555 bistable (LTspice)

Discussion in 'General Electronics Chat' started by TO2, Dec 29, 2010.

  1. TO2

    Thread Starter Member

    Jun 19, 2009
    26
    0
    Hello,

    I'm trying to simulate the 555 timer bistable circuit on Multisim and LTSpice. I may have done something wrong, because I'm not getting the expected behavior.
    When Reset and Trigger is pulled high I expect the output to be low, but it's not.

    Schematic:

    [​IMG]

    (LTSpice schematic file include below.)

    This is the behavior that I expect (java applet):
    Code ( (Unknown Language)):
    1. [URL]http://www.falstad.com/circuit/#%24+1+5.0E-6+10.20027730826997+46+5.0+43%0Av+816+576+816+304+0+0+40.0+5.0+0.0+0.0+0.5%0A165+656+336+672+336+2+0.0%0Aw+720+304+816+304+0%0Ar+848+368+912+368+0+10000.0%0Ar+624+432+560+432+0+10000.0%0Aw+624+432+656+432+0%0Aw+912+368+912+304+0%0Aw+912+304+816+304+0%0Aw+560+432+560+304+0%0Aw+560+304+720+304+0%0Aw+848+368+848+352+0%0Aw+816+576+560+576+0%0Aw+560+576+560+528+0%0Aw+816+576+976+576+0%0Aw+976+576+976+352+0%0Aw+656+464+656+576+0%0Aw+656+576+560+576+0%0Ar+848+400+896+400+0+100.0%0Aw+784+400+848+400+0%0Aw+896+400+944+400+0%0Aw+784+368+848+368+0%0As+944+320+976+320+0+1+true%0As+560+496+608+496+0+1+true%0Aw+560+528+560+496+0%0Aw+624+432+624+496+0%0Aw+624+496+608+496+0%0Aw+976+352+976+320+0%0Aw+944+400+944+576+0%0Aw+944+576+976+576+0%0Aw+848+352+848+320+0%0Aw+848+320+944+320+0%0Ao+20+64+0+39+5.0+9.765625E-5+0+-1%0Ao+5+64+0+39+5.0+9.765625E-5+0+-1%0Ao+18+64+0+39+7.62939453125E-5+9.765625E-5+0+-1%0A[/URL]
    2. Scope 1: Reset
    3. Scope 2: Trigger
    4. Scope 3: Output
    5.  
    Please shed some light :p
     
    Last edited: Dec 29, 2010
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,792
    946
    Try it with a small value capacitor on pin 6 and ground. a single resistor from pin 6 to 7. another resistor from pin 7 to +5 V. Finally tie the trigger pin(3) to pin 6 and fire it up.
     
  3. TO2

    Thread Starter Member

    Jun 19, 2009
    26
    0
    That's for a astable circuit. The bistable circuit should work like a flip-flop.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,792
    946
    Sorry, my mistake.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,792
    946
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Keep in mind that the 555 timer that comes with LTSpice is an "idealized" timer that you can't buy. I have a more realistic approximation of a bjt (transistorized) LM555 timer at the house; I've uploaded it here before.
     
    TO2 likes this.
  8. TO2

    Thread Starter Member

    Jun 19, 2009
    26
    0
    Yes, I also think it's because of the model.

    But the model posted by you doesn't work for me as well.
    (If you mean the model from this post: http://forum.allaboutcircuits.com/showpost.php?p=229723&postcount=14)

    I have used the LTSpice example NE555.asc (in the example folder) and have modified the inputs accordingly to the way that I want to have.

    And it works,
    Reset high, Trigger high -> Output low.
    Output low is in the mV range, in the previous cases the output is above 3 Volt, that's not low (using 5V for Vcc).
     
    Last edited: Dec 30, 2010
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    With the connections you have, the output can be in either state. It may come up in a consistent state when you power it up, though.
    I ran a sim with the NE555 model provided by LTspice. It originally came up with the output high. When I added Startup (see attachment) to the .tran directive, the output came up low.
     
    TO2 likes this.
  10. TO2

    Thread Starter Member

    Jun 19, 2009
    26
    0
    Thank you. It also works for me.

    So I should always use the Startup option of the .tran directive then?
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I almost never do. It all depends on the circuit you are simulating.
     
  12. TO2

    Thread Starter Member

    Jun 19, 2009
    26
    0
    Oke. Thank you.
    Also thanks to everyone for the help.

    Problem solved.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The Startup option is a good one to use, particularly in simulations where ambiguity may otherwise result from having voltages applied from time zero.
     
    TO2 likes this.
Loading...