False trigger problem in a 555 circuit

Discussion in 'General Electronics Chat' started by vale90, Aug 6, 2015.

  1. vale90

    Thread Starter New Member

    Aug 6, 2015
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    MOD NOTE: Split from a hijacked necropost:

    http://forum.allaboutcircuits.com/threads/false-trigger-in-a-555-based-on-off-circuit.67016/page-2

    Hi all,

    Apologies for opening the issue again but I still have false triggering problem even I've tried your suggestions. It seem better after connecting a cap between 2&6 to ground and changing voltage from 5V to 12V but I still have problems. I have 250 circuits in my hand already assembled :(

    Now have a big headache! They will be used in a hospital with a wired hand terminal but sometimes lighting or even a hand touch triggers. I am sure its somewhere from switch input side.

    I dont care start-ups but false triggers making me crazy.

    Your help will be much appreciated!!!
     
    Last edited by a moderator: Aug 6, 2015
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Used in hospitals? For what, electroshock therapy?
     
    Reloadron likes this.
  3. vale90

    Thread Starter New Member

    Aug 6, 2015
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    No i dont want to kill people :). Its just for lighting controlled from hand terminal (button).
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Post a schematic and a picture of one of the boards. Is there a pattern to the false triggering?
     
  5. vale90

    Thread Starter New Member

    Aug 6, 2015
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    Hi,

    Attached you will find X0001.jpg (original produced) and X0001_revised (which I made revisions shown in Green color).

    It seems it is working much more better now.

    False triggerring was in different ways before:

    - once switch wires not connected (open)
    - once touching to wires by hand
    - once an electrical appliance are turned on/off

    Revised circuit seems much better now but I am still afraid :( X0001.jpg X0001_revised.jpg
     
  6. vale90

    Thread Starter New Member

    Aug 6, 2015
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    And I removed R5 resistor and LED
     
  7. dl324

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    You aren't using a timing capacitor. What is the purpose of the timer?
     
  8. vale90

    Thread Starter New Member

    Aug 6, 2015
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    Toogling relay on each button press
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
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    You're probably getting a spike in your voltage divider when the relay switches and it became less sensitive when you increased the supply voltage to 12V.

    Do you have a decoupling cap on the timer power pin? You might also try one on the control pin.

    Is the connection to B1/B2 a mechanical switch?

    I built a timer circuit that controlled a mechanical relay and I couldn't get it to work because switching the relay affected the timer. I switched to a SSR and put the circuit with the mechanical relay aside...
     
  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    How far are switch contacts from the ckt.? Are you using shielded twisted pair cable? Might reduce R2 & R3 to 2k or even 1K.
     
    ronv likes this.
  11. vale90

    Thread Starter New Member

    Aug 6, 2015
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    Thanks to all, it seems problems solved with the updates shown above.

    Customer is happy with the changes.
     
    Wendy likes this.
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