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Square wave help

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Discussion in 'General Electronics Discussion' started by jknott, Mar 5, 2012.

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  1. Mar 5, 2012 #1
    jknott

    jknott

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    Oct 23, 2011
    Is it possible to achieve a wave with transitions from on to off and off to on (see wave A) from a wave with no transitions from on to off and off to on? (see wave B) If so what type of circuit would I need. I am trying to control a motor drive meant for an rc boat with a servopos output from a pic18f88. I am able to control a servo using the same output but when I connect the motor drive nothing happens. The scope settings for both signals are 1 volt/cm and .2 ms/cm. Any help or suggestions are appreciated.
     

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  2. Mar 5, 2012 #2
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    There are always transitions. Do you mean that you want a slower rise and fall time?
     
  3. Mar 5, 2012 #3
    jknott

    jknott

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    Oct 23, 2011
    Yes. I basicly want to convert wave b into wave a. I believe in doing so will allow me to be able to contol the drive.
     
  4. Mar 5, 2012 #4
    siggy

    siggy

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    Mar 3, 2012
    Need to change duty cycle
     
  5. Mar 5, 2012 #5
    jknott

    jknott

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    Oct 23, 2011
    So would i be better off using a microchip with a pwm out pin and a pwm command so i have control of the duty cycle? Im not sure if there is the same control in a servo command?
     
  6. Mar 5, 2012 #6
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, the easiest way is to turn the intensity up on your scope.

    It has nothing to do with duty cycle, unless your question is hopelessly confused.

    The fact that the lines don't appear to join up is normally a good thing.
     
  7. Mar 5, 2012 #7
    jknott

    jknott

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    Oct 23, 2011
    The only major difference i can see between the two waves is the transition and the voltage is slightly different. Just want to match wave B to wave A to allow me to contol the motor drive. The wave showing the transitions is the signal from my rc reciever that allows me contol of the drive but the second signal is from a pic18f88 microchip using a servopos command but will only allow me to contol a servo and not the drive. Do you know what I need to be able to control the drive using a microchip? I've also tried a pulsout command with no luck.
     
  8. Mar 5, 2012 #8
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The only discernible difference I see is that the one on the left is 3V and the one on the right is closer to 5V.

    So, with the PIC you can control the servo, but not the motor?

    Let me guess that the servo has power connections and that this waveform goes to separate connections. And that the motor is just connected to the output producing this waveform?

    If so, the answer is possibly that the RC receiver is capable of supplying a much heavier current required for the motor.

    Perhaps if you can give further information about what else is connected, the power requirements etc.
     
  9. Mar 5, 2012 #9
    jknott

    jknott

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    Oct 23, 2011
    The motor drive is an esc (electronic speed contoller) and uses the same signal that is used to drive a servo. Both the signals in the images in my original post sucessfully drove the servo but only the 3V signal from the rc reciever was able contol the esc. Do you think its the voltage?
     
  10. Mar 5, 2012 #10
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That seems to be a possibility.

    If the input is a fairly high impedance, you could try using a resistive divider, or perhaps power the PIC from 3V (some will operate down to 2.7V)
     
  11. Mar 7, 2012 #11
    GreenGiant

    GreenGiant

    834
    6
    Feb 9, 2012
    if you are worried about the rise and fall time you can just find a digital transistor that has a long rise/fall time, it will still provide the same voltage(s) but it will make a more visible transmission from low to high/high to low like you have in the pictures
     
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