A VCO based on a schematic I found in an old magazine article titled "Inexpensive voltage-controlled oscillator is rich in harmonics over entire audio range". I built a breadboard version during a Hackathon in 2016, then decided to finish it.The magazine article circuit had two flaws in it. The 555 timer was hooked up wrong to be a schmitt trigger and the BJT invertor had a resistor in an incorrect position. I replaced the 555 timer + bjt with an op-amp to simplify things.When debugging the original circuit, I found it really helpful to seperate each circuit block and feed dummy signals into them. I kept that in the PCB as well with the three jumpers.I did an algebraic analysis of the integrator because it's non-intuitive why switching in that resistor would reverse the output. It's uploaded in the Files section of this page.