Varying 555 one-shot pulse length in regards to trigger pulse

Discussion in 'The Projects Forum' started by bobsuper, Oct 5, 2012.

  1. bobsuper

    Thread Starter New Member

    Mar 14, 2011
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    Hi,
    I have a 555 that outputs a 12 volt PWM signal (call it source). How can I trigger another one-shot 555 (call it slave) that will go HIGH when the source is HIGH ( following the PWM signal ) , but shorten (or lenghten) the length of the pulse that slave 555 outputs???
    The purpose is to relatively accurately match the speed of 2 dc motors that must spin at the same (low) speed.
    Each 555 drives a MOSFET that powers each motor.
    Any ideas?
    Thanks

    Denise
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    Is this what your want?

    You can adjust the value to suit your need.

    [​IMG]
     
  3. bobsuper

    Thread Starter New Member

    Mar 14, 2011
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    Wow, thanks!
    I will try that on a simulation and see what happens
    Thanks a lot!
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    He said he wants to be able to lengthen or shorten the pulse. You need to add a differentiator between Q2 and pin 2.
     
  5. bobsuper

    Thread Starter New Member

    Mar 14, 2011
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    Scott,
    your circuit is very clever, and allows me to control when the one shot 555's pulse ends. That will be very handy to control the motors at independent speeds from each other.
    I can't believe you just whipped that up today! You must be a very knowledgeable person about electronics, congrats!

    However, if I adjust the first Potentiometer to change the global pulse width, the one shot 555 still keeps the same pulse length.
    I would really need to be able to adjust the proportion of the one shot's pulse to the Astable 555.
    So that dutycycle(MOTOR2) = dutycycle(MOTOR1) X "controllable amount"
    so regardless of dutycycle of motor1, motor2 spins at a certain percentage of that dutycycle.

    Basically to make fine adjustments to the speed of the 2nd motor, to match it as best as possible to motor 1 (perfect accuracy is not critical since the motors will spin rather slowly)

    It is for a jewelry coil winder by the way.

    Do you have any brilliant clues on how to achieve my proportional dutycycle effect?

    Thanks so much
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    Thanks for reminding me, I knew that he want to adjust the second motor, I just thought maybe he will change something.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    The Motor need to add a diode to protect the coil when the power of motor is off.

    [​IMG]

    >So that dutycycle(MOTOR2) = dutycycle(MOTOR1) X "controllable amount"

    Because the first duty cycle of the motor1 was controled by hand, but the second duty cycle of the motor2 is set by R,C, but they are fixed, they can't auto adjust follow the first square wave of NE555.

    If you want their duty cycle are the same, how about this:
    Two Motors and mosfets use the same NE555, and then they will get the same duty cycle.

    [​IMG]

    If you have the wave form about two duty cycle and motors, maybe the conception will more clearly.
     
    Last edited: Oct 7, 2012
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Do you have control of both PWM signals?
     
  9. bobsuper

    Thread Starter New Member

    Mar 14, 2011
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    Thank you both
    Yes , I have control of both pwm signals,
    but what I am shooting for is: control the PWM signal with ONE potentiometer (control speed of motor 1), and have motor 2's PWM signal be a certain RATIO of motor 1's PWM signal's dutycycle.
    So both motors's speed is adjustable with one Potentiometer (global speed),
    but speed of motor 2 can be fine-tuned to be a bit more or less than motor 1's..
     
  10. Ron H

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    Apr 14, 2005
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    What is your preferred supply voltage for this circuit?
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Here is one way to do it. The lower graph shows the range of adjustment on PWM_B. It can be increased by increasing the values of R9 & R10.

    EDIT: I should point out that you could probably do this with much less hardware if you use a microcontroller.
     
    Last edited: Oct 8, 2012
  12. bobsuper

    Thread Starter New Member

    Mar 14, 2011
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    12 volts, as the power supply is an ATX psu.
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Check out post #11 for a circuit that should work for you. 12V should be fine.
     
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