zener diode hystersis and digital input

I'm working on the second article in the "Beginning robots for $50 a
Month" series that I plan to submit to Servo.  In this article, I walk
the reader through building a complete CIRC BOT robot.  Since the
definition I am using for "robot" requires the bot to be able to sense
its environment, I have included two CDS cells in separate voltage
divider circuits that are each read by an A/D channel on the
microcontroller.

At the end of the article, I explain that there are different ways to
do many of the circuits on the CIRC BOT.  I give the reader hints at
what some of those alternative circuits are so that they can go Google
for more information and learn on their own instead of just following
my directions.

For the CDS cells, I tell them that there is a way to read the
difference between the two CDS cells on one A/D line by putting them
into the same voltage divider.  I go on to tell them that they could
also which cell is recieving more light  using a common digital input
since both digital 0 and digital 1 actually cover a range of voltages.
I warn them that there is a "no mans' land" range of voltages that they
will need to stay clear of by putting hystersis into their circuit.

Here is where my problem is: I seem to recall that a zener diode has
hystersis so that once a voltage where breakdown occurs is reached, the
diode will continue to allow backwards flow until another voltage that
is lower than the breakdown in reached.  However, I am having a problem
locating examples of this on the web.  Does anyone know of a website
that gives an example of this?

Paul Pawelski