I am trying to interface an optical mouse sensor (OM10B) with an Arduino Uno. The sensor is 3.3V and the Arduino runs at 5V, but also has the 3.3V source. I found this very detailed homepage , where someone has done this and I am trying to follow it. However he is not using an Arduino and I do not understand the final circuit well. Since he does not write at which voltage he is operating his ATMega (3.3V or 5V), I am unsure whether I can follow his instrcutions without modifications. Here is the circuit diagram from that homepage:
I don't understand the usage of the Zener diodes at the bottom in combination with the 150 Ohm resistors for the SPI interface. Is this a logic level converter to connect it to an ATMega at 5V or what purpose does it serve? If it is not, how could I go about to connect it to my Arduino Uno?