555 Timer Question

Discussion in 'General Electronics Chat' started by Sonoma_Dog, Dec 29, 2010.

  1. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
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    Hi all

    I am tying to design a 555 circuit that can control the duty cycle by an input voltage. Is this possible?

    I have looked into many 555 circuits in the pass two days, but non of them are voltage control.

    Thanks in advance
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    924
    Set up a standard astable with two resistors and one cap.

    Give an AC voltage input on pin 5 (CV) and the freq will change within a range of 20-50%(or so) from the capacitor set astable freq. Duty cycle changes some as well.

    This is a Freq modulation scheme and has much less affect over the output than other control methods
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In post 6 of this thread by Bill_Marsden you will see a PWM circuit:
    LEDs, 555s, Flashers, and Light Chasers

    The control voltage will be about 1/3 Vcc to 2/3 Vcc at the comparator input.

    Bertus
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Set up the standard 5-95% variable PWM circuit that uses two diodes and a pot.

    Substitute a digital pot for the variable resistance one and you would have a voltage controlled duty cycle 555
     
  5. mbohuntr

    Active Member

    Apr 6, 2009
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  6. Wendy

    Moderator

    Mar 24, 2008
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    As Bertus said, LEDs, 555s, Flashers, and Light Chasers has it in chapter 5.

    [​IMG]

    The frequency remains stable, and the duty cycle can be adjusted from 0% to 100% with tweaking of the resistors. It is fairly linear to boot, but if it isn't linear enough it can be made more so with a transistor/resistor addition.

    The rest of this article also has many examples how to use this configuration in other circuits.
     
  7. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
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    Thanks for all the reply~

    I think The circuit Bill Marsden posted is what i am looking for.. Thanks!!
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    A LM393 is a better comparator, since it is an 8 pin dual.
     
  9. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
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    some more questions.

    I was just looking at the comparators , and they can not sink too much current. Compare to the 555, it can sink at least 100mA.
    I can easily throw a BJT or MOSFET into the circuit and make this work, but i am trying to make this circuit as cheap as possible.

    is there any other ways that will make this possible? I don't really care about remaining the frequency stable. I just need the duty cycle change over voltage.

    John Lau
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    You did notice the transistor driver? It will go to 100 ma easily, or more, even though it is a PNP device to Vcc. Just remove the LED and resistor on the emitter. Are you needing to ground?
     
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