Monostable 555 not timing out correctly

Discussion in 'The Projects Forum' started by GunsNTulips, Nov 24, 2013.

  1. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    I'm trying to set up a simple monostable 555 timer to give me a voltage for a specific time after a trigger is pressed. This is actually meant as a hack to those musical christmas cards, so that I can press a button and play the song through. To test the circuit out on a breadboard I am using an LED with a 100 Ohm resistor off of 3.

    I am following the diagram on Wikipedia precisely. I have a 1 microfarad capacitor (PMG105 2K) and a 1 MOhm resistor. Off of terminal 5 I have a 10 nF (103Z) capacitor to ground.

    This should give me a time out of about 1 second. Instead, the timeout is inconsistent. It's never just one second, and it increases each time the button is pressed often, not timing out at all.
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Could you post your schematic?
     
  3. tracecom

    Senior Member

    Apr 16, 2010
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    You know that the trigger should be a negative going pulse...right? And do you have the switch debounced?
     
  4. GunsNTulips

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    Nov 22, 2013
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  5. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    The switch button works properly. It just triggers while it is pressed. The trigger goes to the ground.
     
  6. tracecom

    Senior Member

    Apr 16, 2010
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    Build it like this and it will work. 4.5V is on the low edge of what a 555 needs to operate.
     
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  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Do you have a 10K pull-up resistor from Pin 2 to Vcc?
     
  8. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    No. Pin 2 goes to the button that goes to ground.
     
  9. MikeML

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    Add the 10k, and your timing cycle will begin from when you release the button.
     
  10. tracecom

    Senior Member

    Apr 16, 2010
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    I don't think so. The 10k pull-up is required, but pin 2 needs a negative going pulse, so the timing cycle will start when the button to ground is depressed. The problem is there is bounce on the button.
     
  11. wayneh

    AAC Fanatic!

    Sep 9, 2010
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    That's normal; the 555 output cannot get much closer to the supply voltage than that. It gets closer to ground on the low end.
     
  12. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    I figured the voltage was normal, but I thought the information could be relevant. Tracecom's diagram works like a charm, but that circuit is going to be bitch to etch. I wish I understood what all the extra stuff did. I'm hoping the Circuit Analysis course I'm taking my group through will teach me.

    Now I will need to get the voltage and timing right for the greeting card circuit and shrink it down so I can wear it to win my ugly sweater competition.
     
  13. tracecom

    Senior Member

    Apr 16, 2010
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    R4 and C4 can be eliminated and circuit will still work. C2 and C3 are often left off as well, but I always include them.
     
  14. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    By eliminate, do you mean short or break?
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    #2 pin must be high when it is not low, this is not a default.

    Also, when pin 2 is low the 555 will time internally, but the output will be stuck at high.
     
  16. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    I see, so some of the elements added in tracecom's design were meant to bring 2 back to Vcc. Is that right.

    Looking at it, by eliminating R4 and C4, Tracecom must mean break R4 and short C4.
     
  17. MikeML

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    Post deleted, contained wrong information.
     
    Last edited: Nov 25, 2013
  18. MikeML

    AAC Fanatic!

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    Post deleted.
     
    Last edited: Nov 25, 2013
  19. tracecom

    Senior Member

    Apr 16, 2010
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    The reset pin on a 555 is pin 4, not pin 2; pin 2 is the trigger pin, and is activated by a negative-going pulse.
     
  20. MikeML

    AAC Fanatic!

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    I got the Trig~ and Reset functions confused.
     
    Last edited: Nov 25, 2013
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