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servo control via lpt problems

Discussion in 'Electronic Design' started by Mitch__, Mar 21, 2007.

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  1. Mitch__

    Mitch__ Guest

    Hi all
    i am working on a small project in VB, Controlling Servos by voice.
    everything is working great except for one thing.
    If i use one power adapter to control more than 1 servo, then
    following happens
    While 1 servo moves , second servo seems to get enough of a voltage
    fluctuation that it will move one way or the other.
    currently for this to work for me, i need to have a separate power
    supply for each servo, then
    everything works great.
  2. would it be possible for you to post the LPT interface circuit
    somewhere, so we could have a quick look?
    Otherwise we could all be collectively be pissing into the wind

  3. Mitch__

    Mitch__ Guest

    ok, well there is not much to it, i do not have a diagram, but can be
    easily explained.
    LPT ground to Servo Grounds and to 5Volts ground
    LPT Pin 2 out to Servo White wire (and pin3 out to second servo white
    Both servo red wires to +5 volts
  4. what sort of current are you using for the servos?
    what does it look like on a scope?
    where are you getting the 5V from, and how good is the supply?
    whats the current through the servo ground to PC ground?

  5. DaveM

    DaveM Guest

    What kind of servos are you using?
    How much drive current do these servos need? (Hint: LPT ports usually can't
    source more than a few milliamps of current. Most have TTL or LSTTL
    Post the relevant part of your VB code.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
  6. Mitch__

    Mitch__ Guest

    futaba rc servos
    using a dc adaptor to power them

    Dim icount As Integer
    For icount = 1 To 75
    Call PortOut(888, 1)
    Call PortOut(888, 0)
    Next icount
    like i had mentioned, everything works fine if i use separate power
    source for each servo, if i use one power source then when 1 servo is
    instructed to new position then the second servo glitches alot.

  7. Genome

    Genome Guest

    Life is like a roll of toilet paper.

    You buy your own they last for months.

    You enter into a purchase share scheme. You will experience at least one
    shit a month with nothing to wipe your arse on...... after you have made
    your deposit.

    And then they will philosophize about the origins of War!

  8. DaveM

    DaveM Guest

    Ummm ... sounds like a classic case of insufficient current capacity from the
    DC adapter or insufficient filtering.
    Have you actually measured the voltage level from the DC adapter under load?
    What is the voltage/current capacities of the adapter (should be on the label)?
    What are the servo voltage/current specs?
    Have you actually measured the current that the servos need?
    Have you actually measured the drive voltages from the LPT port when the servos
    are being commanded?

    Those are very basic things that you should do, and those measurements will tell
    you nearly everything you need to know about your problem.

    One suggestion... connect a 100uF capacitor across the DC terminals of each of
    the servos and see if the operation improves. Voltage rating of the capacitors
    should be a bit above the output voltage from the DC adapter.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
  9. Mitch__

    Mitch__ Guest

    thanks all
    problem solved with resistors between servo and +5
    everything is working great now
    thanks agin
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