Step 2: What you need
The datasheet is what tells us how the microcontroller works and it is very important to keep it if we want a lower lever of control.
The datasheet can be found here: http://www.atmel.com/Images/doc8271.pdf
The hardware that I added to the Arduino is partly necessary, its purpose is just to form the signal for the ADC and to provide a voltage level for the trigger. If you want, you could send the signal directly to the Arduino and use some voltage reference defined by a voltage divider, or even the 3.3 V given by the Arduino itself.