Mad Scientist Jr said:
I basically want to control a video game (Atari 2600 paddle controls
are 1 MOhm pots connected to 5V DC) with my stereo.
See "paddles"
http://www.hardwarebook.net/connector/userinput/atari2600joy.html
I don't think it has to be zero ohms exactly but a low amount.
I'm not clear whether you want the resistance to be related to the envelope
(average volume) of the signal, or to the instantaneous voltage.
Are you trying to make something like an oscilloscope, to display something
like a Lissajous pattern (ever-changing squiggly thing that shows phase and
frequency relationship between two channels)? Or are you trying to make
something that just moves the paddles depending on how loud the sound is?
If the former, you're probably out of luck; I don't think the Atari itself
will respond to paddle movement at 20kHz.
If the latter, then Rene's light bulb idea is not a bad one. One problem is
that light-dependent resistors (LDRs) with 1MEG dark resistance are usually
pretty slow, taking as much as several seconds of darkness to get all the
way to 1MEG. The resistance will go low in a hurry, but it takes a long
time to recover to high - it's sort of like your eyes when they dark-adapt.
Either way, you probably only need a current or a voltage, not a resistance.
That's considerably easier. And I wouldn't worry about getting the
(effective) resistance any lower than 1k ohms, since the pots in the
joystick are almost certainly linear (as opposed to log-taper): the
difference between 1k and 0 is 0.1% which is less than the pots themselves
can distinguish.
Give Rene's idea a try, as a start. If it doesn't work you can evolve it.