[1] P"Multivibrators." Electronics-tutorials.
[2] "Key Boards and Frequencies.".
[3] "Sedra, Adel S., and Kenneth C. Smith. "Chapter 12 Operational Amplifier Circuits."Microelectronic Circuits. New York: Oxford UP, 2011. 991-92. Print.".
[4] "555 Timer datasheet.".