Easy to build DIY VCO
An analog sawtooth-core oscillator with a reasonably accurate volt/octave tracking, made from just a handful of basic components. It features sawtooth and pulse outputs as well as 1V/oct, FM, and PWM inputs.
https://www.ericasynths.lv/shop/diy-kits-1/edu-diy-vco/
This Module is currently available.
$70