555 Timer

Discussion in 'General Electronics Chat' started by Bertus2, Mar 22, 2009.

  1. Bertus2

    Thread Starter New Member

    Mar 20, 2009
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    Hi

    Is there a way of generating a fixed frequency signal, but with a variable dutycycle, with a single LM555. The Dutycycle must be variable from about 10 - 90% .
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    You betcha, it is easy as toast.

    [​IMG]

    Ignore and remove R2, it is bogus. There is another website that came up with it, when/if I find them I'll post the site.

    ******************************

    Here it is:

    http://www.dprg.org/tutorials/2005-11a/index.html
     
  3. Bertus2

    Thread Starter New Member

    Mar 20, 2009
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    What do you do with pin7?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Pin 7 is not required since the output square wave on Pin 3 is charging and discharging the capacitor through the diode and resistor network.

    hgmjr
     
  5. Bertus2

    Thread Starter New Member

    Mar 20, 2009
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    0
    Thanks. I'll give it a go.

    Bertus2
     
  6. Metalfan1185

    Active Member

    Sep 12, 2008
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    ok, just wondering...


    is it possible to do the same, but have a variable frequency and the fixed duty cycle?
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    That is a subject for another thread, hijacking is discouraged.

    It could be done, but not with a single 555.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually Bill, if R2 is 1/10 the value of R1, it will limit the PWM range to what the O.P. requested.

    I suggest that R2 is not bogus; it limits the peak charge/discharge current through the diodes to/from C if R1 is at one end or the other.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The schematic shows R2 as variable though, so it too can be zero Ω. The way it's drawn doesn't make it stick out as a pot right away. It should be a small fixed value, not variable.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    It was a recycled brain fart, and we all know how those smell. The base circuit is a variation of this, but the diodes force different charge/discharge paths. Since the resistance always adds up the same, the frequency shouldn't change much.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    That's an excellent 555 tutorial, cool pitchers too!
     
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