An experiment to test an newly installed kit from Dr. Blankenstein with photocell controlled ring-modulator. I put it in a cheap guitar from a charity shop.
Christmas fairy lights control the resistance of the photocell and later I use the motor of a fan to give a signal through the guitar pickups to feed the ring-mod. A ring-mod gives you the sum and difference of the two frequencies you put in it - often along with a chunk of the original signals... It's one of the oldest audio effects, invented by Frank A. Cowan in 1934 Further info here!