Following on from a popular video a few years ago on my YouTube channel I have found a schematic online for a Voltage Controlled LFO.
The design is one that I have seen floating around, I believe originally came from an LM358 datasheet. I have taken this one that works well for me.
Searching for LM358 VCO brings up a few images with this design.
The main additions I have made is an adjustable PWM comparator, this currently has a + and – 12V potentiometer (zero is 50% duty cycle) This can be connected via a summer if required, as can the input frequency to allow for additional sources of control.
The resistor to ground on the frequency pot kind of gives the control a small bias, so the voltage never goes to zero which stops the oscillator.
The output is a triangle which is a through zero waveform, to give you a 0 to 5 volt triangle, the top right op amp gives a DC offset. The square wave is easy, the diode clamps off all the negative voltage.
For the frequency control bias, as mentioned on the youtube video there are other ways of doing this. You can also supply these inputs from voltage summers to have multiple CVs or DAC control. It is quite flexible