From
http://www.johnhearfield.com/Eng/Schmitt.htm
Adapted for low power operation. This designed for Vn at ~2.8V and Vp at ~3.5V.
A useful concept for introducing hysteresis. Omit the base resistor of the leftmost BJT if the known driving voltage is known to have an acceptably high source impedance to apply to the base of the BJT.
