Best way to eliminate noise from logic input?

Discussion in 'The Projects Forum' started by SpiderSpartanju, May 22, 2010.

  1. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
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    I recently posted some questions about getting a L297/L298 stepper driver circuit working. After remaking our PCBs using photo paper instead of magazine pages the board is finally working. (Video Proof) Thanks for all the help with that. There is one slight problem though. If you watch the video you'll see that when we first power up the circuit the motor just starts spinning due to what I assume is noise on the logic input to the board. I'd like to eliminate that noise, but I'm not sure what the best way of doing this would be.

    I'm leaning towards adding a 10K pull-down resistor right by the input. I know normally you'd want to use a capacitor to clean up noise, but we plan on sending input pulses between 30Hz and 3 KHz and I'm not sure what capacitor value to use here. Any suggestions on this? Thanks.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    Did your code work properly with the steppers PRIOR to mounting to the board?

    The thing is, the PCB typically reduces noise because conductors(traces) are shorter than a breadboard jumper wire. I think you should check the wiring again.

    Also, you may want to add a delay of a few hundred mS to get everything warmed up, after power on, prior to sending the pulse trains.

    If it were noise, It would continue through out. every time you stopped the motor, it would keep going. The noise wouldn't just stop
     
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  3. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
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    Thanks for the response. We haven't actually hooked it up to our Arduino controller yet. I made a simple 555 timer box with N.O. push buttons to test the boards. I suppose the arduino would simply connect the input to ground when not sending step pulses, where the timer box has the output tied to ground through an LED + resistor which may not help to eliminate noise just large enough to trick the board into stepping.
     
    Last edited: May 22, 2010
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Your 555 circuit is the reason the stepper motor moves at power up.

    If you hold the 555 reset low for a period of time (a couple of seconds) you won't see the stepper move on power up.
     
  5. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
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    The 555 circuit isn't connected until we press the N.O. pushbutton. We tried hooking up the Arduino controller tonight and it worked well. I guess the noise was just coming from the floating connection when the pushbutton was not being pressed. This being the case, is there any reason to still add a 10K pull-down resistor on the input? Or is there any reason not to add a 10K pull-down? Thanks again for the help.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I would add 10k pull-downs to guard against situations like this. At least you will have a known state if the inputs are not connected.
     
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  7. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
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    Thanks. That's what I'll do. I added the resistor to the PCB, but wasn't sure if there was any reason not to add it.
     
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