If the Luminiferous Theremin kit doesn’t work after you have built it, then there are a few things you can do to diagnose the issue.
Check for solder bridges and un-soldered joints, and check that the solder has flowed on to the bigger pins (dry joints)
Connect your USB cable to a Linux or windows machine and run TeraTerm, minicom, or your favourite terminal emulator. Connect to the USB serial port that has appeared, Something like USB Serial Device , a com port like COM6, or a ttyPort /dev/ttyACM0 or similar.
The Terminal emulator needs to be set at 15200 baud ,8 bits ,1 stop bit ,No parity
Unplug the Theremin, and either press both buttons together and power it up, or if you haven’t soldered on the daughter board yet connect pins 5 & 6 of the daughter1 connector to gnd and power it up.
You should get a display similar to the one to the left.
This confirms that both sensors are detected and should give a real time display of the output of left and right sensors.
If one or the other doesn’t show, then you need to focus your debugging around those sensors.