problem with 555 monostable

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

  1. peorge

    New Member

    Apr 8, 2013
    13
    0
    I asked this yesterday in a quick reply and either I did not submit it or it was deleted for some reason. What is R7, the 47k resistor for?

    Thanks,

    George
     
  2. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    i found the problem
    there is a home device which connected to power with it in the same socket
    and when this device turn on the 555 triggered
    now any help
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,940
    1,428
    Get the mains voltage off the PCB, place the filter capacitors adjacent to the regulator, and add a .1μF capacitor from pin 1 to pin 8 on the 555.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    6,762
    1,047
    I had handled the similar case, but my suggested on #14 didn't get the answer.
     
  5. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    i don't have o'scope but with voltmeter imeasured input = 12.5v
    output= 11.8v
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    6,762
    1,047
    If no waveform then hard to say it's easy to get to the point, but you still can try, you can using a coil with ferrite core that the values as 22uH~50uH in series with the input of 7812, if you doing this and the 555 became normal then it's done, otherwise you have to in series another one with GND of 7812.

    Toroidal Ferrite Core Choke Coil.

    Ferrite Core Coil Inductors.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    6,762
    1,047
    I recheck the circuit, the coil also can be in series with the output of bridge rectifier, and if you need to adding the second coil then you just in series with the GND of the output of bridge rectifier.
     
  8. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    so i don't need any capacitors
     
  9. ScottWang

    Moderator

    Aug 23, 2012
    6,762
    1,047
    The capacitors are for the positive voltage and some over the Vcc as 12V, it means that the pulse could be over 12V.
    If the pulse are negative or low less 0V then the capacitors maybe useless, and during this time the 555 can be triggered, if we using the coil that it can be isolate the pulse to interfering the right side of the coil, and the right side of the coil will keeping in a stable status.

    If you in series the coil as the described on #27 then it will become a LC filter.
     
  10. pheonix992

    Thread Starter New Member

    Jun 23, 2013
    21
    0
    can i use inductor which its shape look like the resistor
    or i must use toroid coil
     
  11. ScottWang

    Moderator

    Aug 23, 2012
    6,762
    1,047
    If you using the inductor as the resistor type, you should care about the current that is the choice at here, and the watt is enough or not, the coil is a choke coil here, against the rush current.

    So you can try and to get some Experience.
     
  12. soda

    Senior Member

    Dec 7, 2008
    174
    13
    Hi

    C4 discharge through R7 to gnd
     
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