The Basic Triangle/Square Comparator/Integrator Oscillator
For such a simple circuit, I'm surprised that it works as well as it does. But it does work well, as long as you don't need to be too precise. I use this circuit a lot, and found that I kept having to look up those formulas every time I built a new circuit, so I thought it would be good to put them all together on one jpeg:

for pretty much every oscillator topology I've ever seen, the frequency calculation involves a 2*pi term somewhere to convert radians/second to hertz. But not this one. I don't know why, tho it hasn't kept me up at night very often.
Recently I needed an application in which I could vary the pitch from about 30 to 80 Hz. Using the math shown above, here's what I came up with:

Ken Stone uses this basic design in his excellent "Utility LFO". He's added two diodes and a potentiometer between the comparator and the integrator to allow the user to vary the triangle shape from a ramp, through a triangle, to a saw-tooth.

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