Simple 555 problem: Input high=>Output constantly high

Discussion in 'The Projects Forum' started by Selportion, Mar 21, 2011.

  1. Selportion

    Thread Starter New Member

    Mar 21, 2011
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    Stand-alone 555 problem (monostable)
    ((I bet this question has been answered before and sorry for that, but all previous titles are like "555 help" and i cannot find the answer to that specific problem, i ask for your understanding))

    I am using 555 monostable timers for a project, and all of them have output constantly high (1) when the input is constantly high(1), which I think is against the expected responses. If I change the input to (0) then the output lowers to (0) after the expected time and remains so.


    Using:
    • "CHN NE555N KA6735"
    • all with capacitors non-electrolytic (i think ceramic) C=0,01uF
    • and some of them with R=270k and C=10uF electrolytic
    • and the others with R=3,18k and capacitors non-electrolytic (i think ceramic) C=0,01uF
    Schematic as [​IMG]
    http://www.google.gr/imgres?imgurl=...D4gby98DgCA&page=1&ndsp=30&ved=1t:429,r:1,s:0

    From your experience is it broken/burnt or some sort of wrong use of capacitor types/values?
    Any help appreciated!:)
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    Hey Welcome to AAC

    A monostable has only one stable state. A trigger will cause it to switch and after the time constant it will revert back to it's stable state
    Recheck ur wiring
     
  3. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    I suppose when you say "it remains so," you expect it to go high again (on-off-on-etc.)? That would be astable operation which requires this circuit:

    [​IMG]

    I hope this helps :)
    Der Strom
     
  4. Selportion

    Thread Starter New Member

    Mar 21, 2011
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    0
    Thanks for welcoming, I've been reading around for a while but never bothered registering till now!

    Hm, I've messed up my initial description, and i see i cannot edit it, I got it wrong. So here's more clear:
    • I need monostable operation, like LowLowLow...-High for X secs-LowLowLow...
    • I get for Input HIGH => Output LOW
    • Input HIGH->LOW->HIGH => Ouput HIGH for x secs
    • Input steadily LOW => Output steadily HIGH <<Unwanted response
    So could it be something like too small/big capacitors or not/yes electrolytic capacitors or destroyed 555? The wiring seems fine.
    PS>Vcc 5.2V
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    When pin 2 of the 555 goes lower than 1/3rd of the power supply voltage then the timer starts and the output goes high. Pin 2 must become high again or the timer will never time out. When the timer times out the output goes low.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    You can solve your problem by edge triggering the 555.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Most datasheets for the 555 show how to edge trigger it.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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  9. Selportion

    Thread Starter New Member

    Mar 21, 2011
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    I didn't know that was true..
    So if i want: 50ms after hitting a switch, one led to get light up for 3s, i cannot use 2 monostables (50ms / 3s) in row? Cause you are saying that after the 1st monostables finishes its flash (=>output low), the second one (input low) will never stop (output constantly high) ??
    I see the whole edge-triggering idea, but doesn't it require a manual switch on the input? Or is it just a schematic?
    Thank you all for your help!
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    The exact details are explained in the theory of operation in the link I gave.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    The switch can be a transistor, a Mosfet, a comparator IC with an open-collector outout or a logic IC with an open-collector outout.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    No, the switch can be the collector emitter junction of a transistor, Fet or relay contacts.
     
  13. Selportion

    Thread Starter New Member

    Mar 21, 2011
    8
    0
    Thank you very much guys, edge triggering works and it's easy if you understand it.

    So, to anyone with the same problem,
    put any small capacitor to your 555 input, 2 reletively big resistors from the capacitor legs to +Vcc and connect your input before the capacitor, just like the image on previous post.

    Topic closed
     
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