You can detect pitstop by using a webcam and a special program who can detect mouvement in different area(s)
I try Web Zone Trigger , this program is perfect for pitsop detection because Web Zone Trigger have the possibility to execute a program when a mouvement is detected in a specify area (you can specify more than one area).
So in version 4.26 i add a simple program called pitstop.exe that can be used by Web Zone Trigger to send the pitstop detection information to Pc Lap Counter (the program is in the directory c:\Pc Lap Counter).
The program pitstop.exe accept lane/car information as a parameter

Here is a little documentation of  how to set Zone Trigger for Pitstop detection:

1 - Add a Spot to detect pit stop

2 - It’s a Single pattern Spot (available only with zonetrigger pro version)

3 - Click ‘set’ to take a picture of the place without car

4 - The spot will run the program pitstop.exe

5 - Specify the program name to execute and the lane number

6 - The spot have to execute the program only when the car stay on the zone during 2 seconds

Here is a video of Pc Lap Counter and Zonetrigger in action:

download original video here