> hopefully not stupid question..is it possible to modify the circuit so the sound frequency is controlled with only one pot?
If R.G. had wanted that, with sine-wave output, he would not have used this nice simple oscillator.
It's a real problem for simple audio sources, and I'll have to work it out.
What do you want? If you just want a semi-tone of pitch and only "tone" output, change R3 to like 6K8 plus 5K pot.
Even then, because of how R4 trims the gain to 1.00000...., any change in R1 R2 R3 will want a change in R4's trimmers.
And if you take this idea further, the oscillator will get unhappy, quit or go square.
The "right" way is to change all three capacitors C1 C2 C3 in proportion. You can't get trim-caps this value. You could try switching values, but the C and R interacts so if the ratio of the capacitors changes even a little, you are back to trimming R4 every time you switch.
Not "only one pot", not with this plan.
The real "problem" is to get a Sine with one pot frequency. Most wide-range solutions use two pots on one knob plus an automatic gain trim to keep it oscillating despite minor mis-match between pots.
For many audio workbench uses, a triangle would be just as good, sometimes better,
But not here. R.G. uses self-oscillation and marginal-self-oscillation to get the "Notes" mode. The simple triangle generators don't oscillate like this, they are function generators.
Sure, it is "possible" get just what you want. Let the tone generator run steady, with whatever to control frequency. Use a variable gain scheme to make it loud/soft according to an LFO or whatever shape/rate you want.
But design thought has cost. If we pretend that R.G. worked 10 hours and is worth $10/hour that's $100. He's surely worth much more. Your ideas might take longer to think-up. Would the result save $100 in your time and labor?
A gated square is dead simple, two 555 chips. You may find plans as "555 beeper" or "2-tone siren".