I originally built this setup for use with a large air cannon with a 3" bore.
The sensor assembly is attached to the end of the air cannon barrel. It contains infrared sensors used to detect the passage of the projectile. A microprocessor measures the time it takes for a projectile to travel a known distance and displays it on an LCD. From this information, the velocity can be easily calculated.
Step 1: Overview of Operation
The sensor assembly consists of a length of 3” diameter PVC pipe, with the two IR sensor pairs mounted 6 inches apart. The sensor assembly has a threaded fitting on one end, which allows it to be easily connected to another threaded fitting on the end of the barrel.
The timing circuit initially monitors the first sensor, waiting to detect that the beam has been broken by the projectile. After the first beam is broken, the processor starts an internal timer to record the time that has elapsed since the beam was broken. The software then begins monitoring the second sensor. When that beam is broken, the timer will be stopped. The processor then displays the time elapsed between the breaking of the beams on the LCD.