These kits include parts for a photogate that uses a 555 timer as a  Schmitt trigger.  This provides a clean output signal to trigger a flash unit. SPG1. The matched infrared emitter and detector have separate housings. This allows the two parts to be placed different distances apart, depending on the application. SPG2. The emitter and detector are housed in a single interrupter-style unit.  The interrupter, shown to lower right, has a slot of fixed width between the emitter and detector posts. The separation of the posts (5/8" or 1.5 cm) is wider than for most interrupters. This is large enough for liquid drops and small projectiles to pass. Both SPG1 and SPG2 include 3 feet of 2-conductor cable to connect the output to a flash unit and 3 feet of 3-conductor cable to move the emitter-detector pair to a location away from the breadboard.	Above: An assembled Schmitt trigger photogate kit with separate emitter and detector (the SPG1 option).  Click on the photo for a larger view. Below: The interrupter-style emitter-detector that is included with the SPG2 option.  It replaces the separate emitter-detector pair of the SPG1.
An electronic delay unit is commonly used with the photogate to achieve timing synchronization.  You receive a $1.00 discount when purchasing an SPG trigger with a delay unit.  To order this combination, go to this page.
When purchasing a SPG kit with a breadboard, there is a $1.00 savings over buying a kit and breadboard separately.