http://en.wikipedia.org/wiki/555_timer_IC
http://www.kpsec.freeuk.com/555timer.htm
http://www.sentex.ca/~mec1995/gadgets/555/555.html
http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/
http://teacherweb.com/WI/AshlandHighSchool/ElectricityElectronics/555Timer.ppt