Vernier Photogate Sensor
Description:-

Vernier Photogate Sensor allows for extremely accurate timing of events within physics experiments, for studying free fall, air track collisions, pendulum periods, the speed of a rolling object, among other things. 
The laser may be some distance from the gate so that you can measure the speed of larger objects such as a rocket, model car.
The Vernier Photogate includes an accessory rod for mounting to a ring stand.
The Vernier Photogate can be daisy chained so up to four photogates can be connected to a single channel, connecting the last one directly to the interface.
The laser gate mode requires the addition of a common pen laser, which is directed into the laser port. 

Vernier Photogate Sensor