Kai:
It is funny that you refer to the circuit as "Michael's circuit". I think the honors actually go to Lee Cross who posted it as ASCII in the post of 08-01-03 19:12. (See http://www.8052.com/forumz/read.phtml?id=51944 ). A short bit later he was miffed at the fact that the "\" characters he put in the ASCII circuit were stripped out by the forum software. I saw the problem and decided to help out simply by drawing it as a graphic on 08-02-03 08:21 (See http://www.8052.com/forumz/read.phtml?id=51959 ). I already had the opto isolator graphic from a posting from some months back and simply added the Zener. I did take the liberty at the time to interchange the positions of the opto LED and the 100 ohm resistor becasue it was easier to draw that way and the circuit is essentially equivalent to what Lee had put together.

Now as a further discussion, if I were designing an input that needed ESD protection, I would have used a transzorb for the protection device anyway!! In addition to the transzorb I would also have used a series ESD filter component to slow down the effect of the ESD event as it entered my piece of equipment. My favorite two types of components for the series ESD filter are the surface mount types from ACT (i.e. YCB1206 See http://www.act1.com/emi_hiimp.html ) to be used for single ended signals and the surface mount common mode chokes from Murata (i.e. PLM250S20 See http://www.murata.com/ ) for differential signals.

Here is a snippett of a circuit board design I did that uses components such as these for EMI/RFI filtering and protection:



MICHAEL