Available as an assembled Module and as a DIY project.
This Module is currently available.
Light-dependent voltage generator
This is yet another somewhat lazy design, kind of fitting for 2020: four resistors, two photodiodes and a dual op amp. The simplest NLC module, so far.
The photodiodes produce current when light falls on them. This is fed to an op amp which spits out a voltage that reflects the light intensity. The design is really supposed to be an ambience and movement sensor to reflect changing light conditions (if you were playing outdoors for example) and it responds to your movements as you move around tweaking and patching.
It can be used as an optical Theremin, tho the closer your hand (or other appendage) is to the photodiode the lower the output voltage.
These merchants probably sell this module. Huh?