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Transistor Help Required Please!

Discussion in 'Electronic Basics' started by SME, Nov 11, 2003.

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  1. SME

    SME Guest

    Hello Group

    I wish to reverse the logic from a parallel port output using transistors.
    So when the output is high the line into the transistor is high

    For example :-

    Collector 12V

    D0 -----|

    As I understand it the above circuit when D0 is high (+5V) will sink the
    collector bring the 12V input to Low (I understand there should be resisters
    in the above circuit etc etc)

    As mentioned above I which to have a circuit that when D0 is High the 12V
    input is High and when D0 is Low it pulls the 12V input Low.

  2. Luhan Monat

    Luhan Monat Guest


    Why not reverse the sense in the software?

    Otherwise, you are actully describing 'non-inveted' output: High in =
    High out.
  3. If you need less output pull down current than the parallel port can
    sink, you can use a common base amplifier to increase the output
    swing, without inversion. Tie the port pin to the transistor emitter,
    and bias the base with a resistor to a voltage that is between port
    high and port low voltage. For instance, if the port swings a full
    five volts, you might tie the base to +5 with a 10k resistor. This
    will limit the base current to less than a half milliamp when the
    emitter is pulled low, while letting the collector sink 5 or 10 ma, if
    the port output can sink that combination of collector and base
  4. SME

    SME Guest

    Thanks for all your help......

    Additional info!

    The original controller board (not the stepper Drivers) has three dual opto
    isolator's each one handle the Step and Dir.

    Step is defaults to hi on the led side which sets hi transistor side these
    then appear to go via two resister arrays which in turn connect to a
    transistor (NPN) this then connects to a cable connector which is a 16way
    (similar to an IDE cable but smaller) the ribbon cable from this connector
    connects to a backplane that then connects to the X,Y,Z stepper drivers.

    ------ ResArray Connector
    Step X (H) -| o |- | |..|
    Step X (H) -| p |- StepX(H)--|-(TR)-|..|-StepX(H)---StepCtlr-(H)
    Dir X (H/L)-| t |- DirX(H/L)-|-(TR)-|..|-DirX(H/L)-- StepCtlr(L/H)
    Dir X (H/L)-| o |- | |..|

    You made need to copy the above into a text editor!

    The Step pin on the Stepper Controller is HI by default when the motors
    moves a LOW appears for as long as movement is requested.
    This LOW is witnessed all the way back to the frontside of the optoIsolator
    when the motor completes its task the Step go's HI again.

    The question is how is this done? (regarding the circuit above)

  5. norm d.

    norm d. Guest

    One way with transistors is to use 2 NPN transistors to get the logic you
    want. Put a resistor in series between your +12VDC and the collector to keep
    from frying the transistor. Use the junction of the resistor & collector as
    the output to drive a second similar transistor circuit. A series resistor
    going to the base of each transistor may be required also depending on the
    transistors you use. The output of the 2nd transistor will be just the
    opposite of the 1st transistor, but will follow the parallel output
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