Removing Potentiometer from 555

Discussion in 'The Projects Forum' started by andrew2020, Apr 9, 2010.

  1. andrew2020

    Thread Starter New Member

    Apr 3, 2010
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    0
    Hello I am in the process of building a 555 timer circuit. I was wondering if i am able to remove the potentiometer and replace it with some type of resistor. I would like my led to produce around an 60-80hz signal.
    This is the drawing I am using.
    [​IMG]
    Any suggestion would be greatly appreciated:).
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, you can.

    Unfortunately, the pot in that schematic will only adjust the ON vs OFF time.

    R1 is really a bit too small; it should be more in the range of 2.7k to 3.3k.

    Do you want the ON vs OFF time to be over 53%, or exactly 50%, or somewhere between 3% and 97%?
     
  3. andrew2020

    Thread Starter New Member

    Apr 3, 2010
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    Anywhere from 50-60% is fine. So I can remove "P1". Replace R1 with around 3k resistor. But what should I do with "D1" and "D2"?
     
  4. Bychon

    Member

    Mar 12, 2010
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    41
    Change R1 to 27k and attach it to the junction of 6,2, and C1. The diodes aren't necessary because they were used to steer the different duty cycles.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, this is the first really bad advice I've seen you give.

    It just won't work like that.

    Take another shot at it.

    There needs to be resistance between Vcc and pin 7 (R1), and there also needs to be resistance between pin 7 and the junction of C1/pin 2/pin 6 (R2).

    You can use a diode from C1 to pin 7 to match R1/R2 pretty well to get a 50/50 duty cycle.
     
  6. Bychon

    Member

    Mar 12, 2010
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    Oh #$*^&. I was looking at the monostable drawing.
     
  7. whatsthatsmell

    Active Member

    Oct 9, 2009
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    Just for background, the circuit that the op posted is a pwm circuit to dim LEDs, and works well for that. The only limitations are that the duty cycle is roughly between 5-95%, meaning the LEDs can not be turned all the way off or to full brightness.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    OK, the attached schematic should provide a 70Hz output frequency with a fixed 50% duty cycle.

    If you want the duty cycle to be variable but keep the 70Hz flash rate, that can be done, too.
    (eta) Oops - snipped off the ground symbol from the bottom. Trust me, it WAS there before I ham-fingered the editing.

    [eta]
    Andrew,
    If you're thinking of using this circuit in your truck - please don't do that. It's not suitable for automotive use.
     
    Last edited: Apr 9, 2010
  9. Wendy

    Moderator

    Mar 24, 2008
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  10. andrew2020

    Thread Starter New Member

    Apr 3, 2010
    3
    0
    Thanks all for the advise;)
     
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