# zener diode hystersis and digital input

• posted

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

• posted

Probably because it's not true. Maybe you're thinking of a diac or SUS or something of that ilk. The voltage-current curve is single-valued (no hysteresis).

Best regards, Spehro Pefhany

• posted

"Probably because it's not true. Maybe you're thinking of a diac or SUS or something of that ilk. The voltage-current curve is single-valued (no hysteresis). "

True enough. I was thinking Schmitt trigger. That is what happens when I post questions too late at night.

Paul

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