You can build a VCO from a single thyristor and a capacitor (and some resistors): http://home.debitel.net/user/jhaible/pe1000_1.gif That's from the Korg PE-1000. Note that the 1/4 LM324 is not part of the oscillator core - it's the _waveshaper_ section! (and it buffers the oscillator, of course) JH.