/diy/ - Do It Yourself

this is a classic hex oscillator. You can control a neat sounding effect by varying the resistance of the right resistor. http://tiny url com/wvub22a

my thing is this: Obviously it's well and dandy that I can control the resistance by hand. I want to modulate the resistance via another signal (like a sine or saw LFO). I have no fucking clue how. I *thought* that was exactly what a transistor was for, but I tried substituting an npn transistor in place of the resistor, and putting a saw sweep on the base, but it did nothing at all.

What is the standard way of modulating resistance (or current flow I guess) via another signal?
And maybe, what is the abstraction above that? What is the general solution to trying to modulate carrier X with signal Y when it comes to electronics?