I did this when I made the bent MM-4. You are going to want to first build an oscillator, then feed the oscillator into an H11F3 or photocell to get the variable resistance. Sine vs. Triangle modulation is basically the same 90% of the time, it is definitely not worth the extra work to get a sine wave. My oscillator of choice is this one:http://experimentalistsanonymous.com/di ... %20LFO.gif
You can get a different range out of the LFO by using a larger pot and changing that cap value. You should feed the output of this into a simple, noninverting buffer:http://www.seas.upenn.edu/~ese205/Labs0 ... age019.gif
You now have three op amps, might as well put it all in a quad op amp package. Then, you might want to wire a simple volume pot (one side lug to the noninverting buffer output, one side lug to Vb, and the center lug is the out) into another noninverting buffer - this is the LFO amount pot. Wire the output of this second noninvering buffer, whose output should go into a 470 ohm or so resistor, into the + input of the H11F3. - input to ground, then you have 2 pins for the varying resistance. If the expression pedal uses 3 connections, one for center, and two sides, then you will need to wire an inverting buffer, and a second H11F3.
What does this do when you take the output of the LFO and run it through a non inverting buffer before sending it to the h11f3?
Is it crucial?