Here is the schematic for the 3rd revision of my vactrolSynth. It uses only one 4093 quad NAND gate chip. Try experimenting with different capacitor values for the control and audio oscillators to achieve various tonal capabilities. Note: this schematic omits a potentiometer before the vactrol LED... add this for a pseudo pitch control. Try to use a super-bright LED and a photo-resistor with a wide range for excellent frequency response.