http://www.falstad.com/circuit/e-multivib-a.html

http://www.electronics-tutorials.ws/waveforms/astable.html

http://www.circuitsgallery.com/2012/03/astable-multivibrator-using-transistors.html

http://www.qrp.pops.net/multivibrator.asp

http://www.tpub.com/neets/book9/36a.htm

http://www.davidbridgen.com/astable.htm

http://www.jamia-physics.net/lecnotes/lab/multivibrator.pdf

http://www.scribd.com/doc/12862517/Designing-an-Astable-Multivibrator

http://circuitelec.blogspot.com/2009/04/pulse-generator-circuit.html