pit lane reversal

Hi all, I have a pit exit that comes out at my start/finish line. I was planning on fitting a sensor to the exit hole to rectify my lap timing (have 3 lanes crossing the start/finish) issues as the inside lane currently doesn't time & count laps. Does anybody know how hard it is to wire an extra sensor into the APB to fix my problem? A schematic at the very least would have the soldering iron warming up for the experiment.

Thanks Riko, PM sent. Will check out your 3 lane start thread when I get home tonight.