Dual-channel passive optocoupler VCA/filter/low-pass gate (4HP - perspex or FR4 panel)
BE AWARE! This is a deliberately lo-fi module - if you want crystal clear sound reproduction, fidelity or whatever then you won't find it here.
The module relies on the properties of a photo-transistor to generally dirty up any signal coming passing through it; think of it as kind-of like a low-pass gate with built in distortion and a few other properties too.
In the beginning there was the OptoThing - it was an interesting piece of kit in that it sometimes behaved a bit like a VCA, or maybe a filter, or maybe somewhere in between. It was a classic example (I think) of how something with a relatively low parts count could do weird and wacky things to your sound. Things have moved on a bit since then but not a lot has changed, really, except that the new version of the module packs two optocoupler gates into a 4HP module - double your opto, double your thing.
Operation is the same - give it a CV, and input and an output and twiddle the pot until you get a response that you like. The two CV inputs are normalled together, so any CV presented to the top half of the module will also appear in the bottom half _unless_ there's something plugged into the lower CV jack.
Use it as a 'ghetto' VCA with an envelope-type waveform or use swept voltages to create bizarre, filter-like effects. Whilst the opportunities aren't quite endless, there are still quite a few of 'em.
Panels are made from laser-cut and engraved, 3mm, opaque black perspex or FR4 (PCB material)- also provided are suitable mounting screws (M3x10 or M3x6). There's no power cable as this is a fully passive module
If you'd prefer to fab your own panels, you can get the design files here: https://bitbucket.org/yorkmodular/eurorack-panels/src/master/OptoThing3-panel/
You can hear it in action here: https://soundcloud.com/yorkmodular/optothing-ghetto-vca