problem with 555 monostable

Discussion in 'General Electronics Chat' started by pheonix992, Mar 7, 2014.

  1. pheonix992

    Thread Starter New Member

    Jun 23, 2013
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    i making a 555 monostable circuit which its output get high when trigger pin gets low put the problem is that sometimes the output get high without push the button here 's the circuit and the 400 ohm resistor is relay


    [​IMG]

    any help
     
  2. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    The schematic looks correct, except for a local capacitor or 2 on the 555 chip, but that won't cause a false trigger. I can only guess you have an intermittent connection. Have you tried building this with solder?
     
  3. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    it 's already on pcb
    what is the problem of capacitors ?
     
  4. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    a 555 chip can throw glitches in the power supply to itself if it doesn't have a 10 uf and a .1 uf capacitor attached very close to the chip.

    Perhaps try an improved trigger?
     
    Last edited: Mar 8, 2014
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    When will it happened that it has random trigger, when the power up or power on already?

    Could you show your pcb?
     
  6. AfdhalAtiffTan

    Active Member

    Nov 20, 2010
    117
    11
    What kind of switch did you use?
    I was once tricked by having moisture in the switch itself...
     
  7. soda

    Active Member

    Dec 7, 2008
    174
    13
    Hi
    This is a more reliable way to trigger a mono stable
     
  8. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    when the circuit power on it triggers , and when i touch the pin which attached to pin 2
    it triggers


    [​IMG]
     
  9. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    here 's my key

    [​IMG]
     
  10. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    why there is an capacitor between the 2 resistors
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    The schematic looks pretty standard, but the PCB layout is not good.

    T1 on the schematic appears to be transformer, but T1 on the PCB appears to be a TO92 transistor. I don't get it. Are you really bringing 220 mains voltage onto the same PCB as your 555 circuit? Where is the mains transformer?
     
  12. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    This is not a trigger problem at all. First is a start up problem. You can fix that by adding a 10K resistor from the positive supply to pin 4 (reset) and add a 10 uf capacitor from Pin 4 (reset) to ground. That will keep the chip from responding for a tenth of a second when the power starts up.

    If you don't want it to trigger when you stick your finger in the circuit, don't stick your finger in the circuit!
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    I do not see the decoupling capacitors on the power regulator on your PCB:

    [​IMG]

    They should be placed real close to the regulator.

    Also there are no decoupling capacitors near the NE555.
    A 10 μF and a 0.1 μF parallel on the power points (pin 4 and 8) of the NE555 will give a lot more stability.

    Bertus
     
  14. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    If you have the O'scope, maybe you can measure the input and output of 7812, and the P.2 of NE555, you should measure them when the power up.
     
    nerdegutta likes this.
  15. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0

    t1 is transistor and 220out and 12v these are the transformes wire



    the 3 decoupling capacitors you will find it under the regulator



    if you think that it 's not a trigger problem so why it trigger without press the button
     
  16. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Because you stuck your finger in it!
    You triggered the timer with your finger.
     
    Last edited: Mar 8, 2014
  17. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Do you mean C1 and C5 on the PCB?
    They are VERY far away from the regulator.
    C1 is even on the very end of the trace.

    Bertus
     
  18. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    it doesn't trigger when touch it with my finger but when touch it with any metal
    and the problem is that it triggered without touching it with any thing


    they are c1 c3 c5 are filters for power supply
     
  19. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    I made some remarks on the PCB drawing:

    [​IMG]

    Bertus
     
  20. nerdegutta

    Moderator

    Dec 15, 2009
    2,515
    785
    Why don't you upload the Eaglecad schematic file....?
     
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