This kit includes parts for a photogate that uses a transistor to switch a flash unit on.  The circuit has a rapid response. The matched infrared emitter and detector have separate housings. This allows the two parts to be placed different distances apart, depending on the application.	Click on the photo to enlarge.
The kit also includes  i) 3 feet of 2-conductor cable to connect the output to a flash unit and ii) 3 feet of 3-conductor cable to move the emitter-detector pair to a location away from the breadboard.
When purchasing a TPG kit with a breadboard, there's a $1.00 savings over buying a kit and breadboard separately.