DESCRIPTION:
Ever wondered how an analog synthesizer actually generates sound? Using some basic components and a 7555 IC, we'll show you how to make your own square-wave generator, allowing you to create small yet powerful and versatile synths of your own!

For a more in-depth look at the 555 timer IC family and what you can do with it, check out this pdf: http://ecee.colorado.edu/~ecen4618/lab1.pdf

Here is another approach to the 7555, its just a wire diagram/schematic.
https://www.circuitlab.com/circuit/dgv878/variable-hz-7555/