What we really need for this project is an Arduino board and the datasheet of the ATMega328P.
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.