555 timer help needed

Discussion in 'The Projects Forum' started by mistercheif, Jan 3, 2009.

  1. mistercheif

    Thread Starter New Member

    Jan 3, 2009
    2
    0
    Hi all, I have just recently got into experimenting with electronics and have very limited knowledge so if this is a really dumb question I apologise in advance.

    I have built a small timer circuit on breadboard, the purpose is for a 555 timer to recieve a trigger pulse which then causes a relay to close. I have it working fine, but now I want to have the trigger to take priority so that if a new trigger is recieved the 555 finishes the current cycle (opens the relay) early and starts a new one.

    The circuit here:

    http://www.doctronics.co.uk/555.htm#Images_swf/p555_retriggerable

    is roughly what I have based my circuit on, using a BC557 to allow the retriggering, but this does not produce the desired result, in that it does not cause the relay to open before initiating a new cycle so the result is that the relay is closed for the added duration.

    So say my pulse is like this " ___ " and my trigger is this "."

    So I send three triggers, each after a pulse cycle is complete, I get .___.___.___

    But if I send three triggers, and 2 of these are before the cycle is complete I get ._______.__ But what I actually wanted was .__.__.____

    I hope that makes sense and forgive my crude diagram, but if this is possible and someone could help I'd appreciate it. I am just in the breadboard stage with a very limited grasp of these concepts so if you don't mind explaining in simpleton terms that would be great :)
     
  2. duffy

    Active Member

    Dec 29, 2008
    44
    0
    You need to tell us the minimum "off" period of the relay. You are actually looking at two timing periods, one when the relay is on, another when it is off.

    If the off time is comparable to the width of the triggering pulse, using a logic gate or a couple of diodes or something to lock out the load for the duration of the trigger pulse might be an easy way to do it.
     
  3. mistercheif

    Thread Starter New Member

    Jan 3, 2009
    2
    0
    Ok thanks, the off period needs only to be slightly longer than the trigger pulse (which is about 20ms), if possible I would like to keep it as simple as possible so the diode route may be the best option, but I have no clue how to achieve it.

    As an example of what I am trying to do, lets say my 555 cycle duration is set at 700ms duration, and I want the relay to be on for 700ms, then 2 of 100ms then 1 of 700ms then 3 of 50ms. I am using a normally closed reed relay with a response time of about 2ms.

    Hope this explains it better, thanks for your help.
     
  4. duffy

    Active Member

    Dec 29, 2008
    44
    0
    Now you've changed the "on" period from something that was (apparently) fixed to something that is variable. It would help a lot if you would explain what you are using it for.
     
  5. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    This should do http://That.Homepage.dk/PDF/555_Retrig+.pdf

    D1, R2 and C2 resets the 555 (thereby releasing the relay) via pin 4 for a short time with each retrig. pulse.

    The rest of the circuit should be what you have allready, so I didn't draw that.
     
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