RE: Commodore joystick ports

From: Scott McDonnell (
Date: 2007-04-09 04:25:28

Yeah, Daniel, no can of worms. It just wasn't completely clear what you
were looking for. The discussion has just been about methods, one that
has been proven to work, and others that should work in theory. Your
application is going to require the theoretical solutions, I think.

John/Lori (which is it?) I agree an op-amp solution would be great. Of
all the analog theory, op-amps are my very weakest point. Any chance you
could take the numbers and come up with something?

I will see what I can come up with using the transistor and maybe move
to FET. The FET should give us more linearity, but I'll have to brush up
on my math. The advantage to figuring this out is that the POT inputs
could be used for lots of different analog sampling purposes as long as
an interface can be made. Of course, some would say it would be better
to just have an ADC on the user port. They would be right, but where's
the fun in that? :)

Anyway, I'm in. Just got to find enough time (and ambition) to work
something out. I was getting some crazy ideas yesterday like using an RL
circuit with a varying frequency, etc... Mostly interesting just for the
academic factor.

I should re-install multisim (lets me play with theoretical analog
circuits on the computer.)

Scott McDonnell

-----Original Message-----
From: []
On Behalf Of john/lori
Sent: Sunday, April 08, 2007 9:59 PM
Subject: Re: Commodore joystick ports

Daniel O'Shea wrote:

> I apologise, I wasn't intending to open up such a can of worms!

hey, that's what we're here for, and it's a question that comes up
fairly often.  But I still haven't seen a good answer. (I still like the
PWM idea, but I'd use a 555. haven't tryed it yet though)

> As I
> stated in my original post, I have a variable voltage of 0 to 5V (no 
> microcontroller involved) that I want to translate in to a variable 
> current using a transistor. And so, I am lead back to my original 
> question - what kind of transistor and what value would work best to 
> provide a variable current which duplicates the potentiometer's
> current? and do I need any extra components besides just a transistor?

> thanks!

Use an op amp it won't be any more complicated than using transistors by

themselves and it will be a lot less tricky to get right.

The closest to a single component would probably be an FET.
I believe they make FETs taylored to use as voltage controled resistors.

There's a problem in that any really simple circuit I know of to source
current will (roughly speaking) want a voltage referenced to the
positive supply.  That may or not be a problem, if it is, by the time
you get through taking that into account, you might as well have used an
op amp circuit.

However, it might be tricky getting an op amp current source to work 
with a 5V supply

Another possibility with a 555 would be a V/F converter and a charge

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