Hi there all,

I need to make up a X/Y Joystick that uses a center-tapped 3k Pot for each axis.
I cant find pots online to suit this, so does anyone know how to get round the problem?

Thanks

Simon

 

What are the joysticks connected to?

 

That's not easy. What load is the pot driving? How is the pot connected?

 

Its for a Virtuality video game machine from the 1990`s.
The joysticks connect direct to a custom `I/O` board, that has a set of MAX,CEN,MIN trimming pots on it for each axis, then ultimately heads down a connection to a Commodore Amiga 3000 computer`s joystick port.
It looks like the wiper goes to the Amiga via a resistor, and the tap and outer connections go to the trimming circuit part of the PCB.

 

Perhaps I made an incorrect assumption at first. What do you mean by "center tapped"? What difficulties have you experienced when searching for a replacement? What electrically distinguishes the desired pot?

 

Hi,

What I mean is I need a 3K pot, that also has an extra connection to the resistor, half-way along its length,

So really instead of a 3k Pot, it could be referenced like two 1.5K pots in series, with a single wiper that spreads across both resistors.

The use for this kind of pot has almost become obsolete, as have the components.

Hence I`m asking if there is a way of `faking` the output from one, as my VR system requires that kind of input.

 

Good clarification. Unfortunately, I don't have any ideas. I searched myself and was also unsuccessful. Most of the results were for audio pots and much greater resistances.

 

yeah I know...

thats why I want to `fake` it.
Other than that, I have recently found information on how to mod a standard Pot to a have a center Tap, so I`ll try that
Then I have design and build a pair of joysticks

 

You could try this fake. Might need to tinker with component values so that both the C and W impedances/voltages are convincing to the Commodore.

 

You could try this fake. Might need to tinker with component values so that both the C and W impedances/voltages are convincing to the Commodore.

I did see a suggestion for using this circuit on an audio forum. Thanks, Alec

 

Thanks Alec,

it being a 3K pot,

I then assume I`d need R1 & 2 to be 1.5K ?
and U1 as the 3K ?

or at least something like that?

 

No. With the component values I showed, R1 + R2 = 9.4k. That sum is in parallel with 5k, giving a combined resistance of 5k x 9.4k / (5k + 9.4k) = 3.26k between points A and B.
So the Commodore will see an apparent '3.26k pot' between points A and B, and the voltage at 'centre-tap' C will be half the AB voltage. The voltage at wiper W will be a variable fraction of the AB voltage.
This arrangement should fool the Commodore, providing the impedances of its inputs which connect to C and W are considerably higher than 5k.

 

ah yes..... resistors in series / parallel ! - missed that when looking at it - DOH!!!

I will give that a try soon! thanks

I have a test program that is used to trim the pots, using a graph, so I`ll use that to see how it reacts.

Simon