Please see the attach circuit. I try it using it as a substitute for the resistance/pot on my PC game port (Joystick) but cannot make it work. I apply voltage to the gate that are less than +5V (no more than 1 volt) but the resistance does not change. Do I need this gate voltages to be negative? Thanks a lot. I am using a MPF102 FET.
I can't tell which is the source and drain on your JFET because of the component symbol, but if the drain is on top and source is on the bottom, the gate voltage you apply (if it is referenced to ground) will not be the same voltage that appears across the gate-source of the JFET because of the voltage drop across the 1K resistor. Maybe try moving the 1K resistor to the drain instead and connecting the source straight to ground. THis is just a guess- I've never done this before.
THe first link I gave you also says that the resistance-gate voltage curve is non-linear, so you need two resistors to linearize it.
Many JFETs are symmetrical and their drain and source can be swapped without changing the performance. Then their symbol is drawn with the gate at the exact center as is shown in this thread.
When is the pretty girl in your avatar going to entertain us?