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Wait a moment for it to get going.
Change the pot to change the frequency. The capacitor can be changed as well, but it is usually easier to change resistance in real applications. I got this circuit out of "Engineers Mini-Notebook: Op Amp IC Circuits"
Blue is the output, green is the non-inverting input (reference voltage), and orange is the inverting input (capacitor voltage).
In this example, the feedback resistors are both equal to 100k, making the reference voltage half of the output voltage. That means the frequency will be 1/(2ln(3)RC), or about 1/(2.2RC). R is the pot and C is the cap.
This circuit also uses a second op-amp in a voltage follower (analog buffer) configuration, which provides a virtual ground at 2.5v, which is used as a reference voltage for the first op-amp's circuit. This allows you to make a clock generator with a single-ended 5v supply.
I have built this in real life using a TL072, and it works.
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