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Voltage Follower

Discussion in 'Electronic Basics' started by suntraveler, Jul 11, 2003.

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  1. suntraveler

    suntraveler Guest

    Hello,

    I am building a parallel interface for a joystick. The output
    impedance of the joystick is about 140Kohm, so I am using a voltage
    follower (with LF356) to buffer the analog output of the joystick. My
    voltage follower works when I test it with a pot, but if I connect it
    to the joystick, the output of the voltage follower is always the same
    even though I measure different voltages at the input of the voltage
    follower. (The input to the voltage follower is 5.04-5.105V, and the
    output is fixed at 5.107V. V+ of LF356 is 9V. I am doing this on a
    breadboard.) Can somebody please suggest me what I could do to fix
    it???

    Thanks

    Johnson
     
  2. First of all, you are getting very little voltage change out of your
    joystick. I suspect you are connecting to only two terminals of the
    pots, making a variable resistor (which looks like a low resistance,
    regardless of position to the almost infinite input resistance of the
    opamp), instead of using all three terminals to make a variable
    voltage divider across the supply.

    Secondly, the opamp you are using does not work very well with a 9
    volt supply. I suggest you use an LM324 (quad) or LM358 (dual) that
    is designed for this sort of thing. Its inputs and outputs work all
    the way down to the negative supply rail, up to within about 1.5 volts
    of the positive supply rail and are stingy with battery power. They
    are not nearly as fast as the LF356, but how fast can you move that
    joystick?
     
  3. Rich Grise

    Rich Grise Guest

    Are you sure that the game port is looking for a voltage? This is
    almost a WAG, but I seem to remember seeing a schem. where the
    joystick pots were actually a part of an oscillator, with the
    cap on the controller board. (think 555.) And the freq. of osc.
    is how it figures out the pos. of the stick. But I could be
    hallucinating - I don't really know if there's a way to tell.

    Good Luck!
    Rich
     
  4. suntraveler

    suntraveler Guest

    Are you sure that the game port is looking for a voltage? This is
    Rich, you are right. According to
    http://www.epanorama.net/documents/joystick/pc_joystick.html ,
    I should connect the joystick with a 555 oscillator to measure
    frequency of oscillation in order to determine the postition of the
    pot.

    Thanks for everybody's help!!!


    Johnson
     
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