pulse generator

Discussion in 'General Electronics Chat' started by samjesse, Aug 17, 2009.

  1. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Hi

    I built a pulse generator with some components for the local radio shack, 555 on a breadboard and ...
    I tested it and it puts out a 50% duty cycle square wave. With a variable resistor I am able to control the frequency.
    I want to build one where I control the duty cycle as well as the frequency.

    how do I do this? any diagrams?

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Thanks.

    the breadboard shows one op-amp but the schematic shows 2.
    I am thinking to use a resistor instead of 3 diodes to drop the voltage. actually I have a 4.5v dc supply and may not need much resistance. and instead of the 12v light, I have a LED.

    I will be looking for another schematic to replace the op-amp with comparator(s) since this is my first go and I am learning.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The 1458 is a dual 741 opamp. It has two complete operational amplifiers in a single package. You could use an LM324 instead; it is a quad opamp that Radio Shack carries.
    That won't work. The output voltage of a 741-type opamp won't go low enough to turn the transistor off. That's why they used three diodes; each will have around 0.7v drop across themselves until the current level gets very low.
    You will need to use a resistor in series with the LED to limit current.
    Rlimit = (Vsource - Vled) / DesiredCurrent

    Why not just build it as-is for the moment?
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    Greeting samjesse,

    I think that sgtwookie's suggestion that you build the circuit as shown is best. I sense that you are just beginning to get acquainted with electronics. There are enough variables in any circuit that you are better off avoiding the introduction of any new ones. Once you have successfully built the circuit as it is shown, you can then indulge your curiosity and investigate the effect of the changes you have suggested.

    hgmjr
     
  6. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    OK, I will have a go at it.
    but which is the control for the frequency and which is for the duty cycle? I see only one potentiometer.
     
  7. jRaskell

    New Member

    Aug 4, 2009
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    That circuit has a fixed frequency and can only vary the duty cycle.
     
  8. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    My original post said:
    "I want to build one where I control the duty cycle as well as the frequency."

    I was going to buy more components today to finish this project, but how can I get it to vary the frequency as well as the duty cycle?

    thx
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    I have made some changes to the eBook schematic.

    [​IMG]


    The 1M pot will change the frequency.
    The 10K pot will change the duty cycle.

    Greetings,
    Bertus
     
  10. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Thank you.
     
  11. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Will the updated schematic make the frequency and duty cycle adjustment independent?
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Yes they are independend.
    The 1M pot will change the frequency only.
    The 10K pot will change the duty cycle only.

    Greetings,
    Bertus
     
  13. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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  14. Wendy

    Moderator

    Mar 24, 2008
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