http://en.wikipedia.org/wiki/Schmitt_trigger

http://www.falstad.com/circuit/e-schmitt.html

http://www.play-hookey.com/digital/experiments/rtl_schmitt.html

http://www.visionics.ee/curriculum/Experiments/Schmitt%20Trigger/Schmitt%20Trigger1.html

http://www.johnhearfield.com/Eng/Schmitt.htm

http://web.mit.edu/Magic/Public/papers/00260219.pdf

http://www.daycounter.com/Circuits/Schmitt-Trigger/Transistor-Schmitt-Trigger.phtml

http://www.indiabix.com/electronics-circuits/schmitt-trigger-with-transistors/