To change the frequency of 555 Timer to control a Servo motor

Discussion in 'Digital Circuit Design' started by bacdoc7, Mar 18, 2019.

  1. bacdoc7

    Thread Starter New Member

    Mar 18, 2019
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    Hi all,
    Newby question of the day! I have a circuit board which controls a SERVO motor but it makes it move 180 degrees. I would like the motor to only move 90 degrees and I was told that its the PWM that controls the limits. How would I adjust this schematic to get to 1.5 ms PWM to make the motor move from 0 to 90 degrees only?
    Thank you for any input or assistance! It is much appreciated!
    I will try to post schematic pics
     
  2. bacdoc7

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    Mar 18, 2019
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  3. mvas

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    Jun 19, 2017
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    Model # and link to the Servo Motor ...
     
  4. jpanhalt

    Expert

    Jan 18, 2008
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    Welcome to AAC,

    Are you using a potentiometer (2nd schematic) or switching between fixed resistor values (1st schematic)? Do you have a way to read duty cycle (many multimeters have that function) or pulse width? Do you want to use a pot or switches?
     
  5. bacdoc7

    Thread Starter New Member

    Mar 18, 2019
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    The schematic I’m using is a little different . It uses two tactile buttons. But the component values are the same as in schematic two. I just couldn’t figure out how to upload the exact schematic I use so
    I posted one I
    Found online. I ordered motors from
    China and when they moved 180
    Degrees instead of 90 degrees, they told me the problem is my board and not the motors. So I found a formula online to adjust the range but not sure if it’s correct. Any help
    Is appreciated
     
  6. jpanhalt

    Expert

    Jan 18, 2008
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    Please answer: Can you measure pulse width or duty cycle?

    Your desire to move "90°" is unclear. Do you mean, for example:
    1) Move 45° to each side of center?
    2) Move 90° from one side of center? If so, CW or CCW?
    3) Move 90° to each side of center? Presumably, you want the servo in center with no button pressed. Does that happen?

    What do you mean by it moved 180°? (Essentially same set of questions as above substituting 180° for 90° and 90° for 45°, respectively.)
     
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  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    RC hobby servos are designed to move through an arc of 180°. A PWM signal running at 50Hz is used to control the servo. The pulse width varies from ~1mS to 2mS, where the former positions the servo at 0° and 2mS is 180°. A pulse width of 1.5mS is considered to be centered. Note that the servo may extend further than 180° and hence the actual upper and lower limits for the pulse width may vary by servo brand.

    To limit the range from 90°, you have to calculate the resistances necessary to produce that range and replace the pot with one that has the same value of the difference in resistance that you’ve calculated.

    To help in your calculations, search for “555 calculator”.
     
    Last edited: Mar 19, 2019
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  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I don't know why posters always seem to omit what type of SERVO they have questions on , there are many types and uses, evidently this one again turns out to be a RC SERVO!
    Max.
     
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  9. djsfantasi

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    Because noobs don’t know that there are other types of servos. I was guilty of this at first. Now, I try to remember to qualify my comments with “RC” when appropriate.
     
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  10. bacdoc7

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    Mar 18, 2019
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    Sorry,
    Yes, I want it to move 45 degrees in each direction.
    Thanks for any help!
     
  11. djsfantasi

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    You do realize that with hobby servos, moving 45° in each direction is the same as moving 90°? The servo horn can be positioned wherever you want.

    The only consideration is what resistances you use to generate the control signal with a 555. Different ranges may result in better resistances required. So you could calculate resistances for 0°, 45° (center) and 90°. Or 45°, 90° and 135°. Or 90°, 135° and 80°. Or even 27°, 72° and 127°!
     
  12. BobTPH

    Senior Member

    Jun 5, 2013
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    1.5 ms is center. 1.25 ms is -45 degrees and 1.75 is +45 degrees.

    So change the circuit to limit the range of pulses to 1.25 to 1.75 ms. The frequency is not critical but should be about 50 Hz.

    Bob
     
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  13. djsfantasi

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  14. jpanhalt

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    @bacdoc7

    I suspect if you are using the component values in the 2nd link (one with pot) and design of the 1st link (one with buttons) , you will have travel problems. In particular, note how the pot is wired.

    1) Please confirm you are using schematic in the first link.
    2) Please list the component values you are using, including tolerance. That is not as important for the resistors, as they are probably 5%. But the capacitor might be almost anything.
     
  15. bacdoc7

    Thread Starter New Member

    Mar 18, 2019
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    Thank you! much appreciated!
     
  16. bacdoc7

    Thread Starter New Member

    Mar 18, 2019
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    Thank you, I will check them out.
     
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