You can play music just like on the original theremin, just waiving your hands, but sensors are not made of the antennas, but the modulated infrared light. Diodes LED13 and LED14 are IR emitters, and photodiodes (marked as FREQ and VOL on the schematics diagram) are receivers. The signal is amplified, filtered with bandpass filters, demodulated and amplified again as the DC signal. One channel is used to determine the period of the divided frequency, and the other one for PWM modulated volume of the output signal. Simple as can be.
The instrument is built on two PCBs, the upper one contains the analogue circuitry, and the lower one is with the microcontroller PIC16F18426 and USB interface. However, USB is not used in this project, so it is for power supply only. But the FTDI chip is still in its place - who knows, maybe I'll upgrade the firmware once.
Two groups of 6 white LEDs each, driven by T3 and T4, are just for visual effect, and they do not affect the audio operation.