I needed a VCO for a project and stumbled on an old Reddit post about symmetrical waveforms using an LM358 opamp. Pretty interesting, and dead simple to build. However, down in the comments I ran into the KASSUTRONICS version and that led me even further down the rabbit hole of the LM358 synth.
Further Google searching and forum reading led me to the page The Exponential VCO from the DIY Synth Series at AllAboutCircuits.com. This design uses a couple of LM358 opamps and a handful of other mostly passive components to create a Square Wave and a Triangle Wave. Small schematic, and looks easy enough to convert to Vero.
Almost looked like what I needed and the circuit explanation is outstanding, even tho I moved on to another schematic for the build, this Expo VCO is the root of what I ended up using. I suggest everyone take the time to read the linked article.
More searching led me to my final destination, the “Dual Shape LM358 Oscillator” This was it, an excellent improvement to the AllAboutCircuits design by Benji Jiao. It follows the Hz/volt standard but can be easily modified using the Exponential Converter on the AllAboutCircuits site.
This version is a really great mixed-wave VCO, you can even route the triangle and square waves to independent outs sans mixer (or in addition to).
My layout uses a 93mm x 55mm stripboard, which is 35 holes x 19 holes. Sure it could be smaller, but I like using the same size boards on multi-board projects.
I have this built and running inside a sequencer build that is almost finished, it requires some kind of CV to drive the VCO and produce sound.
Here is a quick test I did using my old Baby 8 sequencer.
You can see the whole build live in action on October 21st at the Torrance Art Museum show.
Reference Links
Reddit Post that started me down the Dual Wave VCO path
KASSUTRONICS – Variable wave shape LFO
Original Schematic – “A very common layout” according to the author, Robin Miller at AllAboutCircuits