Monostable 555 not timing out correctly

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

  1. GunsNTulips

    GunsNTulips Thread Starter New Member

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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

    tubeguy Well-Known Member

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    Could you post your schematic?
  3. tracecom

    tracecom Well-Known Member

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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

    GunsNTulips Thread Starter New Member

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  5. GunsNTulips

    GunsNTulips Thread Starter New Member

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

    tracecom Well-Known Member

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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.

    Attached Files:

    GunsNTulips likes this.
  7. MikeML

    MikeML Well-Known Member

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

    GunsNTulips Thread Starter New Member

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    No. Pin 2 goes to the button that goes to ground.
  9. MikeML

    MikeML Well-Known Member

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

    tracecom Well-Known Member

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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

    wayneh AAC Fanatic!

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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

    GunsNTulips Thread Starter New Member

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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

    tracecom Well-Known Member

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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

    GunsNTulips Thread Starter New Member

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    By eliminate, do you mean short or break?
  15. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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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

    GunsNTulips Thread Starter New Member

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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

    MikeML Well-Known Member

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

    MikeML Well-Known Member

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

    tracecom Well-Known Member

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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

    MikeML Well-Known Member

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