Voltage Follower

[suntraveler]

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

 

[John Popelish]

suntraveler wrote:

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

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?

--
John Popelish

 

[Rich Grise]

suntraveler@mail2engineer.com (suntraveler) wrote in message news:<ff3262f0.0307101921.3f0a5515@posting.google.com>...

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

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

 

[suntraveler]

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

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