Help understanding VCF schematic
The heart of the design is a twin T notch filter: -



This type of circuit normally rejects a small band of frequencies but, when placed within the negative feedback loop of an op-amp, The lack of negative feedback at those frequencies means the gain of the op-amp circuit is very high - thus a band pass fitler is created.

The LED is acting as a voltage controlled resistor varying R/2 in my circuit. This of course pulls the centre frequency up or down. It's a bit crude but relies on the slope of the LED V-I characteristic: -



Look at the red curve - with barely a volt across the LED the slope of the curve is very low but progressively gets more as the voltage is increased. If you translate this to V/I (resistance), you get a variable resistor controlled by a DC voltage. There will be a little distortion of the superimposed AC but as it is an "effect" it probably doesn't matter a great deal.