This synthesizer has 4 individual voices. Each voice consists of a fan, LED and LDR-based light sensor. Fan blades interrupt light hitting the sensor that causes alternating voltage at its output. So the speed of the fan is proportional to sound frequency of the voice.
Each of 4 voices goes into a mixer. Mixed signal then goes into delay module.
This synthesizer has 3 operation modes: push buttons to turn LEDs on; push buttons to turn LEDs off, random sequence.