Optical sensor to detect the Mark in The House Electric Meter spinning Wheel.

I need to calculate the time in seconds (20- 60 sec) of the House Electric Analog meter spinning wheel. By knowing AND recording the time it takes, the Black Mark on the wheel t spin one revolution; I can calculate the KWH consumption at a specific moment.

At the moment I record the time manually using stop watch.!!!

I hope to start this project one step at a time. By that I need to build the circuit to just display the time in my laptop.

I am sure this type of project has been tackled many times , so a link will help me great deal to start this project.