To make it a voltage controlled oscillator (=VCO) we need the tweak it a bit.

As mentioned before, one possibilities to change the oscillators speed is to change the incoming voltage levels.

My idea is to split the squarewave into positive and negative, clip the maximum of both, and add them together at the end.

The negative part gets inverted after and before clipping. This way i can clip both halfs with just one control voltage.

I built it up with some opamps and .....

It actually works!

Yellow is the triangle output and blue the control voltage.