'm thinking there's a 555/556 timer in there somewhere with a flip flop to do some latching that's reset with a button. I'm also thinking a variable resistor can give me some adjustment on the timing. I'm just drawing a blank on how to hook these up. I'm also wondering if I should be using different components or there's something out there that will do the job for me in one IC.

    Any help would be appreciated.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Where does your initial trigger pulse come from? What is the polarity, and how wide is it?
    Are you planning on running the relay and the oneshot on 5V also?
     
  3. mcgski

    Thread Starter Member

    May 6, 2009
    13
    0
    The pulse is sourced from a buffer chip that's connected to a PIR sensor. There's a lot of bouncing there and I only want the relay to open for the duration of the pulse the first time the sensor's triggered AFTER the sensor has been turned on with a delay (10 s). Yes, everything else can be at the 5v.
     
    Last edited: May 22, 2009
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Is the bouncing duration shorter than the width of the oneshot pulse? Does the input trigger start at 0V, and then transition to 5V?
    If you could draw a timing diagram of what you want, it would help. Include the bounce in your diagram.
    I don't understand the 1.5 secs and 10 secs requirements.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Have you read this?

    555 Monostable

    Just some ideas, they don't necessarily apply...

    555 Power up relay

    [​IMG]

    Debounce circuits

    [​IMG]
     
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Sounds like you want a initial pulse from your sensor, to trigger a oneshot keep it's duration "on" time for around 1.5 sec. then shutdown, and not pick up any more sensor inputs until you manually reset it.

    Logic diagram could be, ...

    [RS latch] connect to 1 input of [AND gate] input from sensor to 2nd input of [AND] gate. output of [AND] gate connect to first [oneshot] short pulse output connects back to [RS latch] to flip it's state. output of same [AND] gate feeds a signal also to a second [oneshot] with delay off time wich outputs to a load (relay)...

    something like that..
     
  7. mcgski

    Thread Starter Member

    May 6, 2009
    13
    0
    Ron, attached is a timing diagram (best I could do with pencil and paper).

    Bill, thanks for the links. I think I've read those. They're close but not sure they apply.

    hobbyist, Your right on with simplifying the description. The attached timing diagram should also show a delay in picking up another sensor input for minimum of 10 seconds. The sensor is likely a PIR and would start going off once I reset it so I need some delay.

    Thanks again for your help.
     
  8. hobbyist

    Distinguished Member

    Aug 10, 2008
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    You could interface a timie delay circuit at the input of the RS latch, that could be adjusted in the amount of delay required before the RS latch is set back to it's ready state for another input.
     
    Last edited: May 22, 2009
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Use a second relay, also driven by the same pulse as relay1, but the second relay has a "holding contact" that keeps itself on, and at the same time disables the 555.

    This kind of circuit was very common in the old days on idustrial contactors etc before PLC's started taking over.
     
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